Accuracy of our air source heat pump proposal

by Mars

This post is in response to an interesting question that was posed by Mark Crooks on one of our YouTube videos regarding our air source heat pump proposal and it’s accuracy.

Mark commented: “Personally I’m dubious about the figures that are provided by the system retailers for the obvious reason that it’s in their best interest to give you the best/cheapest possible figures. It would be interesting to know what running cost figures you were quoted before installation.”

So we’re going to pull the curtain back on our proposal and see how close it was. I have no idea what the answer is as I start to work on this post, but I am very intrigued.

As a starting point, air source heat pump performance is typically averaged over one year (encompassing all four seasons), so we will see how accurate the estimates were for our first year, which in our case ran from April 2019-May 1, 2020. Luckily, we’ve documented our entire process, so I have the data.

Let’s get stuck in.

Proposal recommendations and assumptions

So, let’s start at the beginning. As part of the proposal, Global Energy Systems recommended the 18kW Caernarfon air source heat pump for our property that would be integrated with our existing oil boiler to create a bivalent system.

This was done in accordance with guidelines set out by the Microgeneration Certification Scheme (MCS) that specified that the combined system must be capable of providing sufficient heating for our property down to our local area’s design temperature (for us that is -3.3°C).

In light of this, our air source heat pump would supply all the heating and hot water needs for our property down to -0.5°C; beyond this temperature the oil boiler would provide supplementary heating if required, and the heat pump would continue to provide heat down to -20°C (albeit it very inefficiently).

Estimated running costs and breakdown

As part of the estimate, Global Energy Systems estimated that our property would require 15.7kW of heat to keep it warm at -3.3°C. This, according to them, suggests an annual running cost of £1,670, offering a potential saving of £547 against oil.

Estimated Property’s Annual Energy Use (based on EPC)37,782 kWh
Current Annual Running Costs based on Oil at 52p/litre£2,218
Air Source Heat Pump Annual Running Costs based on electric at 14.5p/kWh£1,665
Annual Running Cost with an Air Source Heat Pump + Supplementary£1,670
Annual RHI (Renewable Heat Incentive) Payment based on 10.49p/kWh*£1,459
Annual Savings£2,006
Full breakdown of costs

In addition to this, they provided us with heating fuel comparison graphs to see how we’d perform against other fuel sources.

Heating fuel comparison graphs

Our real world data versus the numbers in the air source heat pump proposal

Looking back at the proposal, the outline of base costs was very accurate. We were paying 14.5p/kWh at the time and the price of oil was around 48-50p per liter.

What I’ve always found odd is the ‘savings’ derived from the RHI that have been factored into the calculations – this simply isn’t a saving. The RHI is repaying us for the investment of the air source heat pump and related equipment, so I really don’t take that into account. This is definitely an exercise in loading the numbers.

So what were our numbers?

Energy consumed for heating12,890kWh
Running cost at 14.5p£1,869
One year of oil£2,400
Saving in year one£531

Essentially, we saved around £530 in the first year on heating. Yes, the outlay for the air source heat pump was significant, but using renewable, clean heating is a massive step for us on the path towards sustainability.

Another massive difference between the ASHP and the oil boiler is that with the ASHP our heating in the cold months runs 24/7 versus limited timings on oil. We’ve covered this aspect before.

And don’t forget that our solar PV array and iBoost also offset our daytime heating when the sun is shining, especially from spring to mid-autumn.

So the proposal numbers weren’t terribly off the mark, and the proposal was professional and detailed. While we shopped around for air source heat pumps, we also received quotations and proposals from other providers, and I’ve summarised those as a point of reference below.

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What other providers claimed

Provider 1

We have calculated that your building will require 28,909 kWh of heat energy each year and that 28,909 kWh of this will be provided by the heat pump, leaving 11,263 kWh to be supplied by a backup boiler. The space heating seasonal performance factor (based upon the heat emitter guide) will be 3.3. This means that the electricity required to run the heating will be 8,895 kWh.

We have calculated that the annual hot water requirement will be 2,780 kWh, and 2,432 kWh of this will be provided by the heat pump with the rest coming from the immersion (for legionella cycle) or backup boiler. The hot water seasonal performance factor will be 3.1, meaning that the hot water will use 895 kWh of energy to run the heat pump. The maximum time to fully recharge this cylinder will be 8,895 mins.

The seasonal coefficient of performance (SCOP) factor for the combined heating and hot water will be 3.23, meaning the total electrical energy to run the system, including all the circulation pumps, will be 4,020 kWh.

Recommended unit: Hitachi S80 6.0, 25Kw (equiv) high temp high output 80 deg C split heat pump.

Provider 2

This company never bothered to give us a prediction estimate or breakdown but wanted to installed two 11.2kW Ecodan Monobloc Mitsubishi heat pumps.

Provider 3

Also never gave a detailed breakdown. They stated: “We’ve quoted for a hybrid installation including a 14kW Mitsubishi Ecodan to provide the majority of your space heating, with your oil boiler providing hot water and assistance during very cold weather. Your domestic hot water will continue to run from the existing oil boiler and DHW cylinder. The EPC for your property estimates 34,821kWh of space heating is required.”


