Saving Money with an Air Source Heat Pump in an old house – 2024 Report

I have written a while ago about the project to replace a 25 year old creaking gas boiler with an Air Source Heat Pump. Today we had our annual service provided by Cotswold Energy Group, the original installer. All clear for another year.

The main advice we follow is not to fiddle with the as-installed setup at all – we let it do its thing! We don’t even adjust the controls (TRVs) on radiators because the system was well ‘balanced’ as part of the commissioning of the system. The only thing I look at periodically is the performance data via my phone or PC. If there was some malfunction it would no doubt show up in a drop in weekly performance data. Mostly we forget the system is there.

So I thought I’d just provide a summary of the 2024 performance and running costs.

Summary

A recap. Our house is a large semi-detached dwelling with a total floor area of 251 m² over three floors. It has solid walls [1], and mostly sash single glazed windows. Only the loft insulation and brushes on sash windows are additional retrofit ‘fabric’ measures [2].

The total heat delivered to the house over the last 12 months (directly metered from pipes flow and return gauges) was 29,236 kWh (kilowatthours), and that was achieved with the input of 7,942 kWh of electricity (again, using dedicated metering). So the annual performance (the so called Seasonal Coefficient Of Performance, Seasonal COP or SCOP) is found by dividing the first number by the second, giving a SCOP of 3.68 for 2024. That can be thought of as an efficiency (output divided by input) of 368%. This apparently magical feat (obtaining an efficiency of greater than 100%) is achieved because the heat pump harvests energy from the ambient environment (in our case, the air), and concentrates it to raise its temperature.

Looking at data on a monthly basis, I found that the worst month was January with a COP of 3.06. There will be days when it was worse than this but even on a daily basis it rarely drops below 2.5; for just a handful of days in the year.

If we’d stuck with our old boiler which optimistically ran at 72% efficiency [3], then the primary energy required (in form of gas) would be equivalent to 40,606 kWh (that is 29,236 divided by 0.72).

The result of this is that we are saving about £480 a year as a result of ditching the old boiler, and also achieved a more comfortable evenly heated home (rather than the roller-coaster heating we had with the gas boiler).

With the old system, hot water to our shower came via a gravity fed system and needed a little pump to improve the pressure (noisy, and pressure not that great). With the heat pump and new water tank we now get our hot water under mains pressure. This was one of the most surprising benefits of our move to a heat pump system.

Running cost comparisons

Taking the unit prices for gas and electricity that applied for us for the most of 2024 (5.9p/kWh and 22.7p/kWh, respectively), and the standing charges (28.21p/day and 58.63p/day, respectively), the cost of heating (mainly space heat, but some water heat too) was £2,017 in 2024.

Had we stayed with our old gas boiler, it would have been about £2,500 to do the same job. Probably more because the system was creaking and unlikely to have performed according to the published performance figures [3].

Conclusion

Yes, you can heat any old building with a heat pump without having to make any significant or disruptive changes to the insulation.

Of course, where you can add insulation to a house heated by a gas boiler you can reduce the rate of heat loss and therefore the heating bills. The same is true of heating with a heat pump. But you will find that as you try to reduce the heat loss further and further, the costs will escalate, as I discussed in Insulate Britain: Yes, But by How Much?.

No, it wasn’t difficult to install (whatever ‘noises off’ you may hear from the perenially sceptical ‘You & Yours’ [4], and other naysayers), if you engage professionals with the experience, as you would do for any important job.

We remain very happy with our Air Source Heat Pump and our suppliers. We have a more comfortable house, that is cheaper to run than the boiler it replaced (even given the unjustifiable ratio of electricity/gas unit costs), is very reliable, and we have better showers.

No fiddling, or ‘intelligent home’ tech, required. Keeping it simple.

What’s not to like. You won’t regret it.

(c) Richard W. Erskine, 2025

NOTES

[1] The 200 year old walls are termed ‘solid’, but are actually two course of Cotswold stone with in-filled rubble, providing an element of air gap. The overall wall thickness at ground level is about 600mm. This kind of wall tends to perform better than is often assumed.

In fact, more generally, the heat loss from traditional buildings can be up to three times lower than expected (prior to that found in a 2010 research report, ‘The U-values of Traditionally Built Walls’, Caroline Rye, Society for the Protection of Ancient Buildings (SPAB), October 2010.)

[2] I get a little frustrated with the question “is your home insulated?” If one lives in an imaginary house with no roof or walls then the answer would be no! The fact is that any structure that is enclosed provides insulation. The question is really shorthand for “has any insulation been added to the fabric of the building above and beyond the original construction?”. Most people now have some form of loft insulation which wasn’t original, but it is worth ensuring you have it up to the recommended depth (but going much beyond that is not really needed as there is a law of diminishing returns). Draught-proofing is a really good idea, as it reduces the air turnover in the house, improves comfort levels near windows and doors, and is relatively inexpensive. Extraction fans in kitchens and bathrooms are also important, both in reducing the risk of mould, but also because moist are needs more energy to warm it!

[3] You can find out the estimated efficiency of your old boiler using the Product Characteristics Database search function. Our old boiler was a Glow-worm Hideaway 120B. I’ve taken the slightly higher figure presented for winter of 72% (rounded), even though this is likely to be optimistic for a 25 year old boiler.

[4] ‘You & Yours’ is BBC Radio 4’s consumer affairs programme https://www.bbc.co.uk/programmes/b006qps9 – Today (6th Jan 2025), during a segment about heat pumps we heard about the forthcoming new homes standard, and research being done with Barretts and Salford University. I was astonished to hear that with their new build standards they were seeing a performance, for the air-source heat pumps used to heat the new home (they were referring to the SCOP), of 3. That’s right, worse than our 200 year old house! For new homes, I would suggest a SCOP of 4 is an absolute minimum target, and I’m sure that those clever chaps at Heat Geek would be aiming for 5.

The researchers also “discovered” that the efficiency of heat pumps improves if they are kept on rather than used like gas boilers (being put on on the morning for a few hours and then again in the afternoon). Who knew? Anyone who has any knowledge or expertise in heat pumps, that’s who. With our house, the thermostat in the living room is set to 21C from 0630-2230 and setback to 18C from 2230-0630. The heat pump works only as hard as it needs to (based on the external temperature) to achieve this goal, and does this by changing the ‘flow temperature’ as needed. In our system the maximum design flow temperature of 50C is only for the very coldest days (perhaps a few days a year). In my house over the last month it has averaged about 35C and only once gone above 40C (a few days ago it was 42C).

It then discussed the use of radiant heating for those small dwelling “unsuitable” for a heat pump! I know that Nathan Gambling of BetaTalk would probably be jumping up and down at this point! The Air to Air kind of Air-Source Heat Pumps can be quite compact systems, and can be fitted to any dwelling. For a small flat, at a lower cost than a gas boiler. Direct electric heating may have a niche role is super-efficient PassivHaus’s, we’ll see, but it is not true to claim that heat pumps cannot be used in small dwelling.

So I haven’t a clue about the quality of the research referred to, but based on this admittedly brief segment, it did raise some concerns as to the research brief. And it is clear that the producers and lead presenter on You & Yours are still unable to accept that heat pumps are the primary game in town, so they will continue to find ways to sow doubts.