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Air Source Heat Pumps (ASHPs)
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Topic starter
09/09/2025 2:11 pm
When I had my heat pump installed last year, the installers gave what appeared at the time to be odd advice about how to get the most out of it. Testing has shown the odd parts of that advice to be incorrect. I suspect that the odd advice may have come from the premise that COP (Coefficient Of Performance, or its annualised average, SCOP, Seasonally-adjusted Coefficient Of Performance) is the only relevant measure of efficiency. This idea is mistaken, and I suspect that there may be many people using heat pumps in a sub-optimal way because of this.
The odd advice
The advice that I was given that I thought odd at the time was as follows:
- I should not use thermostatic radiator valves in each individual room; and
- I should leave the heating running even when not in the house during the day.
I decided to use my thermostatic radiator valves in any event, and the installers then set my heat pump (a Vaillant Arotherm 7kW) to a target temperature of 30C so that my Honeywell system that controls the thermostatic radiator valves would have control of the heating. I was advised that the target temperature did not affect the amount of energy that the system would consume. The heat curve was set to 1.0.
The installers stated that the basis for the bulleted advice was that the heat pump runs more efficiently when it is running constantly at a lower temperature rather than cycling, that it will run more constantly with a larger circulation volume (i.e. with all radiator valves open), that running it all day will result in less cycling, and heating from cold is less efficient than maintaining an already warm temperature. It was also stated that heating multiple rooms is more efficient because heat from one room can propagate to the next room, allowing a reduction in the flow temperatures because the radiator in the room that it is desired to heat will not need to reach such a high temperature because the heat will be supplemented by the heat from neighbouring rooms.
The hidden untested premises
What was not made explicit, but which is obviously implicit in the above, is the premise that the heat pump is so much more efficient working constantly rather than cycling (etc.) that that efficiency gain outweighs the efficiency loss in heating unoccupied rooms or heating the house at all at times when it is completely unoccupied.
It could easily be the case that the heat pump does work significantly more efficiently, all things being equal, when heating a larger volume of water than a smaller volume of water, and running constantly all day rather than heating only when an occupant is present, but that that all things are not equal, and that the amount of heat energy thrown away heating unoccupied spaces more than outweighs the gain in thermal efficiency of the heat pump system itself. However, one cannot say with any confidence whether this is the case or not without actually measuring this quantitively, and it was plain that the installers had not done this and had no quantitative basis for the implicit quantitative claims made.
I suspect that these claims are made because, if one measures efficiency only by the COP, following the advice will indeed lead to a better metric, and there is no other metric readily available or well known.
However, the true metric for space heating efficiency is the amount of energy needed in any given period of time to heat the occupied living spaces to a comfortable temperature. All things being equal, the higher the COP, the lower that the energy consumed for a given degree of heating will be - but all things are often not equal.
To take a hypothetical example: suppose that an imagined heat pump will run at a COP of 4.0 in ideal conditions (heating all radiators all day), but at a COP of 2.0 in suboptimal conditions (heating some radiators for part of the day) on a given day. Suppose that the amount of heat energy that the heat pump will produce in those ideal conditions on that day is 100kWh (meaning, with a COP of 4.0, electricity consumption of 25kWh). Suppose, however, that, on that day, heating occupied rooms requires heat energy of only 30kWh. Heating occupied rooms only gives a COP of 2.0, meaning actual electricity consumption of 15kWh. Despite halving the COP, 10kWh has been saved. Thus, the COP alone is an unreliable measure.
My own experiences, setup and data
For reference: I am a one person household in a 1909 built 2 bedroom mid terrace in London. I work full time, but sometimes work from home.
After the heat pump was installed, I read the manual and quickly discovered that the target temperature does affect the energy consumption, so reduced it from 30C to 20C. I also routinely turned the heating system off (i.e., set it to standby) when out of the house, which I later automated using Home Assistant. Through trial and error on cold days, I was able to reduce the heat curve from 1.0 to 0.55 and still maintain a comfortable level of internal heating.
