@jamespa  Thank you for your very helpful insights.

I have been looking afresh at the important effect of oversizing of a heat pump. On the face of it, my Samsung 12kW heat pump was sized fairly accurately. At OATs of < 5C it is supposed to provide between 10.41 and 10.47 kW. My heat loss was estimated to be 9.8 kW at -3.7C and using the Freedom HP calculator I made it 10.4 kW. Due to running the IAT lower than 18C, our actual heat loss will be lower - possibly as low as 7 kW.

My 12 kW Samsung will modulate down to around 3.5 kW, but when I looked at my average daily heating power, there was no day when this exceeded 3.5. In fact on only 15/182 days did my average daily heating power exceed 2.5 kW. Even a 5 kW Samsung would struggle to modulate low enough to run low and slow in my home. The solution to my situation has been to run in room temperature mode, where LWT is controlled by weather compensation, but the thermostat in the wired remote controller maintains IAT within limits using intermittent heating periods of 1-4 hours without any short cycling.

@derek-m suggested some time ago that a heat pump with two compressors, one of which could modulate down to very low heating power, would solve the oversizing problem. I wonder how feasible this would be to create.

Posted by: @mike-h

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@derek-m suggested some time ago that a heat pump with two compressors, one of which could modulate down to very low heating power, would solve the oversizing problem. I wonder how feasible this would be to create.

I don't doubt its feasible, however there is extra cost and (physical) volume.  Many modern heat pumps are pushing the 0.6cu m permitted development limit already and, until this changes, wouldn't have room for an additional compressor without going over.

For me the solution is correct-sizing (or at least correct determination of the heat demand), using something other than (or in addition to) the GIGO spreadsheet method.  Unfortunately the latter is close to mandated by MCS (I'm not convinced its 100% mandated, but its close-to).  The obvious thing to me (in a retrofit circumstance which is where the major sizing problem lies) is half hourly meter readings, which suitably interpreted must tell you more or less everything you need to know, but there are certainly other methods.

One question that is unresolved, so far as I know, is how much does oversizing compromise efficiency.  I have never seen any meaningful data on this for modern pumps.  However even if the effect on efficiency is small I would still argue strongly for right-sizing; there are loads of knock on effects of oversizing including larger than necessary radiators, unnecessary pipe upgrades, a larger and more difficult to locate condenser unit and the possible need for planning permission when a right-sized pump would fall within PD limits.  We should aim to start with good data and make a decision based on that, not start with data which is known to have flaws and try to justify the approach.  Unfortunately until MCS changes its rules, that seems unlikely and installers have little choice.

If you wait a few days and watch the forthcoming video about heat pump sizing on this forum there is a discussion about this, alternative methods for sizing, and future developments!

Posted by: @jamespa

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It is, in my view, inevitable that ToU/sophisticated tarriffs become either compulsory or very heavily incentivised and we should be embracing this transition not resisting it.

Agreed.

On the assumption that @lickofpaint is still following these meanderings, let's follow this thinking forward a bit...

I would like to see a Government/Ofgem initiative to facilitate ToU tariff options for households with pre-payment meters.
This would cover a substantial number of occupants in energy-poverty, and tenants in rental properties.

Yes, this is an ethical point, but I do understand how Smart Meters work, and the required functionality is already present.

There are four main factors which might be used to make up a ToU tariff:

