Climate change is not just an abstract, distant threat; it is an ongoing process with tangible impacts on our weather systems. One crucial element of our atmospheric dynamics that climate change is influencing is the jet stream, a fast-flowing ribbon of air high in the atmosphere that plays a pivotal role in shaping weather patterns. Recent studies indicate that the jet stream is becoming more erratic due to climate change. This shift has profound implications, particularly for regions like the UK, where the jet stream’s behaviour directly affects temperature and precipitation patterns.
The Jet Stream, AMOC & Climate Change
The jet stream is driven by the temperature difference between the polar regions and the equator. As the Arctic warms at an accelerated rate (a phenomenon known as Arctic amplification) the temperature gradient that powers the jet stream weakens. This can cause the jet stream to slow down and become more wavy, leading to prolonged weather patterns, whether they be cold spells, heatwaves or storms. In the UK, this altered jet stream behaviour can result in more extreme and unpredictable weather conditions.
A related and critical component of this climatic interaction is the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is a system of ocean currents, including the Gulf Stream, that circulates warm water from the tropics to the North Atlantic. This circulation plays a significant role in regulating the climate of the North Atlantic region, including the UK. As the planet warms, there is concern that the AMOC could slow down due to an influx of freshwater from melting ice caps and increased rainfall, which disrupts the salinity-driven component of this circulation. A slowdown of the AMOC could further destabilise the jet stream, exacerbating the climatic impacts on the UK.
Multiple studies have shown the link between climate change and alterations in the jet stream. Research published in Nature Climate Change has demonstrated that the jet stream is becoming more wavy, which can lead to prolonged periods of extreme weather. Another study in Science indicated that a weakening AMOC contributes to these disruptions by reducing the thermal gradient between the equator and the poles, a crucial driver of the jet stream’s strength and stability.
The jet stream has traditionally acted as a protective barrier for the UK, moderating its climate by steering severe cold and hot air masses away from the region. The UK’s relatively mild winters are, in part, due to this influence. For example, the jet stream helps to keep the frigid Arctic air at bay, ensuring that the UK’s winter temperatures remain relatively moderate compared to other locations at similar latitudes.
Comparison with Other Regions on the Same Latitude
To understand the jet stream’s protective effect on the UK, it’s insightful to compare it with other regions on similar latitudes:
Midlands, UK (Latitude 52°N): Average winter temperature: Around 3C to -3C.
Calgary, Canada (Latitude 51°N): Average winter temperature: Around -9C to -15C. Calgary experiences significantly colder winters compared to the Midlands due to its continental climate, which is not moderated by the Atlantic Ocean or the jet stream to the same extent as the UK.
Warsaw, Poland (Latitude 52°N): Average winter temperature: Around -2C to -6C. Warsaw, while also benefiting from some moderating influences, tends to experience colder winters than the UK Midlands, illustrating the protective role of the jet stream over the British Isles.
Minneapolis, USA (Latitude 45°N): Average winter temperature: Around -10C to -15C. Despite being at a lower latitude, Minneapolis experiences much harsher winters than the Midlands. This further emphasises the unique moderating effect the jet stream has on the UK.
Challenges with Colder Spells & Their Impact on Air Source Heat Pumps
Given that most heat pumps in the UK are designed and sized based on the national average winter temperature of around -3C (colder as you head north into Scotland), if the UK were to experience a month of -10C or -15C, would many air source heat pump systems struggle to reach and maintain target temperatures? To explore this, we asked several heat pump installers whether this was a fair assessment. Here’s what they had to say:
David Alston explains that some high-quality ASHPs can maintain their output even at very low temperatures, such as -17C, by artificially running the compressor at higher frequencies through the inverter. However, he points out, “the colder it gets, the more kilowatts the house will need, often requiring built-in immersion heaters to supplement the heat.” Alston notes that during the cold snap in 2011, while many boilers failed, ASHPs continued to operate. “Customers did understand they would have to wait a little longer for domestic hot water as a lot of the energy was focused on the heating circuit a lot of the time, but it was an interesting experiment,” he says. He also highlights that colder conditions typically mean less wind, which can reduce ventilation losses, and that snow acts as an insulator, enhancing the thermal envelope of buildings.
Brendon states that while theoretically ASHPs might meet design temperatures they are often oversized in the UK due to the calculations used. “Sufficient secondary flow rate on the house side of a buffer can maintain good performance below -3C,” he explains.
Damon Blakemore expresses concern about operational challenges at extremely low temperatures, such as difficulty in defrost cycles and potential system freezes. “You wouldn’t be able to get out of defrost, and the system would probably freeze,” he says. Blakemore emphasises the importance of backup heaters, like those built into Viessmann units, and suggests that for such extreme temperatures, different design considerations, including the use of glycol, would be necessary.
Sune Nightingale emphasises that extreme cold events, such as temperatures below -3C, are still rare in many parts of the UK. In London, for instance, temperatures reached -9C for only two hours over a 20-year period. “The real issue is not the few hours of extreme cold but rather the potential for oversizing heat pumps, which leads to inefficiencies and higher running costs,” Nightingale argues. He notes that outdoor design temperatures vary significantly across the UK, from 2.5C in the Scillies to around -8C in Scotland. He suggests that understanding historical weather data and considering climate change impacts are crucial in accurately assessing heat pump performance.
Jason Perry acknowledges that while most systems might struggle during extended cold spells, many heat pumps are unintentionally oversized, which could mitigate some of the potential issues. “The assessment is correct, but it’s not as big an issue as the theory tells us,” he adds.
James Law confirms that heat pumps would struggle to reach their target temperatures at -10C to -15C. “The answer is very simply yes, they would struggle to reach their target temperatures,” he says, offering to demonstrate the performance of heat pumps under such conditions using software simulations.
Heat Pump Design and Efficiency: Adapting to New Realities
The design of heat pump systems is crucial in determining their efficiency and effectiveness, especially in the face of potentially more extreme weather conditions. Moving from traditional gas and oil boilers to heat pumps necessitates a fundamental shift in how we heat our homes – transitioning from short bursts of high heat to a “low and slow” approach that maintains a consistent, moderate temperature.
With traditional gas and oil boilers, many households, especially those in fuel poverty, rely on short bursts of heating to quickly warm their homes, often leaving the heating off for several hours to save on fuel costs. This method, while potentially cost-effective with boilers, is far from optimal for heat pumps. Heat pumps are designed to operate most efficiently when maintaining a steady temperature, as frequent on-off cycling can lead to higher energy consumption and increased wear and tear on the system.
For households in fuel poverty, the challenge is significant. If maintaining a set point of 19C over above becomes unaffordable, the fallback option might be to lower the set point to 16C or 15C. While this wouldn’t result in the same cold temperatures experienced when a boiler is off for long periods, it would also mean that the home wouldn’t reach the warmer bursts typical of boiler heating.
Lowering the thermostat set point by even a few degrees can have a significant impact on energy consumption and heating costs. Studies suggest that for every degree Celsius reduction in the set point, households could save approximately 6-10% on their heating bills. This means that lowering the temperature from 19C to a more modest 16C could result in a reduction of heating costs by as much as 18-30%.
This cost-saving measure can be especially appealing to households facing fuel poverty, where heating expenses form a substantial portion of the budget. For these households, reducing the set point by a few degrees might mean the difference between being able to afford heating throughout the winter and facing financial hardship.
However, these savings come with trade-offs. Lowering the set point to save on heating bills can lead to a cooler indoor environment, which may not provide the same level of comfort, especially during the coldest days of winter. The impact is particularly concerning for vulnerable populations, such as the elderly, young children or those with pre-existing health conditions. Prolonged exposure to lower indoor temperatures can increase the risk of cold-related illnesses, such as hypothermia, respiratory issues and even cardiovascular problems.
Furthermore, a lower set point may also affect the overall livability of the home. Reduced temperatures could lead to increased condensation and dampness, potentially fostering the growth of mould and mildew. This not only poses health risks but can also lead to additional costs related to home maintenance and repairs.
In balancing these factors, it becomes clear that while lowering the set point can offer financial relief, it is not without its downsides. The decision to reduce the thermostat setting should be carefully considered, taking into account both the potential savings and the health and comfort of the household members. For those in fuel poverty, finding the optimal balance between energy efficiency, cost savings, and maintaining a safe, comfortable living environment is crucial.
The potential for a disrupted jet stream to bring more severe cold weather to the UK presents a significant challenge for the design and efficiency of air source heat pumps. As climate change continues to alter weather patterns, it is increasingly important to question whether current ASHP designs and sizing practices can handle abnormally cold, sustained spells that could last for weeks or even months.
Heat pumps are typically sized based on the assumption of “design temperatures,” which represent the coldest conditions a location is expected to experience under ‘normal’ climate patterns. In the UK, where winters have traditionally been relatively mild, these design temperatures have informed the standard practice of sizing heat pumps to provide adequate heating in typical winter conditions. However, this approach may not account for the potential of extended periods of extreme cold, which could become more common as climate change disrupts the jet stream and leads to harsher winters.
The efficiency and capacity of ASHPs diminish as outside temperatures drop, and if a heat pump is sized just to meet the needs of a home under average cold conditions, it could struggle to maintain adequate heat during prolonged extreme cold spells.
Even heat pumps that are correctly sized according to current standards may turn out to be undersized if future winters bring more sustained and severe cold temperatures than those experienced historically. In such scenarios, the heat pump may run continuously without ever reaching the desired indoor temperature, leading to uncomfortable living conditions and potentially higher electricity bills as the system works overtime.
I’d love to hear your thoughts on this… how do you think we can best prepare heat pumps for the challenges of a changing climate?
Well, what a bringer of doom, gloom and despond you are today Mars! I feel we have passed the point of ‘no return’ and any form of rectification would be measured in centuries or even a millennium or two.
It is said that the only other similar land mass to experience a similar climate to the UK is the Falkland Isles; being surrounded by a large mass of water provides unique weather conditions. So it looks as though for the foreseeable future, we can look forward (if that is the appropriate term) to greater variability in weather conditions – with greater extremes than experienced in living memory or even within documented historical weather records. When speaking to meteorologist friends, they have always talked in terms of centuries as being but a ‘blip’ in terms of the world’s history – the last ice age being a relatively recent event.
At school (in the 1950’s) we were taught that the gulf stream influenced our weather by bringing warm ocean currents in our direction – and less seems to have been known about the influence of the jet stream, or if it was known, it had not filtered down to the curriculum as far as I know.
The Scilly Isles have had the ‘buffer’ provided by the gulf stream and this has allowed the islands to enjoy a mainly free of freezing conditions climate for many years (might this now be threatened I wonder?) With the jet stream wobbling or modulating erratically, I suppose we can look forward to even greater truth in the saying “The UK doesn’t have a climate, it just has weather!’
The changing weather conditions might well add more strength to the argument to phase out fossil fuels and ‘come clean’ ASAP (and the ‘P’ needs to be far sooner than 2050 as well! Weather conditions could see a dramatic increase in pollution from fossil fuels with descending winter temperatures – we can hardly afford to sit back and think ‘well it might go the other way and we will be fine’. I’m nearly 77 now and will have to weather it out, but my grandchildren surely have a right to a better and more optimistic forecast than we are hearing about?
One more observation: Methinks that a lot of thought went into contriving the acronym ‘AMOC’!😉
Regards, Toodles.
@Toodles
I seem to remember reading once that an ice age occurs every 10,000 years. I think the next one is due a week next Wednesday. 😋
@Derek M Could you be a little more precise please, is that a.m or p.m.?😉 Toodles.
Massive global events usually happen in the morning. 😆
…but in this case you’ll be fine for a bit of lunch but it’ll ruin your barbecue. You’ll need a rib-sticking hearty slow-cooked stew instead but if you time it right you can cook it for free from the solar leccy generated in the morning.
@Toodles You raise several important points, and I appreciate the depth of your reflections, particularly on the long-term view of climate changes and their broader implications.
It’s true that the scale of climate change and its potential impacts can feel overwhelming, especially when we consider it in the context of centuries or even millennia. The idea that we might be past the point of no return is a sobering one, and while it’s easy to feel despondent, it’s also important to recognise that every action we take now, however small, contributes to shaping the future. While we may not reverse all the damage within our lifetimes, we can still make a significant difference for the generations that follow.
Your mention of the Falkland Islands and the unique climate of the UK due to its surrounding waters is particularly interesting. Indeed, our climate is influenced by a complex interplay of ocean currents, like the Gulf Stream, and atmospheric patterns such as the jet stream. The Gulf Stream has long been credited with moderating our climate, but as we’ve started to understand the jet stream’s role more fully, it’s become clear just how interconnected and delicate these systems are. The recent erratic behaviour of the jet stream could indeed challenge the relatively mild climate the UK has enjoyed, as you noted with the Scilly Isles.
Your point about the urgency of phasing out fossil fuels is crucial. The potential for increased pollution during colder winters only underscores the need for immediate and decisive action. As you suggest, relying on the hope that conditions might improve on their own is not a strategy we can afford to take. The time for change is now, and while 2050 is the current target, there is a growing consensus that we must aim for much sooner if we are to mitigate the worst effects of climate change.
I share your concern for future generations, including your grandchildren. It’s imperative that we strive to leave them a world that is not only livable but thriving. While the challenges are immense, the shift towards cleaner energy, more sustainable living and greater environmental awareness offers a path forward.
As for AMOC, I agree that it’s an acronym that sticks with you – whether by design or coincidence! But beyond its memorable name, understanding the Atlantic Meridional Overturning Circulation and its role in our climate system is crucial as we navigate these uncertain times.
Reverse the change.
Yes, I know that’s flippant and far easier said than done but it feels like preparing heat pumps for a changing climate is tinkering with the symptoms instead of tackling the blindingly obvious cause.
@Majordennisbloodnok I completely understand where you’re coming from – it’s a sentiment that resonates with many of us who are deeply concerned about the direction our climate is heading. Addressing the root cause of climate change is undeniably crucial, and you’re right to point out that preparing heat pumps for a changing climate can feel like we’re only addressing the symptoms.