I adjusted the settings for my thermostatic radiator valves. Now, in whatever rooms that I want to heat to the highest temperatures, I always set the target temperatures for the TRVs about 1-2C above the target temperature for the Vaillant system. That means that the valves for these rooms should remain constantly open while the heat pump is running, letting the Vaillant system's flow temperature maintain the heat by running constantly with a low temperature, as is the optimum way for a heat pump to work (all things being equal). For other rooms, the TRVs are set to a lower temperature so that those rooms heat only to the necessary temperature. For example, the study, where I work during the day when I work from home, is heated to a full comfort temperature, but the bathroom, which will receive only occasional short visits during the day, is heated to 18C. The dining room is heated to 18C for most of the day, but increases at breakfast and lunch' time, and I set it manually to increase when I start cooking dinner. At other times, it is maintained at a lower temperature.
My data show a significant difference in heating energy consumption on days when I am out (and therefore the heating is turned off during the day) and days when I am home (and therefore the heating is running during the day).
For example, on the 11th of January 2025, the system was running all day, and consumed 17.7kWh of electricity, with outside temperatures ranging between about -1C and +4C. The COP is shown as 2.0. Likewise, on the 14th of January 2025, the system was running all day, and on that occasion consumed 7.5kWh of electricity with outdoor temperatures of 4-9C with a reported COP of 3.0. On the 15th of January 2025, my data shows that the system was not running in the afternoon from about 1500h to 1900h (presumably because I was out); it consumed 4.9kWh with outside temperatures of 7-9C, with a COP of 3.3. On the 16th of January, the system was not running between 1300h and 2200h, and, in outside temperatures of 7-9C, consumed 3.7kWh with a COP of 3.2.
Moving a little later in the season, on the 4th of March 2025, the system was not running between 1000h and about 2145h; in outside temperatures ranging from about 2-12C (the lower temperatures when the system was running overnight, but with a lower target temperature), the system consumed 3.0kWh with a COP of 2.7. On the 1st of March 2025, by contrast, the system was running all day and consumed 7.7kWh in outside temperatures ranging from 3-11C and with a COP of 2.8.
For space heating only, in the year 2025, my total electrical consumption to date (the 9th of September) is 675.1kWh, with an average COP of 2.7.
I have not specifically gathered data about leaving all radiators on to their highest setting as against my normal regime, but may try that this winter to generate some comparison data.
Conclusion
While COP is a good measure for comparing the efficiency of one heat pump against another, it does not, by itself, effectively measure the overall efficiency of the system, as total efficiency can also be significantly increased by simply generating less heat, even at a cost of having a lower COP.
While this should not generally affect people's choice of what equipment to install, it should affect people's choice of how to operate that equipment, and suggests that careful setting up and automation is optimum for minimising the electricity consumption while maintaining heat comfort in the home.
It would be interesting to see others' data to see the extent to which this holds true over different systems, installs and circumstances.
1
09/09/2025 2:42 pm
James, I would not question your findings and realise that your circumstances are very different to my own as I am home pretty much 24/7 and my wife and I are not comfortable without a somewhat elevated comfort level of 22.5 degrees C throughout the house with the only exception being that we prefer 25 degrees C or more in the bathroom.
You were writing about COP and efficiency, I would argue that ‘efficiency’ also relates to comfort levels and in this house the ‘24/7 applies to the whole house - we do not turn heating down at different times with the one exception of using a TRV to cool our bedroom at night and also to combat solar gain. The room has almost the entire SSW facing wall glazed so that TRV does assist us. I feel that letting the ASHP controls based around WC do the controlling with TRV’s left open is a good scheme. Our is actually slightly different as we have a Homely controller but this effectively does the same thing via a slightly different route.
I imagine that a two bedroom home with one occupant who may be away from the abode during many hours is a very different ball game - nevertheless, comfort is I think a very important element of ‘efficiency’; if I were not warm and comfortable, I would say the heating is not efficient! Regards, Toodles.
