@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.

@derek-m Could you be a little more precise please, is that a.m or p.m.?😉 Toodles.

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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

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.

@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. 

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@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. 😆 

 

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Posted by: @toodles

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@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.

 

Warsaw, Poland (Latitude 52°N): Average winter temperature: Around -2°C to -6°C. 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.

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. 😁

So perhaps you can clear up a BIG ISSUE I have with heat pumps that has been bugging me a lot!

ASHPs are designed and sized to a minimum external temperature. Typically -2C or -5C, given the heat loss data for the house. This is great for a 'typical' (mild) winter as I'm sure everything works to plan!

But what happens in a -10C or even -20C winter, which has a once per decade probability (10%), in the event that the AMOC continues to shut down over the next decade? In this situation, I presume that the heat pump will be significantly under sized and will not keep your house warm! How can this issue be managed with a heat pump without 'over sizing' it? AKA with a design temperature well below what is typically used today!

Riddle me this and I will cease to have doubts! Is having a secondary heating system e.g. A fireplace, the only option here?

 

Edit: here is a video on the subject worth watching.

 

@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.

@editor 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?

 

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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.

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 )

@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.