However, it’s important to recignise that while tackling the root cause – reducing greenhouse gas emissions and transitioning to a more sustainable way of living – is the ultimate goal, we also need to manage the realities we’re already facing. The climate is changing, and even with the most aggressive mitigation efforts, some of these changes are now inevitable. This means we must adapt our infrastructure and technology, including heating systems like heat pumps, to cope with the new conditions.
Think of it as a two-pronged approach: on one hand, we must fight to reverse and mitigate climate change by reducing emissions, transitioning to renewable energy and changing our consumption patterns. On the other hand, we must also adapt to the changes that are already here and those that are coming. Preparing heat pumps for colder, more variable winters is part of that adaptation strategy.
In essence, adapting our technology doesn’t mean we’re giving up on reversing climate change; rather, it’s about ensuring we can continue to live safely and sustainably in a world that’s already feeling its effects. Both efforts – mitigation and adaptation – are necessary to navigate the challenges ahead.
Not saying the content of the article isn’t facts, but it is sensationalising.
Not only heat pumps, most boiler systems are also to have the room radiators sized for -3 deg. They are generally designed for dT50 so very little wiggle room if you start to get -9 degs.
Not really that simple, mine will do -20 on paper, BUT as temp drops to -10 and below the output temp reduces, by -20 the max output temp is 40 degs. Systems designed to 45 or even the latest trend 50+ at -3, stand zero chance with backup heating.
Last heating season, we had 2 weeks at end of Nov/Dec at between -7 and -9 and again the same in January. Not really an issue, little shocking seeing the bills, but the house was warm enough
BUT
So for perspective that is about 7% of the year.
For those that have MCS installations, this is what design guide says
Standard MIS 3005, requires the unit to achieve 100% of the duty at an external temperature condition exceeded for 99.6% of the year.
So in my case an MCS install should be designing to meet those temps – i.e. -9 degs in my area, NE Scotland, same is also true for Highlands which can get colder for longer.
Not really sure that is true if the design follows MCS guidelines, if it designed for -9 (our case) it doesn’t matter if its day or several months, its designed to cope.
@johnmo Thank you for your detailed response.
Regarding the design of heating systems, you’re absolutely correct that many boiler systems, like heat pumps, are designed with outdoor temperatures around -3°C in mind. As you mentioned, radiators are often sized for these conditions with little room for adjustment if temperatures drop significantly. This is a common design approach in the UK and reflects the climate conditions we’ve historically expected.
Your point about high-quality ASHPs maintaining output at very low temperatures is well taken. It’s true that as temperatures drop, especially below -10°C, the efficiency and output of these systems can decline, with maximum output temperatures potentially falling to 40°C or lower by -20°C. This highlights the importance of designing systems with appropriate backup heating, especially in regions like NE Scotland, where colder temperatures can persist for weeks, as you’ve experienced.
You also bring up a critical aspect of heat pump design according to the MCS guidelines, which require systems to achieve 100% of the heating duty at an external temperature exceeded for 99.6% of the year. This standard is indeed intended to ensure that systems are designed to cope with the coldest conditions typically experienced in a given area. If a system is designed for -9°C, as in your case, it should theoretically manage sustained periods at that temperature, regardless of whether it lasts a few days or several months.
However, the article aims to explore the potential challenges posed by even more extreme and prolonged cold spells, which could exceed these design expectations. While MCS guidelines provide a solid foundation, climate change introduces an element of unpredictability, and the historical data we base these designs on might not fully account for future conditions.
The intention wasn’t to undermine the robustness of current standards but to consider the “what ifs" as our climate continues to evolve. It’s about preparing for the unexpected and ensuring that our heating systems, whether boilers or heat pumps, can cope not just with the extremes we know but with those that may yet come.
My reaction is that this is great news for the fossil fuel industry, yet another reason to delay change.
More sanely, the prediction for the shutting down of the gulf stream is between 2025 and 2095 with a core value of 2050. If we ignore the extreme because we can’t do anything about it, 2050 is far enough in the future that heat pumps and other central heating systems being fitted now will be due for an update and also far enough that most of our energy will come from renewables. But even if it’s earlier just add in a 3kW willis heater and that will give enough extra oomph to cover the colder days. Turn up the ft 5C and the emitters will emit sufficient heat. Yes it will cost more, but colder days will cost more whatever the heat source.
Ironically the many over-specified ashp systems that seem to be a feature of the industry will perform better if the climate is colder.
It’s probably less of a problem for a system designed for heat pumps than for one designed for dt50, as @johnmo says.
Ironically the many over-specified ashp systems that seem to be a feature of the industry will perform better if the climate is colder
Of course that won’t stop the fossil fuel industry exploiting fear to delay change.
In summery I think this is scare mongering, tells a one sided and distorted story and is probably encouraged by the fossil fuel industry. Whist the underlying science about the gulf stream is presumably true, the conclusions reached in relation to heat pumps (when compared to fossil fuel heating) are largely Bow Locks.
@JamesPa Thank you for sharing your thoughts, and I appreciate your candid response.
Firstly, let me assure you that the purpose of this article is not to provide ammunition to the fossil fuel industry or to stoke unnecessary fear. Our goal is to engage in a serious discussion about how our climate is changing and what that means for our heating systems, particularly heat pumps, which are a cornerstone of the UK’s strategy for reducing carbon emissions.
You’re absolutely right that many of the heating systems being installed today will be due for an upgrade by the time we potentially face the more severe impacts of climate change, such as a significant weakening of the Gulf Stream. The core value of 2050 for such an event does give us some time to adapt, and we can expect continued advances in heat pump technology and renewable energy integration over the coming decades.
However, my article’s focus is on ensuring that we’re not just passively waiting for those advancements, but actively considering how we might need to prepare our homes for the possible extremes sooner rather than later. For instance, adding a 3kW Willis heater or adjusting the flow temperature as you suggest are practical solutions that could help mitigate the impact of colder days in the future. It’s essential to think about these options now, especially for households investing in heat pump systems today.
Regarding the point about oversizing ASHP systems, it’s true that an over-specified system might cope better with a colder climate. However, oversizing also brings inefficiencies and higher running costs in the short term, which we must weigh against the potential long-term benefits. This is what makes this so complicated. The article aims to strike a balance between acknowledging these challenges and recognising the resilience and adaptability of well-designed heat pump systems.
Lastly, I want to address the concern that the article is “scare mongering" or telling a “one-sided and distorted story." That’s simply not what I do. The intent was to present a thoughtful exploration of the potential risks and how we might mitigate them – not to suggest that heat pumps are inherently inferior to fossil fuel systems or to downplay their benefits. Heat pumps are a vital part of our transition to a low-carbon future, and it’s crucial that we understand and address any challenges that may arise as our climate changes.
The article is not about delaying the shift away from fossil fuels but about ensuring that the systems we’re moving toward are as robust and future-proof as possible. It’s a conversation that needs to be had, and I’m glad you’ve contributed your perspective to it.
Thanks for all the thoughtful comments and feedback above. I’ll respond more directly to address some key points. The goal of this piece isn’t to be sensationalist—it’s about highlighting real issues and urging homeowners not just in the UK, but globally, to make their home heating and cooling systems more efficient. This is essential to reduce reliance on fossil fuels and lower carbon emissions.
With good system design it need not be the case. Our house is new well insulated and pretty airtight with MVHR. Heat pump sizing was done my me on a self install. I chose the heat pump being close enough and it was a really good prices – but it kicks out 6kW at -3 when my heat load is around 3kW. And at an average outside temperature of 10 degs I don’t need heating. Getting a suitable heat pump not that easy and at the time none on the shelf. So not really by design I have a heat pump that’s still ticking over at -9. But I needed to work smart to get it all to work. So no zones, no additional pumps just simple and open. And about 60-70L of system volume.
To support a given kW and not suffer short cycling you need a defined volume of water of 10 to 15L per kW output. Then a bit of fine tuning the running parameters.
I am currently running cooling via UFH – have been for the past month, the heat pump runs for 10 to 13 mins then compressor stops, for an hour or so based on floor return temperature (dictated mostly solar gain) and so overnight there is 3 or 4 hrs between runs. When the compressor runs it has a starting spike of about 1kW and quickly settle down to about 6-700W. The pattern is pretty similar when I start to need heating. As it gets colder the compressor runs longer.
Low flow temperature is key to flexibility for falling temperature, you need headroom to increase flow temps as needed.
You can live with a perceived oversized heat pump. But you need big volumes of water with large heat pumps. So a 12kW heat pump is needing 120 to 180L. So a big system and/or volumiser or a big 2 port buffer, with hydraulic separation, suitably controlled.
@johnmo this is precisely the kind of conversation I wanted this article to create because your insights highlight several important considerations about system design and performance, particularly in relation to managing extreme temperatures.
It’s clear from your description that effective system design, including proper sizing and volume management, plays a crucial role in ensuring that heat pumps can perform reliably even in colder conditions. Your approach of using a well-insulated, airtight house with MVHR and a well-sized heat pump system is a great example of how thoughtful design can enhance performance and efficiency.
The fact that your system performs well at -9°C despite being initially undersized speaks to the resilience of a well-designed heat pump setup. This is the question that I wanted answered, and that many seasoned installer are still grappling with.
Your experience also underscores an important point made in the article: while heat pumps are designed to cope with a range of temperatures, the specifics of their performance can depend significantly on how they are installed and managed. Systems that are well-designed, with sufficient volume and appropriate control settings, can indeed handle colder conditions more effectively than those that are not.
@JamesPa You bring up an excellent point about the adaptability of systems designed for certain conditions. While these systems are optimised for a specific range, many can indeed be adjusted to cope with variations. As you noted, the potential changes due to shifts in the Gulf Stream or other climate-related factors may require adaptations, but not necessarily a complete overhaul – at least not immediately.
One other factor I hadn’t thought of while writing the article is that given the typical lifespan of heat pumps there is room for ongoing assessment of the risks and potential recalibration as our understanding of climate change and its impacts evolves. So, for example, if winters get dramatically colder over say a decade, then installation guidelines surrounding sizing of heat pumps would have to adapt and change to accommodate for these vacations.
Your concern about overwhelming the workforce with too many variables is also valid. The challenge is to ensure that discussions about future-proofing heating systems remain measured and practical, without causing undue alarm or complicating the already complex task of transitioning away from fossil fuels.
Warsaw gets colder winters because it’s further in land. From living in Spain which is 80% mountains I saw how in the summer it is so much hotter inland than the coasts and so much colder in the winter.
We could go snow boarding in Granada on a winter morning and the same day go lie on an Andalucían beach in a bikini in the afternoon. Germany, Romania Poland and Ukraine are good examples of really cold winters and hot summers.
That’s not to deny the impact of the gulf stream on the UK of course but a Spanish meteorologist once told me that the biggest impact on British weather was being a little rock in the sea alongside the major landmass of Europe. He drew me charts of Atlantic weather patterns and how they hit our little rock. 😁
In some ways it’s a major advance that we are actually talking about design. Most fossil fuel central heating systems aren’t designed so far as I can make out!
PS I wasn’t for a moment suggesting you article was intended to help the fossil fuel industry. However it will do anything to preserve itself so far as I can tell, so if it can use a spin on the gulf stream to sow fear it will.
Realistically any system design can only be optimised for a certain range of conditions. If the conditions change then the system may perform sub optimally but often still acceptably, or may need adaption. On rare occasions it will need replacing in toto, but usually only in response to large changes. This is true for heating systems and many other systems.
Also just because a system is ‘designed’ for say -3 at 45C doesn’t mean it won’t (automatically) increase the flow temp above 45 when the temperature is even colder. Many, perhaps most, will, so will just work or may simply need a tweak to the WC curve.
I did think about the gulf stream issue a year or two ago in relation to my own system and concluded that ignoring it was both a safe way forward and probably the best given the timing uncertainty. If it happens in 2025 then various adaptions can deal with it (from a heating point of view). However I suspect heating might be the least of the problems this effect of climate change triggers.
My concern about introducing yet another variable (climate change at an uncertain date) into the design process (other than for geeks like us) is that it overwhelms an already strained workforce which, frankly, is struggling with the current design requirements. If it is to be introduced then it needs to be done through some simple change to the target design temp or similar. Asking installers to cope with the uncertainties is asking for trouble. So I do agree it needs to be discussed, but in a relatively closed group that reaches and implements a conclusion not the general public domain and certainly not in the telegraph or daily mail. Otherwise the almost inevitable result is further to delay the move away from fossil fuels.
@SusannaSaunders, reading your contribution has made me revisit the article I wrote when I started this thread back in August 2024, and I think you’ve identified the concern I was trying to explore at the time. Having thought about it considerably more over the intervening two years, my position has developed somewhat, although I still believe the underlying question is an important one.
The distinction I keep coming back to is the difference between an exceptionally cold spell and a fundamental change in what constitutes a normal British winter.
If a heat pump is designed to meet a property’s heating requirements at -3C, it doesn’t suddenly stop working when temperatures fall to -10C. Depending on the machine, the building and its heating system, it may continue to maintain comfortable temperatures, require supplementary heating or struggle to meet the desired indoor temperature. If those conditions last a few days, I suspect most households could manage, although I wouldn’t want to underestimate the consequences for elderly or vulnerable occupants.
But what if those conditions persisted for six weeks or three months?
That’s where I think your argument becomes much more compelling, and it’s also where my original article was heading. A system that occasionally struggles during an extreme cold snap is one thing. A system that cannot adequately heat its property for a significant proportion of winter is something else entirely. And that’s not necessarily a criticism of heat pumps, but rather a recognition that heating systems are designed around assumptions, and those assumptions may eventually need to change.
Where my thinking has evolved is in how we should respond to that possibility. In 2024, I was primarily questioning whether existing design practices would remain appropriate if climate change produced colder and more prolonged winters. I still think that question is valid, but I’m now more reluctant to conclude that the answer is simply to install larger heat pumps today.
Oversizing introduces its own compromises, including increased capital costs and potentially poorer operation during milder conditions we currently experience for most of the heating season. We shouldn’t create one set of problems while attempting to protect ourselves against another that remains uncertain.
