The honest verdict on the efficiency and sustainability of air source heat pumps in the UK

by Mars
renewable energy heat source

When we committed to installing an air source heat pump (ASHP), we had reservations whether it would heat the house sufficiently. At the same time, we wanted to move away from oil so that we could be more sustainable and reduce our carbon footprint. So we took the plunge.

There are a lot of naysayers when it comes to air source heat pumps. Most say that they’re not good enough to heat homes in the UK over winter because it’s too cold and wet.

Throughout spring and autumn, our ASHP operating costs were much cheaper than oil. The real test was always going to be the winter when there was less heat in the air and where the pump has to work harder.

Luckily for our review, we had an awful December and January that was cold and damp; the worst weather conditions for ASHPs to operate in.

In the video below, we provide an honest verdict on whether we think that ASHPs are effective, efficient and sustainable in the United Kingdom.

It’s obviously worth noting that each air source heat pump is different. We have an 18kW unit from Global Energy Systems, a British manufactured pump, and results may vary from the size of your pump to the make/brand. We have not had an air source heat pump before, so we have no point of reference.

In fact, if you have an air source heat pump in the UK, I’d be very interested to hear your thoughts and feedback, and how it compares to ours, so please leave a comment below.

Consumption highs and lows of our air source heat pump

January 16, 2020, was our warmest day by average at 7.4C over the course of the winter. The coldest was January 20, 2020, at 3C. Relative humidity throughout January hovered at the 40% mark.

On January 16, 2020, we used 53.2kWh electricity (95% for central heating and 5% for hot water) to drive our air source heat pump.

By comparison, on January 20, 2020, the colder day, we used 66.6kWh (95% for central heating, and 5% for hot water).

So we used an extra 13.4kWh to heat our house when it was approximately 3.4C cooler outside. This is not insignificant from a running cost perspective over a long cold spells. The financial costs will add up. If you are going to solely use electricity to drive your central heating, that is some interesting data right there.

But that doesn’t tell the full story if you’ve got a solar PV array. The reason why it was warmer on January 16 was because it was overcast and rainy. So we produced virtually no solar (it was less than half a kilowatt hour) but the clouds didn’t allow the heat to escape.

air source heat pump ideal with weather with solar PV
January 20 was bright and fresh

January 20, however, was a lot sunnier and our solar PV produced 18kWh of electricity – we had to buy in 70kWh. On January 16, we had to buy in all our electricity as we produced close to zero, which was over 80kWh.

DateASHPSolar PVAverage Temp
Jan 16, 202052.3kWh07.4C
Jan 20, 202066.6kWh18kWh3C

It therefore appears that with solar PV assistance, cold bright days are better suited for ASHPs, because they go a long way to offsetting electricity consumption.

If you’ve got an air source heat pump without solar you’re probably better off with more gloomy, overcast warmer days and nights that trap the heat because they’ll run are efficiently. At least that’s what our data is showing us at the moment for our pump and weather conditions.

It’s just an observation at this point, but over the course of the coming year I will look into this more closely and determine whether this correlation is accurate and precise.


If you’re interested in an air source heat pump from Global Energy Systems, you can use this code when you contact them and you’ll get £200 off your installation (and we’ll get some Amazon vouchers): GESRFAF000160


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Mark Rosher 22 August 2020 - 09:29

This post reminded me of the heat pump I built from an old fridge, back in 1975, for my school physics project. Powered by a vertical axis wind turbine on the roof of the science block made from a halved chemical barrel and a car alternator, it warmed the science block in winter and cooled it in summer – after a fashion.

It would help to understand how large your house is and where. Maybe that’s back in the blog somewhere (I’ve not looked, sorry). I checked and my average daily (gas) heating and electricity in January 2020 was 65kWh (4 bed detached, 4kWp solar, UK west country). I have a larger than average electricity demand (my bad).

Mars 22 August 2020 - 22:07

Thanks for dropping by Mark and leaving a comment. Your school project sounds very interesting.