Mark Crooks 15 July 2020 - 13:51

Thanks for taking the time to record this very useful information. I’ve recently been inundated with Facebooks ads for ASHP and some of the marketing claims are frankly absurd. Without honest, unbiased info like this there’s going to be a lot of disappointed purchasers in the near future.

Mars 15 July 2020 - 15:08

Thanks for taking the time to comment. I’m still an ASHP newbie, and learning, but I’ve seen some claims from brands that are ridiculous. We’d also met installers that made outrageous promises. Real life, objective data makes it a lot easier to understand how efficient ASHPs are. This was my motivation to make the videos and posts because I found no useful info when we were buying our ASHP.

Mark Crooks 14 August 2020 - 07:03

It does seem their estimates were pretty accurate. Just out of curiosity did their estimates include temperatures that various rooms would be set at? If so, how do these compare to what you actually have them set to?

I recently visited someone who has had an ASHP system installed for a year. Their original estimate for running the system was £495 with main living areas set to 21ºC. They had those areas set to 22ºC throughout the year and actual running costs were £640.

This was replacing an oil boiler which the owner estimated cost £500pa to run so in this case the ASHP was more expensive but the saving was derived from the owner expecting to having to replace the oil boiler in the near future at approx £6k.

Mars 14 August 2020 - 07:28

Our estimate, and I think it’s an industry standard, was calculated on maintaining the whole house at 21C. Based on the example you’ve provided above, that price hop from 21 to 22C looks about right.

What’s more, I don’t think the person you’ve cited would have run their oil 24/7, but their ASHP is likely running most of the time. That’s another huge consideration. Full time heat versus spikes in heat.

Matt Benton 27 September 2020 - 19:55

This data from your EPC is really interesting. It gives a fairly quick way for someone to estimate the cost of running an ASHP.

I have a 79sqm mid terrace in Bristol. My boiler’s just been condemned and I’m looking to take advantage of the green homes grant and go more renewable. From your videos and posts I was trying to calculate cost by comparing the number of radiators to mine, or your property area to mine. I noticed in one video you said you’d calculated your actual COP as 2.7 in the winter months, based on heat output divided by kwh used. And in this post your EPC heat requirement is 34,821kWh and electricity usage is 12,890kWh. That’s a cop of almost exactly 2.7.

So it looks like anyone wanting to know how much power an ASHP is likely to use can just take their EPC heat requirement, and divide by 2.7. I’ve been quoted for a Daikin Altherma, rated as A+++, slightly better than your A++ unit, so hopefully I’ll get a slightly better COP than that. But it’s reassuring to know that calculating it like this, an ASHP is only slightly more to run than the mains gas combi I’m replacing.

Mars 28 September 2020 - 10:02

Thank you for sharing your comments and feedback Matt.

Your theory is very interesting and would certainly act as a good starting point to working out ASHP requirements. On that note, our COP is not amazing, but one that we are satisfied with in terms of our property. As you are in a mid terrace property, you will already be benefitting from an insulation perspective because you’re not going to be losing much heat to your neighbours, and if your windows are double glazed with otherwise good insulation I would imagine that if you maintained a flow rate of 45C your COP would be between 3-3.5.

Out of curiosity, in your quotation, what is the COP that they’ve published for your pump and system?

Please share your experiences as I know very little about Daikin pumps.

Matt Benton 28 September 2020 - 12:13

Thanks Mars,

I think this post is particularly interesting because it seems to be an example of ‘real world’ COP, and it shows the EPC prediction is pretty accurate. Researching ASHPs I kept seeing comments on videos and forums about them being horrendously expensive. I suspected these were because the system was wrongly specified, wrongly installed or is being switched on and off frequently. Even so it was kind of re-reassuring to see that like mine you have an old property with existing radiators, and getting a COP of at least 2.7 means the cost is a bit higher than mains gas, but not alarming.

I’ve only had one quotation so far, which doesn’t actually give an expected COP, so I’m using Daikin’s own documentation for efficiency ratings, and comparing to yours, which should get a real world COP of at least 2.7.

Actually going forward, I wonder if there’s a case for collecting this data together in one specific post, in the same way that electric vehicle forums collect data for real world range of EVs? Ie. How much energy consumed in a year, how much heat produced, what’s the EPC heat requirement prediction? It could prove useful in a couple of years time to show how ASHPs work in practice, as many will get fitted this year thanks to the green homes grant.

Mars 28 September 2020 - 12:58

Your conservative approach to using COP is sensible, and it’s the way I usually calculate things too.

As an aside, please ensure that your installer is MCS accredited so that you can get can register and get RHI payments.

I’m more than happy to create a post that I can update from users submitting their data and creating a reference document/post that will help people make decisions pertaining to ASHPs. I think there’s a lot of scary misinformation out there, and the more people that transition to ASHPs the better because they are so much cleaner than oil and gas, especially when the UK continues to develop clean energy through resources like wind, hydro, etc.