I’d also be cautious about treating a particular atmospheric CO2 concentration as a definitive tipping point for AMOC collapse, or assuming that the UK will necessarily experience sustained -20C winters within the lifespan of today’s installations. The scientific debate is developing, and I wouldn’t claim to know which scenario will ultimately prove correct. Having said that, none of that means we should ignore the risk.
What I’d really like to see is greater emphasis on resilience within the design process. When an installer proposes a heat pump, I’d like homeowners to understand what happens if temperatures fall substantially below the design point (if even only for a week), whether the machine retains sufficient capacity, what supplementary heating is available and how the installation might be adapted if our winter conditions change over time.
That seems more sensible to me than either assuming our climate will remain unchanged or automatically sizing every installation for -20C.
And if we genuinely reach a point where AMOC disruption and associated changes in atmospheric circulation produce prolonged temperatures of -20C or -25C across Britain, I suspect the consequences will extend far beyond domestic heating. Agriculture, food production, transport, electricity infrastructure and essential services would all be confronting an extraordinary challenge. Some heat pumps would continue operating, others would approach their limits and alternative or supplementary heating arrangements might become necessary, much as colder countries already accommodate different heating technologies and backup systems.
At that point, we would be dealing with a national adaptation challenge rather than simply a question of whether somebody installed a 10kW or 12kW heat pump.
So I do take your argument onboard and I don’t think it’s unreasonable to ask these questions now rather than wait until the climate forces us to confront them. What I’m less convinced about is whether the possibility of a radically different climate justifies designing every British heating system around the most extreme scenario today.
Perhaps the more useful question is, if the climate changes significantly during the 15-20 year life of a heat pump, have we given the homeowner a heating system that can adapt, or one whose limitations were effectively fixed on installation day?
Curious to know what others think.
@Mars Hi Mars,
Thank you so much for responding to my question! It’s very much appreciated!
Please appreciate that I’m a strong advocate for heat pumps and I’m on the journey to electrify my own home with solar and batteries already installed. I spent my entire pension lump sum on it! So a heat pump is very much a logical next step! I would totally agree with the premise that we, as a species, need to transition away from fossil fuels and switch to renewables ASAP! I think that’s a done argument…
However, this idea about designing to a (current) nominal state (e.g. -2C winter conditions) is where I’m struggling… I get that this is to ensure a good SCOP and consequently ensure cheaper running costs! As well as the environmental benefits, this is a win-win! But any dynamic system will change over time and our climate in the UK would fall into that category for me.
I’m a retired pharmaceutical research scientist in my 60s and I’ve seen the weather in the UK change significantly in my lifetime already. Cold winters are a rarity these days and having just endured 5 heatwaves this summer (we had 36C in Somerset), I think it’s now reasonable to say that our weather is a lot more irratic and variable than it was a few decades ago because of climate change. Both in a positive (hotter) direction and probably will be in a negative (colder) direction too. We just haven’t had those wild negative winter swings yet. But that might be due to luck e.g. El Nino, the 11 year solar cycle [that peaked in Oct 2024 with 161 sunspots] and a bunch of other rhythms that are simultaneously occurring that I’ll not go into here as this post is already too long). The AMOC is just part of that very complex planetary weather system. Suffice to say, there are medium to long range rhythms that a heat pump designed to last 20+ years needs to operate within.
Yet I’m not seeing that realization reflected in the ASHP industry. Nor any contingency planning for what happens for homeowners if the UK weather norms significantly change because of issues like the AMOC, the solar cycle, El Nino and La Nina etc. At best, the industry has taken the norms over the last 10 years (which may well prove to be non-typical when viewed over several decades) and used that as a basis for extrapolation for their heat pump design parameters… It’s short termism because the industry doesn’t have to pick up the tab…
From an installer point of view, they probably think that any warranty would have long since lapsed ten years from now. Not their problem. Manufacturers don’t care as long as they make sales… But for the homeowner, they are left struggling with a heating system designed for weather conditions from a decade ago.
Secondary heating options might well be necessary to build robust home heating systems. But it would be preferable that we had scalable ASHP systems that can be added to easily if necessary after the initial install. Much as we have stackable batteries that you can just drop an additional battery on the stack if you want more. Either by a simple add-on unit to boost the power output of the heat pump and use any redundancy built into the emitters around the home. Yet that never gets put forward as an option on the table. Manufacturers are not moving in that direction at all! It’s designed for a specific flow temperature and emitter capacity… It’s designed and fitted once and your done! In a system designed with thin efficiency margins and limited ‘wiggle room’, compared to a gas boiler that can have large redundancies built into it – that concerns me! Am I setting myself up for a whole heap of trouble down the road?
Personally, I think that the ASHP industry and installers need to be thinking more broadly about heating redundancies and how to build this into systems to enable better flexibility down the road. Leaving the customer to sort this issue out in 10 years time is not good enough for me to feel comfortable blowing another large sum of money on it…
Yet I’m seeing little if any discussion on this key issue, which is an elephant in the room for me.
If you know of scalable ASHP options, please do point me in the right direction! What are the best options that currently exist? Currently, it seems that I’m left with one of three options:
A) sit and wait to see what the climate does while more scientific data on climate change modeling is gathered… Which will probably start to introduce adaptation measures in the UK very slowly (why is this not being done already)!
B) Accept a bunch of maths around -2C current winter conditions and cross my fingers that the climate doesn’t change significantly for the remainder of my lifetime.
C) Build in a load of redundancies and secondary heating systems to try to mitigate the deficiencies I see in current ASHP systems. Assuming I can find an installer that would work with me to do this.
I’d very much like to hear your take on this on how the ASHP industry is responding to these longer term issues. Is it really just ‘Not my problem’ thinking?
@SusannaSaunders, I think you’ve put your finger on something that deserves more attention, and it’s the financial commitment you’ve already made that brings this into focus for me. You’ve invested your pension lump sum in solar and batteries, you’re committed to moving away from fossil fuels and now you’re contemplating another substantial investment in equipment you reasonably expect to serve you for the next couple of decades and wanting to understand how that investment might cope with changing conditions is perfectly reasonable.
Where I agree with you most is that the homeowner carries the long-term consequences of decisions made during the design and installation process. The installer completes the job, the manufacturer provides a warranty for a defined period, and the homeowner is left living with whatever flexibility, limitations and compromises have been built into the system. We all know the drill. I’ve experienced that imbalance firsthand, albeit for different reasons, and it’s something that continues to concern me about our industry, but that isn’t going to change.
I would make one distinction, though. Designing a heat pump around an outdoor temperature of -3C isn’t necessarily designing it around an average winter, or deliberately leaving it with no spare capacity. It’s an engineering judgement based on the cold conditions a particular location would normally be expected to experience. The machine may have capacity beyond that point, and there may be scope to increase flow temperatures or use supplementary heating when conditions become more severe. The difficulty is that homeowners are rarely shown what those margins actually are, so they have little idea whether their proposed system would cope comfortably at -10C or begin struggling almost immediately.
I also wouldn’t automatically assume that increasingly erratic weather means dramatically colder winters are inevitable. Current mainstream projections still generally indicate warmer British winters, although the more severe AMOC scenarios introduce a different set of possibilities. I’m not qualified to judge which climate model will ultimately prove most accurate, and I don’t think homeowners should have to become climate scientists to make sensible decisions about their heating.
Your comparison with stackable batteries is particularly interesting because it introduces something we seldom discuss… why can’t future expandability form part of the original heating design?
There are heat pump systems capable of operating in a cascade arrangement, where two or more units work together. Mitsubishi Electric has offered this with Ecodan, and Panasonic has cascade-control systems for its Aquarea range. So the underlying concept certainly exists. The limitation is that adding another heat pump isn’t remotely as straightforward as adding another battery module. It involves the hydraulic design, electrical supply, controls, outdoor space and the ability of the existing heating distribution system to accommodate the additional output. These solutions have generally been developed for larger properties and commercial applications rather than marketed as simple future upgrades for ordinary homes. As crazy as it sounds, the hybrid boiler approach still makes most sense to me. The heat pump does 95% of the winter work, and if we have a crazy winter spell of low temps, the boiler (gas or oil) happily takes over. But in the current grant language, to avail of BUS, the boiler has to go.
I think there are also less expensive ways to incorporate flexibility from the outset. Adequately sized pipework, radiators capable of operating efficiently at lower temperatures, sensible access for future modifications and a straightforward hydraulic arrangement all make a heating system easier to adapt. Better insulation reduces the demand in the first place, potentially creating additional headroom during unusually cold weather. None of those measures necessarily requires installing a substantially oversized heat pump today.
Supplementary heating is another legitimate consideration. Some systems can accommodate electrical backup for space heating, although that would be expensive to operate continuously for a prolonged period of time and shouldn’t be confused with the immersion heater that heats domestic hot water. Another possibility, depending on the property’s layout, might be an air-to-air heat pump serving particular rooms. That could provide additional heating during exceptional cold weather while also delivering something increasingly valuable during our hotter summers… cooling. Given your experiences in Somerset this year, I suspect that dual benefit might be worth considering.
I’m also not suggesting you should rush out and buy two heating systems, particularly after the investment you’ve already made. But these are options worth assessing rather than dismissing.
That brings me to your three choices. I don’t think you’re necessarily confined to waiting, crossing your fingers or spending a fortune on backup equipment.
In fact, I’d also propose a fourth… commission a properly considered heat pump design that includes a documented assessment of what happens outside the normal operating assumptions, alongside an affordable plan for future adaptation.
I’d want the designer to show me the calculated heat loss, the proposed heat pump’s actual output at several outdoor temperatures, what flow temperatures would be required, where supplementary heating might become necessary and what could reasonably be modified in future. Most importantly, I’d want those conversations before the equipment was selected, rather than after the installation had been completed. This wouldn’t eliminate every uncertainty, but it would give you something considerably more reassuring than a sales proposal promising an attractive SCOP and little explanation of what happens when conditions depart from expectations.
As for whether the industry is operating on a collective “not my problem" basis, I suspect the reality is a little more complicated. Installers are generally designing to present-day standards, manufacturers are producing equipment for established markets and neither is necessarily incentivised to develop bespoke responses to uncertain long-term climatic scenarios. That doesn’t automatically mean they are acting irresponsibly IMO, but I do think the industry has been too narrowly focused on demonstrating compliance with current design requirements rather than discussing longer term adaptability with the people paying for these systems.
I would also argue that any future changes in our understanding of the climate should be reflected in national design guidance, rather than expecting individual installers to interpret AMOC research. The industry needs a credible mechanism for reviewing those assumptions as evidence develops. We had a rough summer this year; now let’s see what this winter brings.
Where I think your contribution is particularly valuable is that you’re approaching this as somebody who actively wants to install a heat pump, not somebody searching for reasons to reject the technology. You’re asking for greater confidence before committing your money, and I don’t believe that is an unreasonable expectation. I think this is sensible and this is what I’m all homeowners to deliberate.
Sorry for the length of the reply. I seem to be getting carried away. One thing I’d be interested to know is whether you’ve reached the stage of obtaining any heat pump quotations or heat loss calculations for your own property.
Ultimately, I don’t think homeowners should have to choose between an oversized system today and an inflexible system tomorrow. The better ambition would be heating systems designed to perform efficiently now, with their limitations understood and sensible opportunities for adaptation preserved. I don’t know if that’s right or wrong, but it seems sensible as I type this.
@Mars Thank You Mars!
You Raise some very valid points to my post and I appreciate the feedback!
“The difficulty is that homeowners are rarely shown what those margins actually are, so they have little idea whether their proposed system would cope comfortably at -10C or begin struggling almost immediately."
This is a key point that installers and system designers need to be doing! What is the impact going to be around the heating capacity envelop of a given system design, it’s likely increased costs at a range of operational temperatures [outside the design envelop], additional wear and tear [e.g. increased flow rates increases wear within the pipework] and the expected drop in efficiency as flow temperatures are ramped up to increase heat output. This would be a key consideration I would certainly want to understand! Under what conditions would the system cease to keep my home at the desired 22-23C and what steps can be included to mitigate these conditions, both with the heating system and any secondary heating systems. The discussions I have had with a couple of installers to date have been careful NOT to go anywhere near this territory.
“I’m not qualified to judge which climate model will ultimately prove most accurate, and I don’t think homeowners should have to become climate scientists to make sensible decisions about their heating."
I’m most certainly NOT qualified to judge such issues either! But I can at least be aware of these issues and try to draw some broad opinions about where our climate MIGHT go… As for the Manufacturers/Installers – it’s a s**t sandwich they are very much trying to dodge atm and hoping that the customer simply won’t think to ask."There are heat pump systems capable of operating in a cascade arrangement, where two or more units work together. Mitsubishi Electric has offered this with Ecodan, and Panasonic has cascade-control systems for its Aquarea range. So the underlying concept certainly exists. The limitation is that adding another heat pump isn’t remotely as straightforward as adding another battery module." …
Thanks for this info! I’ll look into these offerings to see what they are about. And yes, I appreciate that adding flexibility into any system design adds cost, complexity and potential redundancy. All issues that the industry would rather duck out of… It’s not going to be as easy as sticking another battery or DCEV charger on top of the stack. But it’s a direction of travel the industry should be thinking about imo.
“As crazy as it sounds, the hybrid boiler approach still makes most sense to me. The heat pump does 95% of the winter work, and if we have a crazy winter spell of low temps, the boiler (gas or oil) happily takes over. But in the current grant language, to avail of BUS, the boiler has to go."
And therein lies another problem we have with the current government initiatives… It’s all black and white thinking [most to avoid cheating and manipulation of the system aka just add a A2A aircon system for free]. But also to benefit the rich that can afford a complete system swop too… You need a significant pot of money to take advantage of the BUS schemes ‘as is’, which might become mute anyway if they are scrapped in the forthcoming budget.
“Another possibility, depending on the property’s layout, might be an air-to-air heat pump serving particular rooms. That could provide additional heating during exceptional cold weather while also delivering something increasingly valuable during our hotter summers… cooling. Given your experiences in Somerset this year, I suspect that dual benefit might be worth considering."