Toodles, heats his home with cold draughts and cooks food with magnets.
1
Topic starter
09/09/2025 6:28 pm
@toodles Your use case is indeed very different to mine, and much closer to the COP optimum, I expect. I agree that comfort is relevant to efficiency: the real efficiency metric is, for any given level of heating related comfort, what is the least amount of electricity that can be consumed in producing that level of comfort?
09/09/2025 9:12 pm
@jamespetts Lots of things to unpack here so I will confine my comments to a couple you may find helpful
After the heat pump was installed, I read the manual and quickly discovered that the target temperature does affect the energy consumption, so reduced it from 30C to 20C.
With a Vaillant the set temperature shifts the WC curve so you were right to reduce it (and your installers, IMHO, were wrong to tell you what they did because it has the effect of putting the thermostats in charge not the heat pump)
Through trial and error on cold days, I was able to reduce the heat curve from 1.0 to 0.55 and still maintain a comfortable level of internal heating.
Thats good, particularly when coupled with the above. I suspect these two together reduced your FT by 10C or more resulting in perhaps 30% saving!
I have not specifically gathered data about leaving all radiators on to their highest setting as against my normal regime, but may try that this winter to generate some comparison data.
I wouldn't do this personally without also turning down the LSVs in the rooms you want cooler, so that they heat to the desired temperature.
In general I would prefer adjusting LSVs over adjusting TRVs in the rooms you want cooler for three reasons:
(1) adjusting the LSVs result in a more constant temperature, which is generally more comfortable
(2) 'bouncing off' the TRVs means that the rads go in and out of circuit which will reduce the volume available for defrost if it happens to occur when they are 'out'. This may have little effect or, at the other extreme could reduce the volume available for defrost to the point where the system cannot recover (unlikely, put possible in principle). oist likely the effect would be to increase the time taken for defrost somewhat.
(3) adjusting the LSVs rather than the TRVs is likely to result in more efficient operation, although its equally unlikely you will be able to measure the difference.
For space heating only, in the year 2025, my total electrical consumption to date (the 9th of September) is 675.1kWh, with an average COP of 2.7.
For example, on the 11th of January 2025, the system was running all day, and consumed 17.7kWh of electricity, with outside temperatures ranging between about -1C and +4C. The COP is shown as 2.0
Unless your design temp is very low (eg -6-10) these two figures together suggest that your house may have a loss which is quite a bit less than 7kW. On a day when its -1 to +4 a true 7kW house (with a design temp of say -1~-3) would operate at an average of ~4kW delivered or perhaps a bit more, whereas yours is operating at an average of about 1.5kW. Of course it maybe that by only heating a fraction of the house you are saving 60% plus, but its unlikely unless the other rooms are absolutely freezing cold. More likely is that you are indeed reducing your load somewhat by heating only a part of the house, but that the house full load is also quite a bit less than 7kW. That said I'm not sure this information is of any practical use, other than as context.
This post was modified 3 weeks ago 4 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Topic starter
09/09/2025 11:51 pm
...
I have not specifically gathered data about leaving all radiators on to their highest setting as against my normal regime, but may try that this winter to generate some comparison data.
I wouldn't do this personally without also turning down the LSVs in the rooms you want cooler, so that they heat to the desired temperature.
In general I would prefer adjusting LSVs over adjusting TRVs in the rooms you want cooler for three reasons:
(1) adjusting the LSVs result in a more constant temperature, which is generally more comfortable
(2) 'bouncing off' the TRVs means that the rads go in and out of circuit which will reduce the volume available for defrost if it happens to occur when they are 'out'. This may have little effect or, at the other extreme could reduce the volume available for defrost to the point where the system cannot recover (unlikely, put possible in principle). oist likely the effect would be to increase the time taken for defrost somewhat.
(3) adjusting the LSVs rather than the TRVs is likely to result in more efficient operation, although its equally unlikely you will be able to measure the difference.