1. A fixed time period (Economy-7 or Octopus Go) usually starting just after midnight. The electricity is substantially supplied by gas-turbine plants, allowing them to remain operating in closed-cycle mode at about 50% efficiency.  A CCGT plant may run at a loss during this period, especially on windy nights when it's competing against wind turbines.
2. A fully variable analysis (Octopus Agile) based on the current wholesale price, or the current supply/demand ratio; those are two sides of the same coin. Because the 'electricity market' is national, there can be anomalies with this variable approach. The large British wind-turbine fields are in the North Sea, whilst the prevailing wind-direction is south-west. If North Sea output is low, the wholesale price rises. So consumers in Cornwall and South Wales get presented with a high price-point which doesn't reflect the generation from their local wind-farms. National Grid use ANM to reject the over-supply in South Wales and the West Country when this happens.
3. Nodal pricing. The grid is built from nodes, which are the transformer locations. Data already exists for the Grid Supply Points (275/400kV) and Bulk Supply Points (132kV) which gives the total throughput and the energy-mix per hour. Electricity could be offered cheaper to those living closer to the nodes where generation (from renewables) is plentiful. That reduces the amount of infrastructure which requires enhancement/upgrade. Nodal pricing for consumers was rejected by the then Secretary of State for Energy on 12th March 2024.
4. Demand Side Response (as understood by DESNZ) is to permit 3rd-parties to turn off Energy Smart Appliances (ESAs) in the home during times of high demand. A householder needs to opt-in to such a scheme, and receives financial compensation whenever their home is selected.

None of the above approaches is a good match for households with a heat-pump.

Option-4 is misleading. We already have "Smart" EV Chargers, and now DESNZ are putting forward the idea of a "Smart" Heat Pump.
And yet neither appliance has any inherent Smartness within it.
They are in fact dumb devices, turned on/off by an algorithm being crafted and controlled by a 3rd party, such as an Energy Supplier.

Posted by: @jamespa

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The grid must be designed to meet the maximum demand and, if that increases dramatically as it will if our use is unmoderated, then the costs will soar.

The 10-year-plan at present is based on the assumption that we will all continue to want electricity on demand and won't moderate our consumption based on time-of-day.

In other words, NESO is ploughing ahead with the System Transformation route within the Future Energy Scenarios.
They ignore the Consumer Transformation option, or acting in coordination with the rest of us to achieve Leading the Way.

I fear that is because there are enormous profits to be made by the commercial companies in the System Transformation Scenario.

The scale of infrastructure upgrades required is going to cost us more than HS2.
That means we can't afford it... but no one in a position of authority has yet asked that question. 😲 

I note that @jamespa favours implementing Homely as the way forward.
I'm less confident that we can optimise the use of the increasing renewables section of the market by that route.
It's a software approach based on price-points.

So I favour the use of domestic-level storage batteries.

But such local (in-home) storage will need to be able to supply the switch-on surge of a heat pump... say 30A or so.
That means the home requires an inverter rated in excess of 8kW.

Posted by: @jamespa

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Currently there is nothing to stop, or even dis-incentivise, the whole country from using a heat pump, charging an electric car and turning on the kettle at precisely the same time during the half time break in a cup final, leading to the potential for massive peaks.

I'm discussing this with NESO staff.

There are a number of technical approaches to reduce the peaks.

But the Electricity Sector would prefer to build a grid with a capacity far beyond the UK's requirements.
That will allow them to export our excess-generation to other countries, creating hefty dividends for the (overseas) shareholders.

Electricity passing through interconnection routes doesn't seem to fall within Ofgem's governance.
The profits can be 'accounted for' elsewhere, beyond the reach of HMRC.
There is no apparent benefit to UK consumers by us building a grid with this level of capacity.

Thanks for that explanation which is very interesting.  Btw I'm not advocating homely, just citing it as an example of a technology which (reputedly) achieves demand moderation without material loss of amenity.

It's concerning (but not in the slightest surprising) that the industry may be taking us in the wrong direction to maximise their profits.  Industry led policy making is never a good thing imho, but successive governments (mostly Conservative, that's just a fact not a value judgement) appear to have dumbed down the civil service to the point where there is little alternative.

My personal view is that senior civil service jobs should be very highly paid to attract the very best ( but very slightly lower paid than the top private sector posts so they don't attract those whose only interest is money).  Their impact is massive and we need civil servants who can run rings round those with vested interests.  

I hope for a change!

Posted by: @jamespa

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Industry led policy making is never a good thing imho

Indeed. And we should hope that things will change when NESO becomes the independent energy operator in ten days time.

May I provide a link to an informative YouTube clip by Dr Rosie Barnes as she explains the interest in building interconnectors operating at 1-million-volts DC?