This is very much the direction I am heading towards given the summer we have just endured! If you had asked me last year when we have the solar PV and batteries installed I’d have said go with A2W and be done. This year I’m very much looking at a combined A2W for heating and DHW and an A2A for cooling and *additional* heating capacity! The points made about the available additional range of operational temperatures discussed above again comes into play, along with the potential impact upon running costs etc. We specifically bought a new build in 2024 with a south facing roof [in hindsight I wish we had bought an East/West roof but that’s another story] for the purposes of solar PV and future electrification with a heat pump to help mitigate the heat losses during colder winters. The house purchase was one of the worse decisions of my life [because of the property developers!] but it is certainly warm and well insulated!
“In fact, I’d also propose a fourth… commission a properly considered heat pump design that includes a documented assessment of what happens outside the normal operating assumptions, alongside an affordable plan for future adaptation.
…This wouldn’t eliminate every uncertainty, but it would give you something considerably more reassuring than a sales proposal promising an attractive SCOP and little explanation of what happens when conditions depart from expectations."
That is indeed exactly the plan I have for myself! It should be the default for all ASHP installations imo. At the moment though, I don’t think we are going to have the money [circa £5-7k cash plus the BUS grant] so it might be kicked into the long grass anyway. But a plan and a vision of where the industry is going would be helpful.
“That doesn’t automatically mean they are acting irresponsibly IMO, but I do think the industry has been too narrowly focused on demonstrating compliance with current design requirements rather than discussing longer term adaptability with the people paying for these systems."
Agreed. System ‘Adaptability’ has an uncomfortable *smell* of eating away profitability so the industry is probably reticent to open this particular pandoras box (especially in a ‘young’ industry in the UK) and is instead turning a blind eye to it. That’s my opinion anyway.
“I would also argue that any future changes in our understanding of the climate should be reflected in national design guidance, rather than expecting individual installers to interpret AMOC research. The industry needs a credible mechanism for reviewing those assumptions as evidence develops. We had a rough summer this year; now let’s see what this winter brings."
This is where I think the government should be doing more – providing national guidance on adaptability both for our current housing stock and for future electrification and progression in the housing market more broadly. We should have an independent government body to help advise the heating (and cooling) industry about likely future housing needs and strategies. But, in the UK, we are lucky if we can even get the property developers to put up some houses with the bare minimum [G98/3.68 kW cap] amount of solar panels! The MCS certification process is I suppose a step in the right direction but it’s barely a minimum and does nothing to address the issues I raise in this thread!
“Where I think your contribution is particularly valuable is that you’re approaching this as somebody who actively wants to install a heat pump, not somebody searching for reasons to reject the technology. You’re asking for greater confidence before committing your money, and I don’t believe that is an unreasonable expectation. I think this is sensible and this is what I’m all homeowners to deliberate."
Indeed! This is where the industry needs to go! Lets have a frank and honest discussion with the manufacturers and installers AND the government… Tell us clearly and precisely what variance we are likely to have to deal with and advise us on the best mitigation strategies rather than offer us Black and White solutions that paper over the real issues that we face as we go forwards with electrification!
As for myself, yes I have obtained a number of basic quotations but without a proper heat loss evaluation as of yet as these cost money and only provide refunds of this expenditure with the installer that you go with. So I could spend £150-250 on a heat loss evaluation and then find I can’t get that knocked off the install bill because I’ve gone with a different installer… Another cart before the horse issue. Coupled with the 1st April VAT deadline etc… Irksome!
I’m starting to work through my own heat loss calculations [or at least hoping too! Help!]. I have two years worth of heating data for this house already and that is also helping to guide my thinking on heating needs. I’ve been looking into degree days https://www.degreedays.net I plan to take a look at Heatpunk over the next month or two as well. https://heatpunk.co.uk What happens in the pending budget though will also impact what we do. I suspect the the BUS scheme will be raided by BS Burnham [I’m a Green party voter] to pay for the increased military expenditure…
I would possibly challenge this statement. Yes you get a lot of defrosts when the OAT is approximately -2c due to the residual damp in the air, however when you get to OAT temperatures of -10c for example, the moisture in the air is very low and hence ice forming on the evaporator is a lot lower leading to less if any defrosts.
I had this experience last Winter when we had a few days of -7c and the HP did not enter defrost at all. In northern Finland you can become very dehydrated in the Winter due to the very low humidity ( OAT -18c average )
@techgeek, I think that’s a very fair challenge and it probably deserves more attention than I gave it in the original article, which I did write a while ago.
It’s almost counterintuitive that a heat pump might experience more defrost cycles at -2C than at -10C, but your explanation is correct. Those damp, near freezing conditions can be particularly troublesome because there’s still plenty of moisture available to accumulate on the evaporator. As temperatures fall further, the air generally contains considerably less water vapour, so frost accumulation can actually become less of a problem. That’s not to say defrosting disappears altogether, because humidity, evaporator temperature and local conditions still factor into things, but it does suggest Damon’s original statement was probably too categorical.
And I think this helps sharpen the discussion we’ve been having with @SusannaSaunders. The challenge of prolonged extreme cold isn’t necessarily that heat pumps become trapped in endless defrost cycles, but whether they can continue delivering sufficient heat as the building’s demand increases and the machine’s available output and efficiency potentially decline. I also continue to have massive issues with manufacture’s labels and published outputs that don’t deliver heat at temps below 0.
Your experience at -7C is particularly interesting because it challenges the assumption that colder conditions automatically mean greater operational difficulties. I’d be interested to know whether you noticed any meaningful deterioration in efficiency or heating performance during those days, despite the absence of defrost cycles. I know our bodged heat pump system failed miserably when we had a prolonged -7C spell 4 or 5 winters ago, while consuming over 100kWh of electricity every day.
It’s also a useful reminder that heat pump behaviour isn’t always as intuitive as we might assume, and that real world events can add important nuance to theoretical concerns and assumptions. I’m glad that this conversation is challenging my original article! Very interesting indeed.
@Mars
Regarding my system performance, the HP did manage to maintain the house to 21c during this cold spell of -7c, which I was very pleased about! My Wife did not notice any change in temperature either, which was a great relief on my my part! 😀 As expected there was a significant up tick in power consumption, my system usually consumes approximately 800Wh with an OAT of +7c however during this period of -7c, the consumption was in the region of 2.5 to 3 kWh. Sadly I do not own heat meters etc so can only go be the electric power consumed. Generally I get a SCOP of 3.9 using the figures produced by the HP controller.I really wouldn’t worry too much about the heating at -20
At a sustained -20 a large number of UK permeable brick and stone houses with water in the bricks will simply fall down, our water supply will fail and lots of people will die. Heating is not going to be the top issue.
@EtchedPixels, jovial thought for the day 🤣
@EtchedPixels
😯
Well that wasn’t a reply I was expecting. Winter temperature’s as low as -20C would certainly cause widespread issues throughout the country [including loss of life and a national emergency more generally] but these changes are likely to evolve over time with increasingly cold winters over a decade or more… It’s not likely to be -2C one winter and -20C the next… although that too can happen.
That said, while you might be right that “Heating is not going to be the top issue." It certainly will be a significant one for millions of people! I’d rather future proof myself to some degree by at least considering that scenario and having some idea of how my house will function during such a winter… Especially when considering purchasing an ASHP solution for my home heating needs.
If you are in doubt of whether such things might happen, the coldest temperature has been -27C and this has occurred on three occasions:
11 February 1895 at Braemar, Aberdeenshire (East Scotland)10 January 1982 at Braemar, Aberdeenshire (East Scotland)30 December 1995 at Altnaharra No 2, Highlands (North Scotland)
@SusannaSaunders
We have gone down the hybrid route a somewhat different way. Having become fed up with multiple nights of poor sleep due to high temperatures, we got an air-to-air system fitted with 2.5kW and 3.5kW units upstairs and downstairs. This is sufficient to easily keep the house (a 4-bed detached) at 24C when it is 34C for hours on end outside (efficiency on cooling 9.3). Then in the heating season, we used cheap rate overnight electricity and solar PV electricity to heat using the A2A for at most 2.4p per kWh of heat (efficiency on heating 5.2) to supplement the gas central heating. At present, our gas boiler uses about 8400 units of gas per year for heating, hot water, and cooking, supplemented by about 6000kWh of heating from the A2A.
When we first moved into the house, our gas consumption was 25000 units, so we are now around 1/3rd of that and hence 1/3rd of the CO2 emissions. Lots of insulation and a new boiler were responsible for bringing consumption down to around 14000 units, with the A2A providing the final reduction.
With the retrofit insulation, the heat loss of the house is now around 6.5kW, while the existing radiators are sized to give an output of about 12.5kW at dT50, so the radiator output is enough at an outside temperature of -20C and the 24kW heat only boiler should still work at that temperature with some extra lagging on the condensate pipe. The A2A (Daikin Perfera) is also rated to -20C, so should keep working, although I would expect the overall output to be substantially down on the headline 6kW.
At some point, the gas boiler will be replaced by an air-to-water heat pump. Most likely that will be a 7kW unit, if done under the BUS grant with a requirement to match the whole house heat loss. If, as seems more likely at present due to better economics under almost all foreseeable circumstances, I stick with the current hybrid of gas boiler plus A2A until they reach the end of their lives, then I’ll likely swap the lot for a combined heat pump that can run both air-to-water and air-to-air. Such technology is in its relative infancy, but some units are now coming onto the market.
It may not be a perfect solution in terms of reducing CO2 emissions to zero, but adding A2A and keeping your existing gas boiler, until it reaches the end of its life can be a highly cost effective approach. It is likely the most economic approach if the BUS grant for A2W disappears or is substantially reduced. It has a lot of other positives, including cooling, redundancy of heating systems, hedging on gas and electricity prices, and capability to deal with extreme of temperature (hot and cold).
I did a deep dive into the economics in this thread: https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/analysis-of-the-economics-of-switching-to-an-a2w-ashp-compared-to-other-heat-sources/ , which might be of interest.
Finally, aside from winters potential being very cold at times, we are all getting older. When my parents were in their late 80s and early 90s, their comfortable set point for the central heating was 24-25C for the whole house. Hopefully, we will all get to that age at some point ourselves and may need a much warmer set point than the usual 19-20C. This is another way in which the design temperature difference can increase. Instead of -3C to 21C (24C difference) for my house, perhaps I should really consider -5C to +25C (30C difference). Then the heat loss goes from about 6.4kW implying a 7kW heat pump to 8kW needing a larger heat pump…
@Rob of York I must admit that when we had a Daikin heat pump fitted in February 2023, we gave little thought to the increasing summer temperatures; we have made do during the hottest times with closed windows and blinds on whichever side the solar energy was shining and making judicious use of floor standing fans.
The time has come* and we now await the fitting of Panasonic AC in four rooms next Jan/Feb. (Supply of equipment is still experiencing long delays due to abnormally large demand this summer). The AC will also provide a back up for the heat pump; it did cope quite happily to provide us with 22.5 degrees inside when it was minus ~7 or 8 degrees C outside a year or two back but is is still reassuring to know we have a backup plan😉.
*Due to nature, we seem to grow a year older every year and biology finds increasing ways of trying us with increasing summer heat.
Looking on the bright side, with our need for elevated temperatures to be comfortable, we will probably not tax the AC so heavily and the heat will be accompanied by sunshine so our solar panels should be able to power the system – no sweat! Regards, Toodles.
@toodlesles Hiya,
Thanks for your contribution to this discussion! It’s most helpful!!!
It is indeed this past summer of exceptional heat waves in the UK and uncomfortable nights, as well as days, that has focused my attention on A2A systems as well as A2W ASHPs… 😓 And it is certainly now part of my strategy for managing and including additional heating redundancy capacity into any ASHP (A2W) solution as a secondary means of heating my home during exceptionally cold spells! As well as obviously providing cooling during our ever hotter summer months. What I’m still struggling to understand and what is NOT explained by the array of verbiage that I see that abounds about ASHPs is the available (spare) heating capacity in any given system…
So – for example: I want to heat my house to around 22-23C. I have my internal doors all open as a rule (I’m just not someone that likes closed doors and we treat the house as a single zoned space for all practical intents and purposes). I have modelled my home in Heatpunk which for a house at 21C (I don’t know how to change the room temperatures in the Heatpunk model yet 😐 ) and it comes out that my house needs around 4.2 kW of heating at an external temperature of -0.2C. I don’t know how to change the external temperature either! But anyway…
The above has the flow temperature set to 40C but again, I don’t know what flexibility a given HP would have to vary this flow temp through the radiators? Presumably from 35C up to around 50C? With a commensurate lose of efficiency! Again, I don’t know what this drop in efficiency curve looks like as you ramp up the HP flow temperature!? These are not things that are readily in the sales info…
Obviously, the current radiators are woefully inadequate for a low flow rate (assuming I got even half of this house plan in Heatpunk correct). The flow temperature needs to be increased to 60C to turn the ground floor green at 21C room temp [Emitter output 3,313W / 2481W)]. And even at a flow temp. of 75C with emitter output at 3,148/2,176W for the first floor, Bedroom 2 is still only at 92% sufficient…
That said, back in the real world of lived experience – the current gas boiler doesn’t warm the downstairs enough during winter and it usually feels colder than I would like downstairs but the upstairs is usually too warm even with ALL of the radiators turned off except for the two towel rails in the en-suite and bathroom. This is contradictory experience to what the Heatpunk model is predicting. Albeit, the Heatpunk model assumes all of the doors are closed! I presume I could model the ‘open plan’ variant by just deleting all of the doors from the plan?
You said:
“My house has a total heat loss of about 4kW when 21C inside and -3 outside, i.e. a difference of 24C. If it got to -20C that difference becomes 41C, 38C if you drop the inside temperature to 18, 36 @ 16C inside. So the heat loss goes up to 6-6.8kW. However if my living room has a heat loss of 1kW (it’s a bit less) that only goes up to 1.5-1.7kW which is still comfortably within the capacity of both indoor and outdoor units in an air to air system."