We’re in the Welsh borders, and we’re heating around 4,000sqft.

Our electricity, over and above the ASHP, is a tad high and I’m fitting smart plugs to find the culprits. Your consumption seems a tad high too – do you know what’s responsible?

Ed 24 August 2020 - 16:57

I have used commercial heat pumps both in Michigan and in Washington State for over 30 years. I have been pleased with the energy savings and efficiency. I got likes from you for comments I made on three of Eddy Winko’s blog dating back to 2017. so I thought I would check your blog out. Warmest regards, Ed

Mars 24 August 2020 - 17:02

Hi Ed. Appreciate the comment and for taking the time to visit our site. Eddy Winko is a few years ahead of us in his country living journey, and enjoy his blog.

Michigan gets a lot colder than us in the Welsh borders. What heat pumps did you use?

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Lucy Milton 28 September 2020 - 17:26

Hello Mars,

I’ve spent a large part of today reading all your information about air source heat pumps which has been fascinating. Thank you so much for taking the time to put all this together.

I am considering using this technology for a holiday home which is based in the Lake District and rented out to guests throughout the year. We don’t’ live nearby so rely on local people to help run the cottage and sort out issues as they arise. The heating system currently uses LPG bottles and I am keen to get away from the problem of ordering and changing gas bottles. I want a system that requires as little intervention as possible!

Given that we do not live at the property, do you think the air source heat pumps are as reliable as any other heating system? I was concerned about your comments about defrosting the air pump in cold conditions. Is this done automatically or do you have to actually intervene? Can you think of anything else we should be aware of in this situation?

I’d really appreciate your thoughts

Mars 28 September 2020 - 22:21

Thanks for your comments, feedback and questions, Lucy.

In response, I can only really comment about our make of ASHP. So far, touch wood, it’s been super reliable, and the few times it did have issues, the Global Energy Team fixed the issues remotely. Their support team are available 24/7. I’m also not sure that all ASHPs have 4G connections though, and whether they are manned by support staff. This was a big consideration for us.

Don’t worry about the defrosting. This is all automated and doesn’t require manual intervention. I’d recommend that the LPG system remains so that it kicks in for a few minutes to assist with a swift defrost. This is why we left our oil boiler connected. Defrosting takes 2-3 minutes on most occasions.

Since this is a holiday home, I would automate it with as many smart thermostats and TRVs as possible, and set these to run efficiently with schedules. Guests/visitors won’t be able to change settings or heating. You will also be able to see whether everything is running as it should.

You will need a new hot cylinder for the system, and they can be quite large. That’s a notable consideration if space is tight.

I hope that helps. Let me know if you have any other questions.

Lucy Jean Milton 29 September 2020 - 10:13

Thanks Mars, got plenty to think about now.

Mars 29 September 2020 - 10:23

Great. Don’t hesitate to post any more questions if you need more clarification or assistance.

Tony Ambler 4 October 2020 - 12:45

We took the plunge and purchased an ASHP for our house in the UK and decided on the Mitsubishi Ecodan. We also purchased 14 solar panels and storage batteries that will store 4.8 kW of power. We did this because we had to renew our 24 year old oil boiler which was well past it’s best by date. We used around £600 of electricity and 2200 liters of oil per year (approx) which cost us, last year (2019) around £1100. Total energy cost £1700. Throughout the summer our electricity cost has been reduced by about 40% and, of course, our oil bill had been removed. The colder months have seen an increase in our electricity consumption especially when we generate very little through the panels. We found that our panels through out the summer powered all of our white goods, lighting and TV’s etc and charged our batteries but heavy consumption units like our range cooker needed the grid. The winter is looking like it will use significantly more electricity but I am reasonably hopeful that it will be far less than the old combined oil and electricity cost. I will let you know.

Mars 4 October 2020 - 12:54

Thank you for taking the time to share your ASHP numbers. These are very insightful and should be helpful to users looking at an Ecodan solution. What size is your Ecodan ASHP, and to help us get an additional insight, what is is your approximate floorspace that you’re heating in your property?