It's not overly long at 13½mins, but if you're short of time, jump to 6m23s and listen to her explain the particularly short Return On Investment achieved in the first full year of operating the UK-Norway interconnector.

Then take note of who owns that marine cable. The Norwegian partner is 100% owned by their government. But UK partner is National Grid Group, which is legally separate from National Grid Electricity Transmission and National Grid Electricity Distribution.

This isn't a particularly good topic for me to post this information because few others will find it.
So forgive me if I repost the YouTube link when I find a better topic to host it.

The main point here is to emphasise what is occupying the gaze of the UK energy sector... high profits from exporting electricity.
And that's why they're spending far too little time appreciating the optimal tariffs we Brits require for heat pumps.

Posted by: @lickofpaint

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I know heat pumps are most efficient when working at a low level to keep a house at a constant temp rather than the peaks and dips from a traditional boiler. But in reality we don't want our house heated at 21 degrees overnight or in the day when no one is home. Does anyone actually deviate from the constant temp for 24 hours and if so how does that work for them

So @lickofpaint I think you have your answer - 

“Your question isn’t so dumb…”

as for lived experience we did some experiments on night time setbacks. Perhaps you might glean some practical answers there:

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/creative-trials-on-night-time-setbacks-to-faster-recovery-improved-comfort-and-even-greater-energy-reductions/paged/2#post-27473

this link starts on page 2 of the forum

hope it helps

Posted by: @jamespa

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Posted by: @majordennisbloodnok

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I know we talk about setbacks being used in an attempt to save money, @jamespa, and you're certainly not the first to be raising the practice for discussion and debunking. However, there is an obvious alternative reason for not heating low and constantly - because many find their ideal comfortable temperature varies during the day, particularly wanting a cooler house whilst asleep - and I've yet to see any evidence that money rather than comfort is the primary driver for setbacks being used..... Do you have any evidence or insights that might help identify the split between savings-seekers and comfort-seekers?

I agree with the first point entirely.

As to 'do I have any evidence that money rather than comfort is the primary driver' then I would say yes some, for some people.  The evidence (in as much as it exists) is that some people come onto this forum (and others) and make extravagant claims about how much money setback is saving, sometimes going so far as to say that they don't mind feeling a bit cold in the morning in order to make the saving.  This suggests that their primary motive is cost saving.  The extravagant claims generally exceed the saving which the thermodynamics tells us is possible with a well adjusted system (indicating that either their system is not well adjusted in the first place, or the measurement underpinning the claims is dodgy).

Although its almost impossible to give general guidance I think this is one of those lucky (and rare) cases where comfort and cost probably more or less coincide for many practical cases.  If we divide the heating season into very cold (mid season), moderate (most of the season in Southern England, less so in east coast Scotland)), and end of season (both start and finish) then reasonable qualitative arguments can be made for the following giving both a good level of comfort and a modest saving (<10%):

• Mid season (very cold)  - operate 24x7.  
• Moderate (most of the season in Southern England, probably less so in east coast Scotland!)  Set back/turn off for a few hours overnight - perhaps 5-6, allowing 3 hours for recovery).  Don't overdo the setback!
• Ends of season - turn off overnight and during daytime, late afternoon/evening heating only (maybe with a midday boost).  

The qualitative arguments, which apply to a reasonably well adjusted correctly sized system run on WC, consider comfort, solar gain, variation in COP due to outdoor temperature variations, the change in the pattern of diurnal temperature variation at different times of the season, the thermodynamics and the fixed running energy due to pumps etc.  For brevity I will avoid going through all the arguments here, but can do so if people want.  As I say in an earlier post a good test is that, if you need to increase the flow temperature for the house to recover sufficiently quickly, you are probably better off reducing the setback time instead.

I stress this is very qualitative, very rough, and may well not apply to any individual case particularly to very well insulated/passive houses which are almost certainly better sticking with slow/low.