How did you calculate the increased heat loss at different temperatures [from 4kW to 6-6.8kW] when going from -3C to -20C external temperature?
What I’m fundamentally trying to understand is the available ‘headroom’ for say a 5kW ASHP for a house with a heat loss of say 4.5 kW @ 0C? How can I calculate the required flow temp for this HP to heat my house to the desired temperature [mostly about 23C in my case – I like it fairly warm! 😊] given a range of external winter temperatures [for example, 0C, -5C, -10C, -15C and -20C] and how this factors into the resultant increased running costs of that system and at what external temperature the HP runs out of headroom in the form of increasing the flow temperature to offset the increased heat loss as the external temperature goes down? So, the External Air Temperature along the X-axis, ASHP Flow Temperature along the Y-Axis and Cost per kWh along the Z-axis… Does such a graph exist and if so where can I find it for some typical ASHP models please? I realise this depends upon the Heat Loss for the property and the pipework bore diameter too – I believe mine is 10mm. There are probably a bunch of other parameters that I’m still completely ignorant of (by all means educate me)! But for arguments sake – lets assume a Heat Loss of 4.5 kW for the house (0C External, 22C Internal] and a 5kW heat pump. What would this look like please?
How would that graph change for an external temp of -10C or -20C?
A further complication [which it might be better to ignore for the moment but anyway – humour me] is the input from any secondary heating sources like an A2A system whose primary purpose is to provide cooling during the summer months. There is again a trade-off in efficiency between running the A2A system in heating mode vs increasing flow temperature of the A2W HP system. At what point does it become cheaper to provide increased heating from the A2A system vs increasing the A2W flow temp? I presume pretty quickly! Even a 5C increase in the flow temp is probably more expensive to run than turning on the A2A system to provide additional heating? Is this hunch correct or am I way off base? But this is just working on a hunch – what would be break points be in this scenario? What additional headroom from a heating perspective would a typical A2A system installed to provide some cooling during summer (as opposed to being installed for heating purposes) provide when working in this mode? I presume a couple of kW of heating capacity?
I get your worked example where you cite in your post above [aka the A2A system needing to provide an additional 1.5 to 1.7 kW of heating for the living room], with a house differential of around +2.8kW in the -20C scenario [4kW to 6.8 kW]. Even if I don’t understand how you got to this figure! I’m unsure if this would work on a whole house though, even with open doors (i.e. no zoning) as would be the case for myself because I don’t understand what additional heating capacity a heat pump could be increased to I guess?
@Mars, if you have some thoughts/info on this I’d love to hear them too!Overall, my expectation is that seasonal temperatures will become more extreme with increasing winter and summer extremes… Possibly as much as the occasional -20C to +40C once or twice a decade. So what does a robust ASHP design look like assuming this long term projection of our weather in the UK over the next couple of decades? Assuming that this holds some water? 😜
@SusannaSaunders
I can help you with some of the fundamental physics.
Heat loss has a linear dependence on temperature difference according to the Heat Transfer Coefficient (HTC) for your house. Assuming that IAT is the Indoor Air Temperature and OAT the Outdoor Air Temperature, then:
Heat Loss = HTC x (IAT – OAT)
Your heat loss worked out at 4200W for a temperature difference of 21.2C, so HTC is 198.1 Watts per degree Kelvin or Centigrade (W/K) for your house. You can lower HTC by improving insulation etc.
If you want the heat loss for a different temperature difference, just multiply HTC, i.e. 198.1 by that new temperature difference. So for an IAT of 23C and and OAT of 0C, -5C, -10C, -15C, and -20C, the heat loss will be 4556W, 5547W, 6537W, 7528W, and 8518W respectively (equivalent to adding 198W for every extra degree of temperature difference).
There is almost no headroom with a 5kW heat pump and 4.5kW heat loss at 0C. Assuming that was calculated for IAT of 21C, you have 500 / 198 = 2.5C to play with. So you can have +23C indoors and -0.5C outdoors and that’s it! However, that would also have the heat pump running flat out all of the time, with nothing spare for your hot water!
Radiator output depends on the temperature difference delta-T (DT) between the room e.g. 20C and the average temperature of the radiator. Assuming a temperature drop of 10C from flow to return of the radiator, the table below shows the multiplying factor to use to find radiator outputs. For example at DT30 degrees, radiators output 51% of the quoted power at DT50. At a flow temp of 40C, the output is just 21%. However, as most heat pumps work on a flow minus return difference of just 5C, you really need to look at the factor where the radiator temperature is 37.5C, which is 0.26 or 26%. Gas boilers typically work at flow minus return of 20C, so 80C flow is needed to get a 70C average radiator temperature. You can work out radiator power output by taking the quoted value at DT50 and multiplying it by (new DT value / 50) to the power 1.3. That’s all the multiplicative factor are in the table.
The reality is that with likely no insulation between your downstairs ceilings and upstairs floors and an open door policy, a lot of the heat in your house is drifting upstairs. We also keep all the doors open and upstairs typically ends up 1 degree or slightly more warmer than downstairs, when the opposite would be ideal.
This table shows how Vaillant Aerotherm heat pump efficiency drops with flow temperature. It also shows how the maximum output drops with outdoor temperature.
There is a double cost hit in coping with lower outdoor temperatures. The efficiency of a heat pump depends on the difference between the outside temperature and the flow temperature required. Lower outdoor temperatures increase that difference directly and also via the higher flow temperature needed to compensate for the higher heat loss.
The point that you are getting at, effectively a sensitivity analysis for both cost and performance capability with decreasing minimum temperatures is a really good one. I’ll give it some thought and see if I can come back with at least a partial answer.
@SusannaSaunders
The only solution I could see is having a secondary heat pump that could be called upon by the controller to help boost the primary heat pump when outside temperatures drop below a certain level. This would increase the volume of air to extract more heat, however I am no expert on the subject! 🙂
The critical bit is whether it’s sustained. Sustained it will crack and destroy bricks and foundations, the water supply will freeze and so on. A couple of days of it won’t, especially if heated.
The lifespan of a heatpump is about 15 years realistically so whilst I agree it will be a longer term factor potentially it’s not yet that urgent. The imminent necessity for air conditioning is going to be much more relevant I suspect.
Will the specs be different a decade from now – probably quite a bit, and the technology somewhat.
Thanks @SusannaSaunders for reviving the topic I find it interesting and there’s been many good points made. I’m in a similar position with a lump sum I’d like to use to decarbonise.
Actually the likelihood of a reduced AMOC and the possibility of colder winters and hotter summers is one of the factors that made me look more closely at an air to air heat pump system for my home.
The summer we’ve just had in the UK seemed like a taste of what might be to come. In Yorkshire the temperatures weren’t so extreme 32 C or so. To me the new factor this year was the overnight temperatures. Previously I’d always been able to keep my house reasonably cool by ventilating overnight. I’m not sure how much this might become a problem with further weakening of the AMOC, or general climate change. So far I’ve only seen the headline that summer maximum temperatures in the UK could commonly be 40C. In lots of ways its the minimum summer temperatures that determine whether a well insulated house with lots of thermal mass (like mine) can be kept cool or not.
Even so I mostly see the cooling side of air to air as a bonus rather than a need. I take the points made in other threads that passive ways to reduce solar gain, blinds, awnings and so on, are more economic and probably sufficient even for a future more extreme UK climate.
I think there’s advantages to air to air heating in extreme cold as well. It comes down to zoning. I’m not sure who it was earlier in this thread who mentioned that when the cost of heating is a serious consideration you might want to reduce the indoor temperature to 16C. I’ve been in that situation, for example during the beast from the east. To keep my bills down I let the spare room and bathroom get down to 10-12C, the bedroom and kitchen to 14C but kept the living room at 18C. A bedroom at 14C is fine with a good thick duvet, especially during the day when I’m not there. But even with warm clothes sitting around at less than 18C is not comfortable. But I found quite a saving on my bills to let its temperature drop overnight only heating it again when needed. That is easy to achieve with air to air but not really with air to water.
My house has a total heat loss of about 4kW when 21C inside and -3 outside, i.e. a difference of 24C. If it got to -20C that difference becomes 41C, 38C if you drop the inside temperature to 18, 36 @ 16C inside. So the heat loss goes up to 6-6.8kW. However if my living room has a heat loss of 1kW (it’s a bit less) that only goes up to 1.5-1.7kW which is still comfortably within the capacity of both indoor and outdoor units in an air to air system.
One final point occurs to me as I write. Are we really looking at -20C as a possible day long temperature or is that a night time minimum? Makes a difference to just how much doom and gloom is predicted.
One solution is a heat-pump with ‘Enhanced Vapor Injection’ (EVI) or the manufacturer’s term for their equivalent, it’s a modification to how the refrigeration cycle works that maintains heat output even at very low outdoor temperatures.
For example, the Samsung EHS Mono HT Quiet units, which are R32 units with ‘Flash Injection Technology’ (appears to be their version of EVI) the capacity tables indicate they can maintain full, or virtually full output all the way down to -25 deg C, even at 65 deg C flow. The CoP falls away, to 1.38 at -25/+65, but I think that’s still pretty impressive.
There are a collection of tools around on the internet that will generate R and U value approximations for walls and give you graphs. It’s never simple though – any battens, window edges and so on will completely change the wall performance at that spot. They don’t generally account for leaks, and for cavities if you don’t know how the cavity is arranged for airflow within the cavity.
Some of them will let you work out what your heat loss is across a given temperature pair, and where the dew point is (also a rather important consideration with high insulation levels).
All of it ends up being very approximate. Houses are not built like physics experiments and something as daft as a leak under a window, or airflow around the edge of a timber floor, or even how the cavity and roof are connected can mess up your numbers a bit let alone all the guess work on how much bridging you’ve got hidden away in lintels and window frames
@EtchedPixels
I agree, the modelling can never be that accurate due to all manner of variations in build even between two ostensibly similar houses. The RdSAP10 manual takes a broad brush approach to estimating the total thermal bridging via a simple factor determined by groups of age bands that is added to U-values. 0.15, 0.11, and 0.08 for homes built in or before 2002, 2003-06, and newer respectively. Similarly, an estimate of 0.5 that is now applied for Air Changes per Hour (ACH) for all homes. On the carefully worked through heat loss calculation for my house, thermal bridging and air changes each account for about 20% of the heat loss, with thermal conduction the other 60%. In practice the effects of thermal bridging and air changes could be significantly more or less than that due to higher variability than thermal conduction based on U-values, which again may not be that precise.
The equation for heat loss = HTC(IAT – OAT) provides the principle component of heat loss, but doesn’t account for differences due to high wind speed, heavy rain etc., nor the more subtle effects around dew point and freezing point that you mention. Nevertheless, it is a valid way of comparing the heat loss that can be expected at different temperatures, e.g. a still day at -15C versus a still day at -5C.
For the BUS grant, heat loss modelling is required to MCS standards. In my opinion, it would be helpful if some account could be taken of practical measurement-based estimates of heat loss. Currently, they do at least provide a way of sanity checking some of the outlandish heat loss estimates that can come out of the models.
@SusannaSaunders
To get a good measurement-base estimate of heat loss needs data to be collected over a heating season, ideally at least 100 days (e.g. Nov, Dec, Jan, Feb). The equation for thermal equilibrium can then be used.
Heat supplied = heat lost = HTC(IAT – OAT)
The data required are daily gas and electricity consumption along with daily average indoor and outdoor temperatures. For each day, an (x,y) coordinate is plotted on a graph of heat supply (y) versus temperature difference (x). Once all of the points have been plotted, a best fit line can be drawn through them (linear regression line). The slope of this line is the Heat Transfer Coefficient (HTC). HTC can then be used to estimate the heat loss at any given temperature difference.
The heat supplied should be calculated as gas kWh x boiler efficiency (e.g. 0.9) plus electricity kWh. Note the boiler efficiency is better slightly overestimated here, so that the heat loss is not under estimated.
The graph below shows this data for my house during winter 2024/25.
The value of HTC comes out at 268W/K, which equates to about 6.4kW heat loss at a 24C temperature difference. This is in close agreement to the modeled value that I got of around 6.2kW. This gives me confidence that the actual heat loss is likely somewhere in the range 6kW to 6.6kW and hence a 7kW heat pump would likely be fine.
Note the spread of points, there are some days above the line where the heat loss was higher even though the temperature difference wasn’t that large. I expect that they were somewhat milder wet and windy days. Other days, below the line may have been very still but cold, perhaps with a little more solar gain, they appear to have lower heat loss.
I did a deep dive into this and included solar gain (using my solar PV output to get a good estimate of this) and also adjusted the gas boiler efficiency based on the weather compensation curve that I use, however, the HTC result was effectively the same as shown below.
If you can get daily indoor and outdoor temperature data from your own hobby weather station, then the above is an excellent way of getting a practical heat loss estimate. If not, then it is possible to use outdoor temperature data from the nearest local Met Office weather station and make an estimate of indoor temperatures. If your heating is on 24/7, then you can use the set point as your average temperature. However, if your heating goes off at night, then you need to account for the fact that the indoor temperature will drop more when it is cold outside. Say the heating is off for 8 hours overnight and when it is 0C outside, the indoor temperature drops by 6C in that time. On average for those 8 hours its 3C colder than the set point, in other words the average is 1C colder than the set point over 24 hours. It is possible to account for this by plotting heat supplied versus 0.95 x temperature difference. The graph below effectively does that. It plots Adjusted Degree Days* (i.e. 0.95 x (15.5C – Outdoor temperature)) against heat supplied. Note this gave a similar value for HTC of 263, which is pretty close given the removal of all indoor temperature information. Also, it only used gas consumption figures, on the grounds that electricity was not directly used for heating and so averaged out at about the same values all of the time, no dependence on hot or cold days.
It is well worth a little time with a spreadsheet to get a practical estimate of heat loss based on real data that can then be used to sanity check values from heat loss models. You can just follow (extrapolate) the line up to the right to much larger temperature differences to see what the heat supply would need to be if it were -10C, -15C etc outside.