The ASHP does draw a lot more power over winter (in our experience), but we find that it’s still far more effective than oil.

Out of curiosity, will you be running your ASHP heating 24/7 over the winter and have you automated your central heating in any way?

Thank you once again for sharing your data.

Haresh Patel 5 October 2020 - 11:53

I wonder if anyone has done a comparative study between air source heat pump and ground source pump. Would be very grateful if you can kindly share your experience.

I am planning to install ground source heat pump for my London (UK) home following refurbishment.

Mars 5 October 2020 - 15:19

Thank you for your comment and questions Haresh. We looked at ground source as a potential solution for our heating, but didn’t pursue it because the installation was going to be very invasive and the costs were prohibitive. So we went down the route of an ASHP which was cheaper and much simpler to install.

As a result, I don’t have any comparative data, but I do know that GSHPs are more efficient than ASHPs. If you have the land and the funds, it will probably be the better option, and the RHI repayments are also much higher.

Andrew Scott 18 November 2020 - 13:20

We built a new house in Cornwall in 2016. We are fortunate in that a small stream runs through our property, so we opted for a GSHP and a plate heat exchanger which is quite small and much cheaper than either horizontal or vertical ground arrays. The GSHP is rated at 6.5 kW, and we have a 3.5 kW solar PV system. The house is a modest 3 bed detached and with just the two of us in residence we are not thrashing our appliances. Being a new build I have no comparative data, but our annual consumption is 3916 kWh with a max monthly in January of 590 kWh and min monthly in July of 156 kWh. Generation is 3444 kWh annually with a max monthly of 484 kWh in July and monthly min of 76 kWh in January. These figures are for 2019, but very similar from 2017 to 2020. The upshot of this is that our electricity bills are around £700 pa and FIT payments approx £550, so our annual costs are £150. On top of that we are claiming our RHI payments which are approx. £1200 pa. This is paid for 7 years so we will recover approx £7k which is the material cost of the GSHP and associated parts. This was all supplied by Kensa heat pumps. Hope this is useful information.

Mars 19 November 2020 - 21:30

That’s great Andrew. Thank you sharing. Some very interesting numbers and great as a comparison.

p harvey 8 October 2020 - 20:00

They are noisy and take too much space. unless you are replacing with oil and live in the country in a detached property – you cannot beat GAS

Mars 11 October 2020 - 09:44

When it comes to central heating, mains gas is the most efficient way to heat a property. But it’s not the cleanest and does have a CO2 impact. I’ve read that many ASHP manufacturers are working on smaller form factor, quieter, more efficient models for urban settings, and the UK government is working on getting ASHPs integrated into new builds instead of gas boilers.

Brian Mallalieu 30 October 2020 - 12:52

Thanks for your informative & interesting video. I take it the ASHP is also providing you with DHWS for washing etc. and it would be helpful if you provided more details about your experience with it and comparisons & costs hitherto?

Mars 30 October 2020 - 21:09

Thank you Brian. We’ve actually just finished recording a new video that covers many of the questions you’ve raised. I will reply here when the new video is ready and will share a link.

Danielle Strachan 30 October 2020 - 19:26

This has been very interesting to read thank you- we are currently looking into getting an outdoor heat pump.

We have just been told the total cost is £11k but after w government grant we only need to pay 3.5k- but I’ll still not sure if it’ll be better for us than what we have. We live in w small town in Scotland and our town doesn’t have gas at all.
We have an old economy 7 electric boiler.

I’m gonna do a bit more research before we make a decision

Mars 30 October 2020 - 21:19

Thanks for the feedback Danielle. You may want to look into this, as I’m not sure it’s 100% accurate, but once you get the government grant, you’ll still be eligible for the RHI scheme so you can get even more money back. Definitely worth checking it out.