I totally agree with James’ approach of keep the heat pump running continuously during the coldest periods and use set back for overnight lower temperatures or even off for a time in the shoulder seasons.

There should be an away option too where you can set a lower temperature such as 15C.

The key to making operation cost effective as well as comfortable for you is to do the reheat after reducing temperature in low price periods. Each morning do the reheating to daytime temperatures before 7am to use the Cosy tariff as an example. Similarly when you’re returning from a trip do the reheating on the previous afternoon when the outside temperature is at its highest and Cosy also has a low period. 
The principles of running the pump efficiently interact with financial impact of available tariffs

A separate reply about ventilation.

The heat loss survey method includes heat loss through ventilation both deliberate and unplanned (otherwise known as draughts). For our house the default was 3 air changes per hour for the kitchen and bathroom, and the default depends on the building regs in force when the house was built. Again in our case that would have added up to 30% of the house heat loss, but that would have been wrong since we have a measured low leakage (using the pressure/ door test and a mechanical ventilation system with heat recovery). So our heat loss through ventilation is only ~2% of the total.

Back to your @lickofpaint’s ventilation specific question. Thermally it is considered more efficient to ventilate a bathroom by opening the window wide and closing the door for 30minutes than leaving the window cracked open continuously. The argument being you only loose the heat of the bathroom (by shutting the door, therefore not the whole house) for a shorter period. The exact trade-off depends on your general room heat loss, type of walls etc and how wide is the crack when the window is cracked open. If you like to have a bath before going to bed nicely relaxed then leaving the window cracked open all night is much more convenient than closing it some time later.

As others have said adequate ventilation is crucial but too much is just cold draughts.

Posted by: @judith

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A separate reply about ventilation.

The heat loss survey method includes heat loss through ventilation both deliberate and unplanned (otherwise known as draughts). For our house the default was 3 air changes per hour for the kitchen and bathroom, and the default depends on the building regs in force when the house was built. Again in our case that would have added up to 30% of the house heat loss, but that would have been wrong since we have a measured low leakage (using the pressure/ door test and a mechanical ventilation system with heat recovery). So our heat loss through ventilation is only ~2% of the total.

I have been told over a period of time by several people in the industry that ventilation loss is one of the major uncertainties in the standard spreadsheet method for heat pump sizing. 

For my house the standard assumption would be 2.5-3 ach.  There is reasonable circumstantial evidence to suggest that the actual value is between 0.5 and 1 ach.  This makes about a 3kW difference in an 8kW house. 

I'm not sure what the average person can do with this information but it clearly is an area where there are known (but perhaps not yet universally acknowledged) unknowns.

Posted by: @transparent

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Posted by: @jamespa

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Industry led policy making is never a good thing imho

 

Indeed. And we should hope that things will change when NESO becomes the independent energy operator in ten days time.

May I provide a link to an informative YouTube clip by Dr Rosie Barnes ...

The main point here is to emphasise what is occupying the gaze of the UK energy sector... high profits from exporting electricity.
And that's why they're spending far too little time appreciating the optimal tariffs we Brits require for heat pumps.”

I think I have only just understood a point that I think @transparent has made a couple of times.

 
As a direct consequence of there only being one UK electricity price the interconnector operator buys at a low price (because lots of wind in Scotland) and sells to France/Norway at a higher price. This benefits them/their shareholders, BUT the electricity isn’t in Kent it’s in Scotland and can’t get to Kent since the National Grid can’t carry it. So the fossil fuel generators have to switch on to supply the drain to France. 
Who pays for this unstable control system? Us -the taxpayers and bill payers and the environment basically. So the regional wholesale markets go one step to correct this fundamental anomaly being exploited and the NESO will be another small step. But the profits that can be generated are so large that the brains will be generating more ways to make more profit and put us closer to shortfall in local supply and yet higher bills.

Have I eventually got this straight?

I just wanted to say a quick thank you for all the answers and discussion. It has all been most helpful.

@lickofpaint we also discussed it on our next podcast episode (recorded yesterday), which will be out on Wednesday next week.