@Rob of York Thank you for this!!!I’ll set this up and start monitoring from tomorrow… I have a bunch of ThermPro thermometers so I can do this relatively easily (one on my desk and one in the garage or garden with a suitable ‘weather cover’).
What I do have is the gas consumption data for Sept 2024 to 25 [6,257 kWh] and Sep 25 to 2026 [6.442 kWh].
Both of these winters were very mild though! So probably atypical figures…
The first year (2024-25) the wall thermostat was pretty much on 22C for the bulk of the winter months. For the 2nd year (2025-26) it was probably nearer 23C most of the time as I was feeling cold much of the time the first year.
I also started looking at this last year: How big a heat pump do you need? A simple Rule of Thumb.
This gives me a suggested Heap Pump Power rating of [6,442/2,900] = 2.2 kW
This seems rather on the small side to me even though this is a new build (2024) small house (83m^2).
Or using the Cardiff [3.34N, 51.4N] Heat Degree Days figure of 2067 = [6,442 / 2,067] = 3.1 kW
That’s a significantly different result!?
Fiddling with the HDD figures in a spreadsheet to get an equivalent to 22/23C heat output put the figure just shy of 4.0 kW heat pump…
and a Heat Transfer Coefficient of [6,442/57.3] = 112.43 Watts per degree C.
Therefore to raise the temperature from 0C to 23C would require 23 x 112.43 = 2,585.89 W/hr (2.59 kW/hr)
Therefore for 1 day = 24 x 2,585.89 = 62,061 W/day (62.1 kW/day).
This sounds a LOT! 😲 Daily measurements are clearly needed … Heading to my Octopus gas data download tomorrow and taking a close look at the daily data! I’m going to be clearly busy over the coming weeks!
@SusannaSaunders
Nice! That is the way to go to get the highest quality estimate you can based on practical data.
The issue with the simple methods based on annual gas consumption is that they make assumptions about what the weather is like during the heating season that may not hold for any given winter. Plotting daily data and a regression line eliminates that issue.
I’m with EtchedPixels. Heat pumps designed for -6C won’t keep a house warm at -20C, that is true, but it is hardly unique to heat pumps. Nothing in our country is designed for -20C. I can’t think of a single aspect of my daily life that would operate comfortably at -20C.
The solution, on a personal level, is to install more heat pumps. Specifically air-to-air heat pumps that will also help when the reversal of AMOC turns our mild, short summers into 4 month long 40 degree hellscapes.
I agree few things would be at all comfortable at that temperature.
A few things are however specifically designed to cope with that temperature and below – petrol cars for example, since they are sold into markets that do experience those temperatures e.g. parts of Germany, Scandinavia etc.
The lowest maximum daily temperature ever recorded in the UK was -19.1C at Braemar, Aberdeenshire on 10 January 1982, the same day and location also holds the record for the lowest daily minimum of -27.2C.
7th to 20th Jan 1987, much of England experienced very heavy snow fall and extreme cold. The maximum daily temperature in Warlingham, Surrey on 12th Jan was -9.1C and the minimum -23.3C in Caldecott, Rutland on the 13th. On the 12th Jan the maximum temperature over most of England was between -6C and -8C. The snow fall was over 50cm where I was living in Kent at the time. A consideration of temperatures in the range -5C to -15C is therefore arguably already necessary for most of the UK. Certainly, I wouldn’t want to end up with no way of adequately heating my house should we get another persistent cold wave like that.
@Rob of York Yes, true, petrol (but also electric) cars sold here work fine in Norway too. I wouldn’t necessarily be comfortable in one though!
The other thing I’ll say is that, from memory, those cold snaps resulted in homes being, well, cold. I think there was a bit more acceptance of wearing outdoor clothes indoors back then.
Again this isn’t to minimise the effect of climate change at all — I got a heat pump precisely because of how much I care about protecting our planet from climate change. I’m saying that heat pumps aren’t uniquely bad at dealing with -20C. Most things in this country are bad at dealing with -20C, including old homes with gas boilers.
Let me put it this way, if someone wanted to replace their old gas boiler with a new gas boiler, we wouldn’t suggest they get better insulation and larger radiators too, just in case AMOC reverses and we get -20C winters. Perhaps it is sensible to do that, but this wouldn’t be a routine recommendation for a boiler replacement.
And for a bit of context 1962-63.
In those days very few people had central heating, most houses were heated with coal fires. Our house had a coal fire in one room which meant that the rest of the house was very cold. Funnily enough we managed to survive with just a few chilblains.
Our ASHP system in this house was designed to maintain 20C with an OAT of -3.4C. On the 5th of January the mean OAT was -6C and the lowest was -10.6C. The mean room temperature was 20.1C, so even an under sized and less than ideal R32 system can cope with low temperatures. R290 in a larger machine would cope much better.
Even if that temperature had lasted for several days, undoubtedly the internal temperature would have fallen somewhat, but adding a layer or two of clothes and a bit of supplementary heat would have let us survive without undue discomfort.
I’m all for forward planning, but trying to design a system to cope with a possible but reasonably unlikely event several years in the future seems a bit much. If we did get to a state where average temperatures dropped to -20C for weeks at a time we’re going to have much more serious problems than an undersized heat pump.
@hml1827 1962 / 3 winter is remembered by me as my last winter at boarding school; going out for walks around Exeter and seeing the River Exe frozen with icicles drooping down below the bridge over the River Exe near St. David’s Station and seeing frozen weirs as if in a photograph. And yes…. our dormitory windows had permanent ice glazing on the inside, it was COLD! Ancient Toodles.
I’m not overly concerned about the potential for really extreme events such as AMOC switching direction and plunging the UK into -20C winters. As others have said, if that happens whether the heating can achieve a warm house without supplementary help is going to be a long way down on the list of priorities.
What this thread has made me think about a lot more though is resiliency. Most gas heating systems have a massively over-sized boiler because no one can be bothered doing heat loss calculations for them and a 30kW model costs two hundred pounds more than a 18kW one and impacts the installation hardly at all. The fact it can’t modulate down as far hurts efficiency, but not massively so and most customers don’t notice anyway since the new one is more efficient than the 15 year old 30kW one that it replaces. It doesn’t last as long as it should due to short cycling but the customer doesn’t know that…
Heat pumps are different in that a lot of time and effort goes into actually designing the heating system for a specific minimum temperature that depends on the region of the country the house is in and prescribed room temperatures e.g. 21C. Now if this is done right, and the heat pump is accurately sized on that basis, then it potentially doesn’t leave much headroom.
I have tried to get accurate heat loss estimates using both theoretical models and practical measurements for my house and come up with values around 6.4kW. Taken at face value, it would appear that a 7kW heat pump would be a perfect fit.
However, in terms of resilience, that may not necessarily be a good idea. As I get older, I may want a 24C rather than a 21C set point, and at the same time, we may get another cold spell like the one I experienced in the early 1980s where we have outside temperatures in the range -11C to -5C, call it -8C average for a few days. Then the temperature difference is 32C rather than 24C, and that 6.4kW heat loss turns into 8.5kW. Suddenly, an apparently over-sized 10kW heat pump looks like it could be a good option after all.
What I’m saying is we could usefully use designs that work very well for the expected temperature range – i.e. achieve a high coefficient of performance and avoid lots of undesirable short-cycling during the expected annual temperature range, but are also capable of providing resilience, i.e. will keep working well when asked to deliver for a bigger temperature difference and cope with colder outdoor temperatures.
Going back to my own circumstances, resilience considerations may means that a 10kW heat pump could be a better choice than the equivalent 7kW unit, which has other implications for retrofit such as pipe sizing to support the necessary flow rates to transport 10kW. (The radiators that I have planned would deliver this at a flow temperature of 45C rather than 40C for 7kW, so no issue there). Unfortunately, there is an issue here somewhat inherent in the physics / mechanics of heat pumps that means they cannot modulate down particularly low – usually a floor of around 30% of headline output, which leads to cycling on and off when the heat demand of the house is below that level. By comparison some gas boilers can modulate down to around 10%, which ends up with the same situation if they are in fact 300% over-sized!
The alternative approach is to recognise that under exceptional circumstances supplementary heating is required, size the A2W heat pump for expected temperatures and deliver extra heat via a separate A2A heat pump when necessary, and hence also have a solution for extremely hot outdoor temperatures.
I agree that you need more headroom than 6.4kW vs 7.0kW, what you’re said is entirely reasonable. I would add that (a) there is an error margin on the heat loss calculation and (b) the risk is asymmetric: if you oversize you have a less efficient heat pump, but if you undersize you have a very cold house. If they get it wrong you’ll know about it straight away. If things change in the future you can always adapt your house in the future. My view is it’s pretty cheap and not too disruptive to add a A2A heat pump down the line.
@Rob of York Thanks Rob!
A LOT to digest here!
Please give me a day or two to work through this and it’s implications!
That table of Flow Temp vs radiator output factor… 😬 🤯 Does this mean you need to increase the radiator surface area by 5 times to operate at a flow temp of 40 [Rad temp of 35C]! Even achieving 45C/40C with a factor of 0.3 so aka about 0.35 or roughly 1/3rd ‘rule of thumb’ of the radiator nominal power output. It seems that operating below this threshold, while theoretically plausible is practically unfeasible because the radiators would need to be enormous…
The Vaillant Aerotherm Plus installer manual graphs are very counter-intuitive as well as power output falls as the flow temperature increases… 🙃 How does that even happen??? 😲
The heat pump efficiency tables are very interesting too. Very Alarming! But very interesting too! To maintain any kind of half decent SCOP you need a fairly good amount of headroom to enable you to run the system at a low flow temp… Above 45C becomes punishingly low SCOP. Increase the HP capacity up to 7 or 10kW gives more head room but presumably higher running costs too. It would be interesting to know by what percentage increase across the size options (a complicated question I know because of the shorter cycling times and increased wear and tear issues). The ‘sweet spot’ is a complicated zone!
Thank you for all of this very valuable info!! 😘 🍻 you have increased my knowledge a fair bit so thank you for this! I’ll come back to you once I’ve had some thinking time.
@SusannaSaunders
Indeed, lots to think about there!
Below, I’ll try and fill a few gaps.
Re: Radiators: yes if the radiator temperature is 35C, then you get roughly 1/5th the power output for 70C radiator temperature. So 35C radiator temperature needs very large radiators and/or a house with a very low heat loss. Note, with a heat pump running the typical flow minus return temperatures of 5C, then 35C radiator temperature would correspond to 37.5C flow temperature and 32.5C return temperature. The table is for flow minus return of 10C, which allows for lower flow rates suitable for smaller bore pipework while still being able to transfer the required amount of heat. Getting to a maximum flow temperature of 35C is often only possible with underfloor heating.
If you look at the Stelrad radiator information (
) there is a big table of power outputs at DT50 for different sizes of K1 and K2 radiators. If your old radiators do not have convection fins, then these are either type 10 (single panel no convector fins) or type 20 (two panel no convector fins). You should be able to find some details for their power outputs online. K2s have a much higher output for the same basic size, so it is often possible to upgrade by a factor of two in power output without greatly increasing radiator size. There are also K3s with 3 panels which are much thicker and represent a further upgrade at a more significant cost. K3s are not available in as many sizes.
Note, a system would only operate at its maximum design flow temperature when the outside temperature is at the minimum that the design caters for e.g. -3C. The rest of the time, the weather compensation curve used by the heat pump would lower the flow temperature with the aim of matching the radiator output to the heat loss of the house at the current outdoor temperature. Thus the system will hardly ever actually run at the maximum design flow temperature, most of the time it will be lower.
Re: Power output: The compressor in a heat pump can run at some maximum input power. As the gap between outdoor temperature and flow temperature increases, the efficiency of the heat pump reduces due to the higher lift in temperature required. Hence the maximum output power (= input power x efficiency), also decreases. This is why heat pump output reduces at higher flow temperatures.
A decent design with radiators can often get to 45C maximum flow temperature. Note a SCOP of 3.77 is not bad. It may be some way down on 4.13 at 40C maximum flow temperature, but the running cost differences are not that big. Assuming say 9000kWh of heating, that’s 2387kWh of electricity input at SCOP of 3.77 and 2179kWh at a SCOP of 4.13, so a saving of 208kWh or about £62 per year at full electricity prices. However, specific heat pump tariffs mean that you may only be paying about half that. Chasing the optimal SCOP isn’t necessarily worthwhile. It could notionally take 25-50 years to payback if you need to spend say £1500 extra on new radiators to get there.
@susannasaunders can I just add to @Rob of York s comment. Before I got my heat pump I monitored half hourly
This gives me a suggested Heap Pump Power rating of [6,442/2,900] = 2.2 kW
This seems rather on the small side to me even though this is a new build (2024) small house (83m^2).
I am not at all surprised by this figure. Modern fairly compact houses can be very low loss.
Can I just add to what @Rob of York says about the value of monitoring consumption. Im a great fan of this. Its real data which, used intelligently, circumvents many of the uncertainties associated with surveys. It does introduce some uncertainties of its own but its still well worth doing. Its effectively how my heat pump was sized and had I not used this method the sizing would have been way out. My loss measured by monitoring consumption (and subsequently verified using the heat pump) is 7kW, when I used MCS assumptions to calculate loss based on my own survey it came to 10.5kW; two full ‘professional’ surveys said 16kW. 7kW vs 16kW is a material difference (which I was able to explain).
That said, with a loss of ~2kW it really isnt necessary to determine the value terribly accurately. With an A2W heat pump you are going to end up with a 5-6kW model anyway, and with A2A your capacity will be set by how many outlets you need to cover the rooms.
@SusannaSaunders I agree with @JamesPa, I think that your house’s actual heat loss figure will be somewhere in the range 2.2 to 3.1kW. A 2024 new build has an excellent level of insulation mandated by building regulations.