Mark Crooks 28 September 2020 - 08:55

@MATT Your theory is very interesting. I guess we would need to see figures from other 1 year old installations to get a better idea.
I’m just about to have a system installed. At 24,000kWh heating demand from my EPC my quote from the installers estimate I will need 6,765kWh of electricity to run the system. Your formula would suggest 8,888. That higher figure may well turn out to be more accurate.

What I have realised during the quotation process though is that these figures should only be seen as a guide. Changing the indoor temp, how long the system is set at high/low and indeed the outdoor temperature will all have an affect on the power needed.

Are you saying that your calculations for running the ASHP are coming in near the cost of running a combi on mains gas?
I’d be quite surprised at that. Everything I’ve read suggests ASHPs are only economical when replacing LPG, oil or electric heating.

Mars 28 September 2020 - 10:16

You’re spot on Mark. By simply dropping your flow rate from 45C to 40C will have a huge impact on power consumption and COP.

Which system have you opted for Mark, and when is the installation scheduled to happen?

Mark Crooks 28 September 2020 - 10:22

We are going with a Mitsubishi Ecodan 14kW pump and a 210 litre cylinder. Installation should be happening late November as we are waiting for some renovation work to be completed.

Mars 28 September 2020 - 10:39

That’s really exciting Mark. Please keep us updated and we’d love to hear how your pump performs.

Matt Benton 28 September 2020 - 11:00

Thanks Mark. And very good point. I should clarify what I mean by “slightly more”!

The old boiler is a 25+ year old Worcester Bosch, non condensing. I reckoned I used 8000kwh of gas for space heating last year, so just over £300 annually. My EPC puts my space heating requirement at 8370kwh. Divided by 2.7 and multiplied by 15p a unit, is £465 a year. But I’d be running a A+++ unit in a city where the ambient temp is probably a couple of degrees higher on average than Mars is getting running an A++ unit out in the country. So maybe a fairly conservative COP of 3 rather than 2.7? Cost annually £418.50.

So was thinking in absolute cost terms rather than percentage, which is low because I have a small house, 79sqm. Also I’m mentally factoring in that because I have an electric shower and I’m going to refit the kitchen, replace the gas hob with electric, meaning I can have the gas disconnected and save £60 a year standing charge. Of course if I were to replace the old Worcester with with a modern gas condensing boiler instead (not going to happen as we’re in a climate emergency), that would cut the gas cost for comparison by a fair bit in itself.

Matt Benton 28 September 2020 - 11:08

One thing I wondered Mark, if you’re keeping the existing pipes and radiators, are the installers going to add lagging to every pipe running under every floorboard? I wonder if that’s standard practice?

I kept reading that ASHPs should run 24/7 rather than be turned on and off, but I wasn’t clear about why. Yesterday I saw a post from an ASHP installer who explained it’s because when the flow temp drops below 15c, then they have to function as an immersion heater for a couple of hours before the condenser can take over, and that’s where the real expense, and some of the horror stories about running costs of ASHPs come from.

So I thought before the installation I’ll be pulling up floorboards and insulating every pipe in the system as much as I can. Maybe then I can experiment with having the ASHP off from 12pm to 6am, if I can do that without the flow temperature dropping too much. Although pipe lagging is a good idea with gas CH, I’ve been a bit half-hearted with that, on the basis that it’s warming the floor anyway. But of course, pipes under floorboards are less effective at heating a room than radiators, so maybe pipe lagging is much more important with ASHPs, to prevent that flow temp dropping?

Mark Crooks 28 September 2020 - 11:57

My pipework downstairs is all being replaced as I’m opting for underfloor heating on the whole of the ground floor.
Upstairs pipework will be retained but no lagging has been specified or discussed.
I believe lagging is only required when the pipework is physically outside or underground.

All first floor radiators are being changed to larger 3 panel versions. This is to compensate for the lower flow temp of ASHP.

We are planning to run the heating 24/7 as we work from home. We will possibly experiment with running at 22ºC and reducing down to 18ºC at night but from what I understood from Mars’s experience this probably won’t save much power consumption.

I personally would leave all necessary lagging and pulling up of floorboards to the installers.
As to whether the lagging is necessary, it’s definitely a question for the installers to answer though.

Matt Benton 4 October 2020 - 18:06

For anyone with a small terraced house like mine, I’ve just found this very detailed video. Tristan here has managed an impressive COP of 3.9!

I’m now leaning towards an air to air system rather than air to water. Air to air does apparently qualify for green homes grant, and installation is slightly cheaper. Unfortunately the green homes grant is subtracted pound for pound from any RHI payments, so as my estimated RHI is for an air to water system is £4900-£5600, it makes more sense to use the green homes grant for something else, or go for air to air system, and deal with hot water separately.

btw Mars, I had trouble finding this post again. I don’t think it’s been tagged with ‘ashp’?

Mars 4 October 2020 - 21:20

Thanks Matt. I’ve fixed the tag.

Mars 4 October 2020 - 21:21

An interesting comment. I’ll check out air to air systems, and also check out the YT video. Thanks for sharing.


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