Can I clarify - is the LSV the manual balancing valve? That would not be much use to me, as there are not rooms that I always want to be colder: rather, I want to vary the temperatures depending on the time of day and my activity, which is why I use the Honeywell system interfaced with Home Assistant.
I should note that I have cast iron radiators, so they have a high termal mass, meaning that, whatever the TRVs do, there will not be quick changes in the room temperatures.
For space heating only, in the year 2025, my total electrical consumption to date (the 9th of September) is 675.1kWh, with an average COP of 2.7.
For example, on the 11th of January 2025, the system was running all day, and consumed 17.7kWh of electricity, with outside temperatures ranging between about -1C and +4C. The COP is shown as 2.0
Unless your design temp is very low (eg -6-10) these two figures together suggest that your house may have a loss which is quite a bit less than 7kW. On a day when its -1 to +4 a true 7kW house (with a design temp of say -1~-3) would operate at an average of ~4kW delivered or perhaps a bit more, whereas yours is operating at an average of about 1.5kW. Of course it maybe that by only heating a fraction of the house you are saving 60% plus, but its unlikely unless the other rooms are absolutely freezing cold. More likely is that you are indeed reducing your load somewhat by heating only a part of the house, but that the house full load is also quite a bit less than 7kW. That said I'm not sure this information is of any practical use, other than as context.
I suspect that this is correct. The installers can only use estimates based on standardised data for heat loss calculations, and putting my information into online heat loss calculators suggested that I needed a 12kW heat pump! The house is old, a 1909 build, with solid, uninsulated walls, but the loft has insulation and the house is double glazed, with the rear windows having been replaced with high efficiency types during the refurbishment.
Also, being a mid terrace, if the neighbours heat their houses to a higher temperature than I, then this will reduce the heat loss considerably and might, in the colder rooms, lead to heat gains if they border on rooms heated by the neighbours to much higher temperatures.
I do notice that, even in colder weather (even, I think, in sub-zero weather), the heat pump's compressor does not run constantly, and I do not think that it reaches 100% except when heating the hot water to an elevated temperature (it is set to heat the hot water to the maximum possible temperature of 70C when the electricity price goes negative).
10/09/2025 8:04 am
Can I clarify - is the LSV the manual balancing valve?
Yes exactly that
That would not be much use to me, as there are not rooms that I always want to be colder: rather, I want to vary the temperatures depending on the time of day and my activity, which is why I use the Honeywell system interfaced with Home Assistant.
OK. You might want to try turning down the LSV to the point where the room reaches the highest temperature you want when the TRVs are fully open, then apply the TRV. This way it will be open most of the time and just shut a bit when you want it cooler. Of course doing this will ,slow down the response (see next item)
I should note that I have cast iron radiators, so they have a high thermal mass, meaning that, whatever the TRVs do, there will not be quick changes in the room temperatures.
Given this Im a bit surprised you get any material variation in actual room temperature!
I do notice that, even in colder weather (even, I think, in sub-zero weather), the heat pump's compressor does not run constantly, and I do not think that it reaches 100% except when heating the hot water to an elevated temperature
My 7kW Vaillant, the capacity of which is pretty much spot on to the house, only reaches 100% during defrost cycles and when heating DHW. It does run continuously however at OATs <11C. If you look at the output curves it needs to run at 60-80% only to achieve rated output.
I think the fact your system is oversized is very likely a contributing factor in your observations. In reality it is almost never running constantly and would rarely ever run constantly even if you told it to, it would cycle. So either it can cycle 'fast' (what it naturally does) or it can cycle slowly, because you impose off periods and it then has to play catch up. The latter is likely more efficient. Similarly your radiators are also likely oversized (assuming they were based on the same calculation) so each radiator that is on has reserve to heat adjacent rooms (at the original design temp) even if those adjacent rooms are unheated.