In terms of heat pumps, a Vaillant Aerotherm 3.5kW could be a possibility, however, the 5kW is very similar in terms of internals. The 3.5kW is restricted in compressor speed compared to the 5kW, and both modulate down to the same minimum output before cycling. For that reason the 5kW may be a better choice as it will be able to heat a tank of hot water faster, and provides much more headroom for dealing with very low temperatures.
According to the graph from the installation manual that I posted earlier, the 5kW unit has an output of about 4.2kW at -15C and 55C flow temperature. Assuming that your heat loss comes out around 2.8kW at a 24 degree temperature difference (-3C to 21C), that would equate to 4.2kW at a 36 degree temperature difference (-15C to 21C). So an Aerotherm 5kW heat pump would do the job at -15C, which is a really good level of resilience, assuming that your radiators can put out 4.2kW at 55C flow temperature.
These heat pumps avoid short cycling via the control logic that and the minimum water volumes that Vaillant specify for the system, so hopefully that would not be an issue even though the amount of heat that you would need could be below the minimum output of the heat pump a lot of the time. With air-to-water heat pumps that may well be unavoidable given the low level of heat loss for your house.
An alternative would be to keep your gas boiler and fit something like a Daiking Perfera 2.5kW single-split air-to-air heat pump. This would cost roughly £2000 and can be fitted in about half a day with minimal disruption, although current demand means it is unlikely that you could get one fitted before about Feb 2027. SCOP is 5.2, and the output at -10C is 2.4kW. These units can modulate right down to about 0.8kW output (125W input). Air-to-air would have the advantage of providing cooling as well as a redundant heating system that can be run alongside your gas central heating and hot water. You could likely halve your gas usage, while still using gas for hot water, and hence avoid the need to add a new hot water tank. With the gas standing charge being removed at the end of 2027, this approach has a lot going for it if you are comfortable with a pragmatic solution that lowers CO2 emissions without dropping them to zero.
@Rob of York Thank you Rob! These are very good suggestions! I very much appreciate the thought gone into your reply! I’ll look into the suggested models and learn more about these options.
I like the suggestion of going with the hybrid approach. It would be much cheaper and still provide utility and a reduction in the gas bill. I’m not sure how a hybrid system would function practically though because the two respect thermostats would be switching each other on and off? Effectively competing to heat the house?
How would a hybrid A2A / gas boiler system work practically? You run the A2A at a set temperature and the boiler at a temperature slightly below this set point (say 1 degree lower) so that the boiler kicks in once the heat pump maxes out and is no longer providing the needed heat? Quite what this heat pump does then is a bit confusing though? Presumably, the house temperature rises above the set point for the boiler and then ossilates between them?
If you have any experience of such a hybrid solution, I’d like to hear it! 👍
@SusannaSaunders For context, our house is a 146 sqm 1970s 4-bed detached with a lot of retrofit insulation. The heat loss is about 6.4kW, annual gas usage of about 14,400 units before installing A2A. Reverse engineering the simple divisor on annual gas usage, this gives 6.4kW = 14,400/2250. So my best guess at the heat loss of your house is 6440/2250 = 2.86kW.
With a heat pump tariff there is a period of cheap rate electricity overnight and often another in the afternoon. For example, British Gas’ tariff (open to those with a heat pump, EV, or home battery) has half price electricity from midnight to 7am and from 1pm to 4pm. We have cheap electricity overnight on an Octopus tariff, and also Solar PV and a home battery, which is used during the day. We have Daikin Perfera 3.5kW and 2.5kW A2A units in the living room and main bedroom respectively. For overnight heating, we only use the unit in the living room.
The way we make hybrid heating work with gas CH and A2A is to schedule them to mainly operate separately, which is what I would suggest that you do using the programmable timer-thermostat for the gas CH. Assuming cheap rate electricity in the above time periods, I would suggest the following. Late evening, set the gas CH to go off about 10pm by reducing the set point from normal (22C?) to say 18C. At midnight, the A2A comes on with a set point say +2C over normal 24C, and runs until 7am. At 7am the set point for the gas CH changes to 22C and the gas CH takes over until 1pm then it’s set point changes to 18C (off again). At 1pm, the A2A comes on, set point 22C or slightly higher until 4pm. At 4pm the gas CH takes over until 10pm, set point 22C.
You can shift a bit more heating requirement onto the A2A by effectively turning the gas CH off a couple of hours before the A2A is due to come on, and then heating with the A2A to a slightly higher temperature than actually needed. A2A in say your living room won’t result in the same consistent temperature throughout the house. The idea is simply to inject some very cheap low carbon heat energy into the house, which reduces your heating bill and CO2 emissions. The gas CH then evens the temperatures out again. You’ll need to do some experimentation to find out what works comfortably given the rate at which your house loses heat, and how well the heat from the A2A permeates throughout the house. The biggest win is simply using A2A on cheap electricity to keep your house warm overnight.
If you have both gas CH and A2A on at the same time, it’s not a problem. One of them will inevitably have a higher set point than the other. The one with the higher set point will end up taking over the heating, and the other one will not run. The only hazard is gas CH on and A2A switched to cooling! That said, it is preferable not to have the thermostat for gas CH in the same room as the A2A (the thermostat for the A2A is in the wall mounted unit), as a lot of heat in that room will then prevent the gas CH coming on while the rest of the house is below the desired temperature. If you can set the TRVs in the living room, so that room won’t overheat just because the gas CH is on and the A2A already put a lot of heat into the room, then it should work quite nicely. Note, I would not let the current siting of the thermostat for your gas CH dictate the A2A location. It is important to have that in the right place, most likely your living room as you are primarily interested in heating with A2A.
@Rob of York Thanks for these additional thoughts Rob! Much appreciated!
In way of background info, we installed 16 solar panels (we did have 18 but 2 had to be removed from the garage roof thanks to our neighbours putting up a trellis that blocked the light along the bottom section of the garage roof that backs onto their garden – we re-orientated the remaining 3 panels on the garage to landscape format but it was annoying!) that gives us 7.44 kWp of solar generation and 3 x 8kWh (24 kWh total) of Sigenstor batteries coupled with a 12 kW inverter.
The batteries are able to provide more power than currently needed [typically we use about 10 to 11 kWh per day). The excess capacity is intended for using a heat pump – once we know what we are going to do and we have the money to implement it! 😆 That’s currently the ‘rub’ I’m here to figure out… We currently run on the Octopus Flux tariff as we fill the batteries between 2-5AM at 16p/kWh and then export anything unused [typically around 12 kWh] at double the cost [30p/kWh] between 4-7PM. It basically means our electricity is free during the Winter and in Summer we make well over £100 a month on export.
I get your point around the TOU and set points for the two systems and I like that approach… How it would work in practice I’m not quite so sure about. But I’m reasonably comfortable that it could be made to work just via managing the set points for each system. The gas central heating (GCH) typically oscillates/cycles between about 21.5 and 23C, which is OK most of the time for us. As I suspect is the case with most wall thermostats, its temperature display reads about 1C higher than the actual room temperature. 🙄 How hard can it be to have a wall thermostat that is actually accurate these days? Tiresome. The GCH thermostat has 0.5C steps, which is kind of a bare minimum granularity as well given the vagaries of hot water system that cycles. Are these kinds of issues (inaccuracies’ and lack of granularity) the same on A2W and A2A heating systems?
I’ve ordered a Switchbot Outdoor Meter Kit with 3 thermometers https://uk.switch-bot.com/products/outdoor-meter-kit?variant=49859900801255 for the purposes of monitoring indoor and outdoor air temperatures over the winter months so that I can calculate the daily air temperature difference vs the gas kWh used per day; with a view to getting a better heat loss figure… I’m still figuring that part of this out but hopefully it will give me some real world data to go on rather than a bunch of vague assumptions and averages. 😏 Hopefully I’ll have that setup and running in the next week.
Thanks again for your help and advice! It’s sincerely very much appreciated!!!
@SusannaSaunders
That’s a serious amount of solar PV given the sqm of your house! You should be in a great position to run a heat pump of whatever flavour off of cheap electricity. That changes the equation somewhat on when to run A2A. You could conceivably run it much of the time, with the TRVs on the living room radiators stopping the GCH from heating that room. Put the thermostat for gas heating elsewhere and use a lower set point for that and the A2A could do most of the work, particularly if you effectively have the GCH off most of the night. There’s definitely quite a few things that you can experiment with to get the right set up that works for you.
To avoid drafts, you can use the indoor quiet mode where the fan is barely audible and you just won’t notice the airflow. The output is lower, but with the low heat loss of your house that is likely still enough to keep your living room warm. Perfera indoor units also have something called “Intelligent Eye" that detects if someone is in the room. With that feature turned on, after 20 mins of no one being there it will drop the set point by 2C to save energy, and automatically up it again when it sees someone return. It also avoids directing air at you to reduce drafts. Overnight, you can let the A2A run in auto mode, which adjusts the fan speed as necessary to reach the set point quickly.
There are A2A systems that do hot water too. Daikin Multi+ is one, but they are bigger multi-split systems, for example 6.8kW capable of connecting to 3 indoor units.
One thing to realise with a semi-detached is that permitted development only allows for one heat pump (outdoor unit). So if you wanted A2A and A2W as separate outdoor units, then you would need planning permission. If they were sited appropriately and the noise calculations worked out fine, then there is no good reason why permission shouldn’t be granted. Having to get planning permission is a PITA though that adds time and cost, and needs to be done prior to installation of the second heat pump.
I suspect that the A2W and A2A BUS grants will be cut sometime in 2027, as the budget just took about £750 million off the overall funding for them and this financial year it looks like the total funding available (£400 million) will run out around the end of the calendar year (only £143 million left at the end of Sept, half way through the financial year). There’s a whole thread here about A2W installs being expensive: https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/why-are-ashp-installs-so-expensive/
The swings in temperature with your gas CH are likely down to the flow temperature being higher than needed for the low heat loss of your house. This means once the thermostat switches the heating off, the room temperature tends to overshoot due to all that (too) hot water in the radiators. You can turn the flow temperature down on the boiler to alleviate this problem. You can probably run at a flow temperature of no more than about 60C (50C right now before it gets near 0C) and that would make the boiler more efficient as it would enable it to operate in condensing mode. Better still you could get weather compensation fitted to the boiler, so that it reduces the flow temperature when it is mild outside. We have this on our boiler and at present for the short times that the gas CH comes on the radiators sit at about 35-40C. This leads to very stable room temperatures and low and slow heating exactly like an A2W heat pump would do.
@SusannaSaunders
When you come to get quotes for A2A, as always get 3 quotes from people who actually visit to see what is required. Let them know that your primary use of the A2A will be for heating as this is a key consideration for where to place the unit and choice of equipment. Also tell them your house’s heat loss is a max of 2.8kW or whatever your best estimate is for it.
They will likely suggest the living room for the indoor unit, however they may have good reasons to suggest somewhere else which they should explain.
I think a 2.5kW unit should be enough, but again an installer may suggest a larger 3.5kW unit and explain why. For example, cooling demand can be larger than heating demand and the cost difference is not that much. A 2kW unit may also be enough, however, 2kW units may simply be restricted versions of 2.5kW units for only a few tens of pounds less, better to get the 2.5kW in that case.
Daikin Perfera units are very good (from experience) and are one of the best in modulating down to very low levels (125W input). I’ve seen this for myself on our solar PV app when the A2A is the only thing running. It does low level heating without cycling very well.
Daikin Comfora may be suggested as a lower priced option and would be perfect if you were only really going to use it for cooling with occasional use for heating. However for a few hundred pounds more, the Perfera is a more robust heavy duty A2A system, better suited to day-in day out use. It has a higher SCOP than Comfora, and will possibly last longer with high usage. I have that one and would recommend it.
Other major Japanese manufacturers also do very good A2A systems, I have no relationship with Daikin other than owning two of their single-split systems.
@SusannaSaunders
I use the A2A for shoulder periods and if we need an instant heat boost in the lounge, this worked really well in conjunction with calor gas reducing our use substantially. Actually did that this evening as our A2W is set at 20c currently and I could not be bothered lighting our log burner, and we were both feeling chilly!
Having multiple options may be a very useful way to cover oneself if we are heading for wildly varying climate.
If you have any specific questions that I can help with please ask. A brief version of my journey here…
https://renewableheatinghub.co.uk/forums/rules-terms-of-use/say-hello-and-introduce-yourself/paged/114/#post-64655
@Watcher Thanks for the input! The option of going with A2A for heating/cooling and keeping the gas boiler for hot water is an appealing one with the daily standing charges being removed next year even tough it means giving up the BUS grant! 🙄 It still leaves a gas boiler that needs annual servicing but I guess that’s not too significant in the scheme of things.
We moved into this new build [built 2024] and one of the consequences of buying a new build, aside from the TINY house footprint in square metres of living space is the attendant severe lack of storage! Despite it being one of the worse decisions of my life because of the piss-poor property developers (as an aside, buying a new build is a seriously BAD IDEA these days!), the houses insulation though has been amazing! Anyway, finding space for a 220L water tank is dubious at best but it could go into the loft. As the house is a semi-detached, presumably it could be attached to either the partition wall or the end gable wall. The roof structure is already under strain with the 13 panels [270 kg] of solar panels though, so I’m unsure about putting another 220 Litre water tank up there as well. That’s half a tonne of additional weight on the house structure it was probably not designed to take… Given the number of short-cuts and dodgy practices that the property developer [Summerfield Homes] has made that I know about (and there have been a LOT of them), I simply don’t trust them AT ALL! But I digress…
An A2A heating system wouldn’t be my ‘go-to’ because of the movement of air causing drafts (rather than radiant heat) but once it’s reached equilibrium I guess that becomes less intrusive. The functionality of home cooling though, certainly outweighs that point! As suggested by @JamesPa and @rob-of-york above in this thread, a 3kW A2A system would be a good viable option in this case. It’s an option that I think I need to get some quotes around and then compare that with the GCH removal option with the BUS grant included… Even with a A2W system, I’m still going to want an A2A system for the summer cooling (I can’t handle the heat waves at all above 25C). So the central problem principally resolved around the DHW supply… To my knowledge, no A2A system will supply hot water… 😢 Only an A2W system will provide this for us. Our hot water bill is only about £15 per month though and that’s with the daily standing charge! Once that’s removed next year, the DHW bill will come down to about half of that! Probably only around £100 per year plus the annual servicing costs. So as much as it pains me, it’s getting hard to justify removing the gas boiler completely, despite the government trying it’s best to push that upon us. Economically, it doesn’t really add up under the current BUS allowances. The allowances would need to be closer to £10k for A2W to offset the DHW issues and radiator replacements.