David 11 November 2020 - 17:14

The green energy grant and rhi payments would come from the same pot of money allocated to your project, so the total amount would be the same. If you use the grant that amount would get subtracted from the total rhi you would otherwise receive. Benefit is a big lump near the start as opposed to bring spread over 7 years. We have 1 property on an ashp installed 2 years ago and it’s amazing…. Rented out currently. Our own house is having 1 fitted in a few weeks, a gen 6 Samsung 16kw monobloc. The other house is a gen 5 in a big 3 bed semi, average Elec cost is 80 quid per month

Mars 11 November 2020 - 21:18

Thanks for the clarification David, and for answering this. We would have preferred the lump sum to be honest.

£80/month is excellent on the three bed semi.

Richard Dobbs 17 November 2020 - 10:54

Very interested in your helpful blog Mars. We are looking to install ASHP. We are a flat and can install on the balcony. We are around 150m2 area overall and have 2.7m high ceilings. Any thought on output needed? We are ‘all electric’ 3 phase with a wet rad system so currently two 9 kW boilers really use a lot of juice! 7 kWh LG R32 is recommended but sounded a bit underpowered to me….

Mars 19 November 2020 - 21:35

Thanks for the comment and feedback Richard. I can’t make a call on the sizing and whether it’d be underpowered, but a 7kW may be on the small side. I’d recommend, Covid-permitting, to get another 1-2 installers to quote and see what size they’d recommend.

Wolfgang Küchler 20 November 2020 - 15:50

No one has mentioned insulation? Can you – and other commenters – tell us anything about how well insulated your homes are, the type of windows, draught stopping? There’s a lot of concern about putting heat pumps into thermally “leaky” buildings. Also ground source is supposed to be better, also in terms of noise – any comments on that from anyone?

Andrew Scott 20 November 2020 - 18:23

We built a new property so were able to design a well insulated thermally efficient house. We have 100mm foil backed PU insulation layer in the floor with underfloor heating in a 60mm screed on top, the internal structure is timber frame with 125mm foil backed PU insulation in the walls and roof. We have high efficiency double glazed windows and doors. We have a Kensa ground source heat pump, which is quite noisy, but no more than the oil fired boiler at our previous house and as we designed the house we created a separate small room to house the boiler UFH pumps, manifolds and all electrical and IT equipment. Being in a separate room, the noise is no more than that from our fridge/freezer in the kitchen

Mars 22 November 2020 - 08:40

We spend a lot of time insulating our period property. We have double glazed windows and foil backed insulation in our walls, and whenever we come across a draft, we address this to prevent heat loss. Our property is by no means perfect when it comes to being insulated, but the ASHP still manages quite well. The better we insulate, the more we’ll increase our efficiency.

RALPH Brunjes 21 November 2020 - 00:21

Before we had our heat pump installed we happy and the anual bill for gas was about £2000-£3000 , since getting rid of LPG and haveing all the house run on electric our electric bill is £1320 we also run two big American washer and drier. We do have solar panels and get money back each reading but obviously depending on weather it is different each time . We also live in the far North of Scotland , wet windy cold hot. Three seasons in one day.
So far we have been very happy with the heating unit apart from the control panel, it goes crazy every time we have a power cut . But we were told that it is an old model.
With us both being disabled we tend to have the heating on 24/7.
So in conclusion I personally think that it is a great unit . We have a double fan unit and it heats a large 4 bedroom 2 reception detached bungalow.

Mars 21 November 2020 - 08:51

That’s brilliant Ralph. Thanks for sharing, and it’s great to hear that your ASHP is getting the job done in the north of Scotland.

steven murphy 26 November 2020 - 15:57

they are noisy – unsightly – and expensive – neighbours complained about fan noise and heat exchanger fan
GAS IS THE BEST IF YOU ARE ON THE GRID – had to have additional heating on in the water and electric boost for the hot water
could not wait to get rid of it
never again

Mars 28 November 2020 - 17:41

Sorry to hear about your ASHP experience. Can I ask what pump you had?


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