I'm a little surprised that your recorded COP is so low, I operate at similar temperatures and get a COP averaging 4 over the season. The use case is different, I want most of the rooms in my house heated 24*7, so haven't experimented extensively with partial heating. You may find that you can get a better performance (energy use) still by turning on all radiators and dropping the flow temperature (heat curve) still further, but retaining the intermittency. This might also be more comfortable. You might even be able to operate at UFH like temperatures! That said your consumption is sufficiently low that you probably have other things in life that are more important!
This post was modified 3 weeks ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Topic starter
10/09/2025 9:53 am
...You may find that you can get a better performance (energy use) still by turning on all radiators and dropping the flow temperature (heat curve) still further, but retaining the intermittency. This might also be more comfortable. You might even be able to operate at UFH like temperatures! That said your consumption is sufficiently low that you probably have other things in life that are more important!
I am a little sceptical of this, not least because, in an old house like mine, the insulation between the rooms and other rooms is not much different from the insulation between the rooms and outside. There is a spine wall of double brick thickness (i.e., the same as the outside walls) separating the main bedroom from the hall and study and the parlour from the dining room. There is also a structural wall separating the bathroom from the hall (which is the only room it adjoins) and also the kitchen from the dining room on one side and the conservatory on the other (which again are the only rooms that it adjoins). The only non-structural walls are between the parlour and the hall, the dining room and the hall and the study and the landing. These walls may have a lower insulative value than the other walls. It might possibly be that increasing the heat in the hall/landing (there is one radiator for the whole space) would reduce the heating requirement in adjoining rooms (but not the kitchen, bathroom or bedroom, which are all separated from it by structural walls), but I am somewhat sceptical as to whether this will make a significant real world difference.
10/09/2025 12:04 pm
because, in an old house like mine, the insulation between the rooms and other rooms is not much different from the insulation between the rooms and outside.
If that's the case then you are right to be sceptical, I did say 'may'!
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
10/09/2025 7:26 pm
@jamespetts Interesting post, thanks. Here are my thoughts:
1. It is your comfort, your system and your money, so feel free to run it how you like!
2. Within the heat pump community, there is a certain "fundamentalist" contingent that believes COP is everything. I posted a question on the Facebook Heat Geek group asking whether we should sacrifice some COP to save money by exploiting variable tariffs. My overnight tariff is 7p per kWh, compared to 27p during the day, so this makes a massive difference. Some people reacted as though I was talking blasphemy, though others happily engaged in the discussion. Some people on https://heatpumpmonitor.org/ seem to be tempted to perform slightly unnatural acts to be top of the charts.
3. I believe the MCS guidelines mandate that heat loss calculations must assume that adjoining properties are at 10C, rather than a more realistic 20C figure. The rationale for this is that the adjoining properties might become unoccupied and so your heat pump must be able to cope with them being at 10C. For someone in a terraced house, this means that 2/4 walls are at about 20C, rather than 10C, so that real world heat loss is substantially less than anticipated.
4. If you let your house settle at a particular temperature, it does not matter how thick the internal walls are, the rooms will reach an equilibrium temperature.
5. Heat Geek's advice is to use setback temperatures, rather than completely turning off when the house or a room is unoccupied. I think you are doing that now anyway. See https://youtu.be/zpTVIeUh04E?si=MceS2aYLuqSi8AgS
6. I found it hard to compare your numbers in line in text. It would be interesting to put them into a table and maybe put them onto a graph.
7. As you implied above, the most efficient way to run a heat pump is with minimal flow temperature and no cycling. The reason for this is that starting the compressor needs more energy than keeping it running - a bit like starting a car engine. I wonder whether you have scope to reduce flow temperature even further, so as to reduce cycling. Maybe you are already at the bottom?
8. To be honest, a year to date SCOP of 2.7 doesn't sound particularly good. I would be hoping for at least 3.5 and maybe over 4.0. If you could get the SCOP that high, would this be enough to change your pattern of usage?
9. It might be worth exploring variable tariffs such as Cosy Octopus. Given your pattern of usage, they might not suit you at all, but it is worth considering.
I hope that helps.
Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation
1
11/09/2025 9:50 am
2. Within the heat pump community, there is a certain "fundamentalist" contingent that believes COP is everything. I posted a question on the Facebook Heat Geek group asking whether we should sacrifice some COP to save money by exploiting variable tariffs. My overnight tariff is 7p per kWh, compared to 27p during the day, so this makes a massive difference. Some people reacted as though I was talking blasphemy, though others happily engaged in the discussion. Some people on https://heatpumpmonitor.org/ seem to be tempted to perform slightly unnatural acts to be top of the charts.
Indeed, this made me chuckle. After comfort (however you may define that), the most important factor is cost. My heat pump would run more 'efficiently' (higher COP) if I opened the windows to increase the heat loss, but I'd be colder and it would cost more. But I'd be Top of the COPs. As you highlight, heating DHW overnight at 7p will result in a substantially lower COP but this is more than offset by the significantly lower unit cost of electricity. When I was on Agile, I created a spreadsheet to calculate cost vs OAT (COP) to work out the optimum time to reheat the DHW based on the unit price difference vs predicted OAT for the day. Your price differential of nearly 4 times is always going to be cheaper overnight.
I find COP to be a really useful tool to monitor as it would immediately allow me to identify an issue with my heat pump. I know from past performance that on any given day I'm expecting a COP of X. If it's substantially different from what I'm expecting, then I know I need to investigate the reason why. I can also use it for fine tuning the system. If I make a change, do I see an improvement in system efficiency whilst retaining cost and comfort. I'm very happy with my running costs and my SCOP is around 3.6-4.0 during the winter months.
This post was modified 3 weeks ago by Old_Scientist
Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.
Topic starter
11/09/2025 9:51 am
For unoccupied rooms when I am home, I do use setback temperatures on the TRVs. When I am not at home at all, however, I turn the heating off entirely. If I were just to use setback temperatures, the compressor would continue to run (and would cycle with very short runs) while I were away. I know as I have tried this. The power used for these short cycles is such that more energy is consumed overall if I do this than if I simply leave the heating system off as I do.
I have already optimised the heat curve to 0.54, which I find is just about sufficient in the coldest weather. The installers had set it to 1.0. I still get cycling with the lower flow temperatures.
The low average COP to date may be related to the low loss header installed as a result of the presence of TRVs on the radiators.
As to the tariff, I use Octopus Agile and use my Home Assistant to turn on the water heating at the optimum point every day, so I suspect that I have already maximised my efficiency. I have set up my Home Assistant to run a daily tariff comparison, and the price that I pay with Agile always comes out less than the price that I should be paying if I were to use Cosy.
As for the fundamentalist contingent chasing high SCOP figures, I suspect that they may see it as something akin to a sport where they get some sort of pleasure out of winning that may be worth the higher energy bills of not maximising actual efficiency. I am not into sport, and I should prefer just to maximise real efficiency.
1
11/09/2025 10:02 am
For unoccupied rooms when I am home, I do use setback temperatures on the TRVs. When I am not at home at all, however, I turn the heating off entirely. If I were just to use setback temperatures, the compressor would continue to run (and would cycle with very short runs) while I were away. I know as I have tried this. The power used for these short cycles is such that more energy is consumed overall if I do this than if I simply leave the heating system off as I do.
I have already optimised the heat curve to 0.54, which I find is just about sufficient in the coldest weather. The installers had set it to 1.0. I still get cycling with the lower flow temperatures.
The low average COP to date may be related to the low loss header installed as a result of the presence of TRVs on the radiators.
We keep all our TRVs fully open using the whole house approach. When we are away in winter I turn the flow temps down to minimum and use the room thermostat to control heating, setting it to something like 16C or 17C. I set the hysteresis quite high (maybe 1C) to maximise run times and preventing short cycling. When we are at home, the thermostat hysteresis is set lower (0.4-0.5C) maintaining a more constant room temp but still maintains a decent balance with run times/cycling when constant heating is still not required (my heat pump is oversized, so we only really get to run constantly when temps fall below 5C)
Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.
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