What are peoples general feelings about the BUS A2W and A2A grant amounts? Are they sufficient or not in your opinion, especially in light of the GCH having to be removed completely?
@SusannaSaunders
If summer cooling is actually the #1 priority for A2A, then almost certainly it would be better installed in your upstairs hallway. Then you can open and close doors to direct cool air where you want it on hot days in summer. Since warm air rises up through the house, it’s much better to have the A2A up there for cooling and let some of the cool air drift downstairs. It will still work well enough for heating and drafts from the airflow are a non-issue in a hallway. By turning down the TRVs in all the upstairs rooms, you can let the A2A effectively take care of upstairs heating and use gas CH downstairs.
My brother has a 67 sqm semi-detached 1950s house, with a good level of retrofit insulation. Heat loss is about 4.5kW. He has a Daikin Perfera 3.5kW single-split A2A system on order (£2200), to be fitted in his upstairs hallway at the recommendation of the same installer that I used last year.
The A2W option is more expensive to install even with the BUS grant, however, given the massive amount of battery capacity that you have, it may still be a good option in the long run. Given your need for cooling in summer, you could take a look at Octopus’ offering (still in development) called “Octopus Cosy Cooler".
https://octopus.energy/cosy-cooler/
This is a circa £500 to £600 add on unit, a bit like a portable AC unit, but it attaches to the radiator pipework to get rid of excess heat via the heat pump rather than trailing a large flexible ducting hose out of a window. It works only in conjunction with Octopus Cosy heat pumps, so you would need a Cosy 6 heat pump, but then you would have a complete low carbon heating and cooling set up running off a single heat pump outside (no planning permission issues). Note there are no details yet on how powerful Cosy Cooler is or its efficiency, other than it says suitable for 20-30 sqm rooms.
I would recommend taking a careful look at the potential running costs for gas CH vs an A2W heat pump. Although you have a large amount of solar and batteries, the fact that you can time shift via your batteries and export at a really good export rate (e.g. 30p) means that your marginal cost of electricity is 30p. In other words, if you take some electricity to run a heat pump, then you have less that you can export and the lost export is 30p per unit, so compared to gas CH that is what your heat pump really costs you to run despite the batteries and solar PV. Given the high capital costs of A2W even after the grant, it is likely that the economics are in favour of keeping gas CH and adding A2A with no subsidy.
There is a distinct risk of overthinking this, modelling these things is extremely difficult even if you have the data and ToU tarrifs vary a lot, are likely to change dramatically as we electrify and are anyway subject to political manipulation. Thus any modelling done now is valid only for a short period of time relative to the lifetime of the kit. Of course if you can and want to do the modelling go ahead, but otherwise I tend personally to simplify it thus:
With an electricity to gas price ratio of 3.5-4 (which is the typical value in recent years) a reasonably well installed A2W heat pump without a battery will cost roughly the same to run as a modern, well set up gas boiler, perhaps a bit less, and should deliver a greater level of comfort with about 25% of the carbon footprint, the latter reducing year on year. If the gas boiler being replaced is old or poorly set up, the heat pump should cost less to run and with a battery and a decent ToU tarrif you should definitely save**. Battery or not, solar panels affect the costs of running principally in the shoulder season when we get bright days of reasonable length, but in the height of the season when its cold, for the most part dull, and the days are short they have a marginal/negligible effect. Overall therefore possibly best ignored at this level of detail.
I accept that this is broad brush and there will be exceptions but most people dont have the time or the data to model it in detail and the uncertainty of future prices anyway cant really be factored in. I therefore suggest that, for most, this is a sound basis for an investment decision.
** The exception is if you are playing arbitrage games (buying at a low price and selling at a high price) with a significant proportion of your battery capacity during the heating season then you should probably ignore the battery when considering the running costs of the heat pump, because any capacity used to power the heat pump will just reduce the capacity available for arbitrage.
More broadly this is an interesting discussion.
I dont think we have fully explored/fully understand A2A yet as a domestic heating solution, there have been too few installs.
With a house that has lots of separate rooms and an existing distribution network it is difficult to see how it is going to work – this is where A2W majors. For a relatively open plan house, particularly if its modest in size, it seems like A2A is a great fit delivering both heating and cooling is a single package at a higher efficiency than A2W. With a decent ToU tarrif and/or solar panels, the simplest DHW solution in this case is probably an immersion heater in a tank. Retaining a boiler, particularly if the standing charge is scrapped, is clearly also an option. By the time you take into account maintenance costs for a boiler dedicated to DHW an immersion probably work out cheaper though!
Regarding cooling this is another area we haven’t fully explored/understood and Im not sure we even have the right products yet!
In the aforesaid open plan house the solution where A2A heating is practical the solution is perhaps obvious – just use A2A. I feel that this could be a major selling point once the industry is ready to deliver A2A in volume in domestic situations, which currently it is not.
In a more fragmented house where A2A heating is a poor fit than then fitting one or two a2a units in the upstairs hallway and perhaps the most used living room seems like a reasonable cooling option, and the same unit(s) also provide supplementary heating if required. However (assuming we have an A2W heat pump) do we really want 2 ODUs/separate systems when we already have an A2W heat pump, and unnecessary wall units. I certainly don’t. Nor do I want the abomination of a tube hanging out the window with a unit that sucks hot air into the house in order to cool it! This is where the product gap lies. The Octopus Cosy Cooler attempts to fill that gap but still leaves you with unit to store for 11 months of the year. I dont really want that either. What I want is a wall hung unit (replacing a radiator) that I can connect to my A2W heat pump that will both heat and cool, without having to insulate all the water pipework in my house! Such units do (sort of) exist designed for swanky office and apartment blocks, but cost about 3K plus each.
This is an interesting question. For me it needs to be asked in the wider context of tarrifs and regulation because if you tweak one it can change the equation dramatically. For example if the ratio of gas to electricity price were 2.5, then the argument for heat pumps as a pure financially driven investment (as opposed to one based on comfort and carbon footprint) becomes very strong.
My personal view is that the current ‘system’ (grants, tarrifs, regulations) is designed to fail. The ratio of electricity to gas prices has risen steadily form 2.5-3 to around 4 during the period 2011-2020, as policy costs were progressively loaded onto unit prices. Meanwhile regulation and customer protection was handed to an industry-funded body which, not surprisingly, designed the regulation to protect the industry more than it protects the customer. The grant certainly helps with the capital cost, and increasing it to £7,500 made a difference, but does little when people do not have confidence in the product and in particular its relative running cost. Finally the industry has not even tried to develop a way to respond to the ‘failed boiler’ scenario, which is the only time most people consider replacing their heating. Furthermore (with the exception of Heat Geek) it sells based on a load of technical specs (flow temperatures etc) which few understand and ultimately none care about, rather than things (cost, comfort) which people both understand and care about. The regulation hampers progress in fixing these! In fairness our understanding of the practical use of the technology, and some aspects of the technology itself, have evolved immensely and thus the technological context has changed, but the underlying fundamentals remain namely a grant/tariff/regulation system that does not encourage the behaviour it is supposed(?) to support.
Whether this ‘design to fail’ was deliberate political choice, lack of attention to detail or just incompetence is a matter of speculation, but until its fixed things wont change materially.
The current government has signalled changes which will improve things. The removal of some policy costs from electricity tariffs and the abolition of the gas standing charge should reduce the electricity to gas price ratio as does the removal of VAT (albeit that this may only be temporary). There is a consultation underway the clear intent of which is to take consumer protection (for several retrofit measures) away from the industry rectifying, hopefully, the fundamental structural error in this part of the picture. There is also talk,, but as yet no concrete proposal, of decoupling electricity from gas prices. However these things take time and more needs to be done if we are serious about converting to electric heating and reducing our dependence on despot states.
Personally I would like to see this go a lot further. We are currently upgrading many parts of the gas distribution network because they are dangerously old. This is ultimately wasted capital investment and, if its possible to construct a suitable scheme, the money could instead be given to households to subsidise them to move off gas. A determined initiative to solve the ‘failed boiler’ problem is essential, the technology is now at a point where its entirely reasonable to contemplate doing this. If we could combine these measures with the current ones aimed at reducing the price ratio, the case for conversion becomes compelling at an individual level, and loans could replace grants to enable those who are less well off to make the switch.
We could even go further and tax the carbon in fuels, recycling the money so that average fuel prices (averaged across all fuels) don’t increase. The costs, short and long term, of continuing to pour carbon into the atmosphere and our reliance of despot states for our energy have to be met somehow, and logically they should be met by those who are responsible. Clearly this would be hugely controversial and would need to be introduced slowly over a period of many years, but there is a strong moral and intergenerational argument to support it. Currently we are consuming what we want and foisting the costs onto future generations, a position that morally is difficult to justify!
@JamesPa
I agree with your general comments about running costs for heat pumps, including the exception if doing arbitrage. The difficulty is that the context @SusannaSaunders has has got to be close to the worst case for making an economic argument for a heat pump vs. gas heating.
Specifically:
Daily arbitrage of 12kWh via a 24kWh battery. While it is likely that flat export rates will drop in the not too distant future, the rate paid for export during the peak at 4-7pm is likely to stay high for a long time. Arbitrage of this sort is exactly what the grid needs and energy providers are willing to pay for. So it seems reasonable to assume this as the marginal rate i.e. the cost for the electricity used by a heat pump.
2024 new build, so a 2-year old gas boiler not in need of replacing anytime soon. Great insulation, so very low gas consumption of 6400 kWh and a modest bill for gas usage of £640 a year at 10p per unit. Combi-boiler and no space for a hot water tank. Radiators sized for gas that need a flow temperature of circa 55C to produce the heat required by the HeatPunk modelling, likely the same as required by MCS (even if we know 4kW is most likely an over estimate).
A BUS grant compliant A2W heat pump would need a hot water tank installing, and likely multiple radiators replacing, then given the arbitrage the running costs would be about the same as the gas boiler that has been replaced.
I just can’t see how the economics can work in this particular case. The install cost would be substantially more than the BUS grant, since a new hot water tank and at least some new radiators would be needed.
The unfortunately thing here is that heat pumps were not mandated for new builds in 2024 as they are now, otherwise there would be one in place, with a hot water tank and radiators of the appropriate size.
Given its a new build and the building regs changed a few years ago to mandate design for max 55C flow temp, it’s quite possible rads don’t need replacing. However I believe builders can game the regs by some sort of pre registration nonsense which locks ttem in to older regs, so it may not be thus way.. In any case I agree totally with everything else you say.
From the floor plans posted earlier the house isn’t particularly open plan, so any a2w heating or cooling might need several units or be limited in scope
As you say heat pumps should have been be mandatory in new hones for several years. The big housebuilders have too much clout.
You are correct that until recently the housebuilders would lay a sewage pipe or similar for each plot once they started and then used that to build all the houses over many years to the date of the sewage pipe. That scam has thankfully now been stopped.
The other two problems are
– Assuming a newbuild has insulation to the standard specified. Lots of people taking IR cameras to their home have discovered they have giant gaps in the insulation or in some cases no insulation fitted at all triggering massive mess
– Assuming the plumbing complies with the specification. You are lucky if it’s water tight.
@EtchedPixels There is a lot to be said for living in a ‘Mature Build’ home, one can move in, assume that much work needs to be done in the way of insulation, re-wiring, re-plastering and decoration etc., but at least one is for-warned that this work may be required to bring the dwelling up to an acceptable standard. The new build should have all this work already carried out but one can never be sure…
@Toodles
I’m sure you won’t have any issues on that front, your multi-split A2A will be massively efficient on cooling (SEER 8.5?).
The hottest day this year, we had to dump 36kWh of heat out of our 146 sqm house to stay 10 degrees below ambient (24C vs. 34C) the efficiency of our Daikin A2A on that day was 7.8, so 4.6 kWh needed, meanwhile the 6.56kW solar PV did 37.9kWh …
Since you already have an A2W heat pump, there may not be that much economic gain in using A2A for heating (the SCOP for the A2A will be a little better). The main advantages are that you can choose to heat just one room with an A2A indoor unit in it, heat different rooms to different temperatures as each indoor unit is individually controllable, and do really fast warm up if you come back to a cold house for some reason. The redundancy could potentially also be useful if your A2W heat pump was out of action for some reason, and vice-versa.
First heat wave of 2027 and you will fall in love with your A2A
@Rob of York Yes, these are the very reasons we feel we are making a wise decision. We can’t ‘hold back the tide’, if the ambient temperature is rising, we can at least moderate the effect on our lifestyle and keep our cool so to speak.😉 Rewards, Toodles.
Money Saving Expert has some interesting insights.
😉
https://forums.moneysavingexpert.com/discussion/6675319/daikin-multi-split-a2a-heat-pump-heating-cooling-and-hot-water/p1
Also, perhaps….as an interim?
https://www.midea.com/uk/air-treatment/porta-split-air-conditioner.portasplit
@Watcher If you have sash windows, or a plastic door you can “re-engineer" a bit then the Midea is an excellent piece of kit. Shove it out of the sash window, close the sash close to pipe level and then cut a pair of pieces of celotex or similar to go round the pipe and done. Possibly also add some sashlocks for the right height.
With a plastic door you take the bottom panel out, shove the external unit through it and then cut a U in one side of the panel for the pipe and add some insulation/duct take around it, then reassemble the door.
Otherwise the fact you can’t split the two units is a major pain in the arse.
@EtchedPixels
Agreed, just a thought for others. I am A2W plus A2A in the lounge with a conservatory abortion!
Why?? Because I use it to warm the glass house if I have a sunny day! Extends its use through the year and stops damp accumulation.