Are air-to-air heat pumps worth it in the UK climate?

First a little background on our situation.

Our property is electrically heated. When we bought the place it was heated by a mixture of night storage heaters and convection heaters. There is no wet central heating, no underfloor heating etc. The only additional source of heat is a 5kw log burner in the main room, Because of this, our electricity consumption is massive, especially during the cold dark months of the winter.

So, in an attempt to make heating our property more efficient, I started to look at air to air heat pumps and booked a visit from a local heat pump installer. During the sales pitch they said that the units had a COP of around 3 to 4 but the manufacturers brochure they left with us clearly showed about 1.25 to 1 at 11 degrees C and less for lower temperature.

This made me a bit wary about claims for this technology so I reached out to the community on sustainability.stackexchange.com (because back then there was no Better Century :disappointed:) and the nice folks over there pointed me towards some fairly technical research papers. I don’t really understand the research but the impression I get is that the efficiency of these units depends on 2 factors:

  1. The outside temperature.
  2. The amount of heat that you want the unit to provide.

This has lead me to the conclusion that they may not be viable for the typical leaky, poorly insulated UK house but that they may work in a well insulated new build.

This is born out by anecdotal evidence from the couple of people that I know that have these units. Apparently the heat exchangers on the back freeze up during the winter if you run them too hard, and then they have to spend energy defrosting the heat exchanger :astonished: .

Just thought I would share my findings so far on this. Does anyone else have any experience with this sort of heat pump in the UK? Can anyone point to proper data about their effectiveness?

I would suggest having a word with Sunamp about using a heat battery in conjunction with a heat pump on off peak electricity.

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Hi @peteroche. Thanks for the link to Sunamp. Hadn’t heard of a thermal battery before. It’s an interesting idea. But from reading their website, my impression is that they work with central heating or underfloor heating. :disappointed:

I reached out on twitter to see if a few people would reply here. But got an answer on twitter instead :pensive:.

Have a look at this @russell

Thanks for reaching out on Twitter @Tommo. The “it depends” answer is sort-of the conclusion that I came to as well. It would be good to get something a bit more concrete so that people can actually make an informed decision on this. Regarding the RHI scheme, believe air to air heat pumps have actually been removed from the scheme (according to the energy saving trust website) .

I posted this on Twitter as I’m the Founder of Low Carbon Homes. The reason I mentioned RHI is that the domestic scheme is closed, however, non-domestic is still running and domestic dwellings can qualify as long as there’s a form of “district heating” - most popular are GSHP schemes where the array (the underground loops) are shared. The scheme pays out for 20 years and can provide payback in 10 - here’s a better explanation: https://finn-geotherm.co.uk/blog/heat-pumps-and-district-heating/

HI @russell Can you tell me a bit more about the style, size and construction of your house, and what ideally you’re try to acheive? I should be able to point you to some more information to help you make an informed decision - or at least cut through the confusion. It is all very complicated but there are some basic principles to follow once you’re clear on your objectives

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Thanks for this Locky and welcome to Better Century. Your support here is much appreciated!

Hi @Locky . Many thanks for your offer of advice.

The construction is a bit unusual here. It’s a large houseboat made out of a converted Admiralty ammunition barge.

Google Photos

This is her at low tide. When the tide is in she floats!

So in terms of the space heating requirements, the top bit is like a timber frame house. The bottom bit is like a boat. The feeling that I get is that, although it is insulated, it is thermally leaky. Building regs do not apply to house boats so the people who built it just did what they felt was good enough. It’s not terrible to heat but not brilliant either.

Because it is a boat and there is no vehicle access we use electricity for all our water heating, space heating, cooking etc. We are on the grid. As I said above there is no “central” heating system. Currently it is heated by a mixture of convection heaters and old-school storage heaters. This year we have also experimented by replacing the older heaters by newer electric radiators that have accurate thermostatic controls and are programmable (by Rointe) and these have reduced out electricity consumption somewhat.

What I’m trying to achieve is to use less electricity for heating. One obvious place I thought to look was replacing resistive electrical heating by air-to-air heat pumps. The reason for air-to-air is because we don’t have any “ground” for a ground source pump and also we don’t have wet radiators. Installing a wet central heating system would be a massive undertaking and something that I’m not keen to do, given the fact that water leaks on a boat are a fairly serious issue because the boat is water-tight!

However, my current thinking is that air-to-air heat pumps might not be a good option for us for space heating, due to the face that the units would have to produce quite a lot of heat in the winter to keep the place warm and that might put them in to a situation where they are actually less efficient than resistive heating. Really not sure if this is the right conclusion though as it’s more based on gut feeling than any hard evidence.

Any thoughts?

Hi Russell

What a fabulous home.

I’m no technical expert but I have a broad understanding of the principles involved, so as layman to layman, I think the experts would advise tackling the leakiness of heat first - have you had a thermal imaging study undertaken? That should highlight the areas of greatest heatloss, so you can focus on reducing loss through draft proofing and insulation measures. There are lots of natural materials available now to consider.

As far as air source heat pumps are concerned, I suspect that they could cope with heating this space if the system is designed correctly, and would provide the added benefit of summer cooling if overheating is a concern. They should certainly not cost more to operate than conventional resistive systems (as that’s their key selling point)

As far as water heating is concerned have you considered solar thermal? Or even better, PVT (solar PV, cooled by fluids to provide hot water as a by-product)

Where are you based? I might be able to hook you up with someone to provide more technical guidance.

Regards

Graham

Thanks for the info about PVT @Locky. I’d not heard about that. Will certainly check it out. I’m near Ipswich, if you know anyone near by who could advise, would certainly appreciate that. Thanks again for all your help.

I know someone who can certainly help with the thermal solar side of things, maybe the ASHP too, and he’s based in Ipswich would you believe!

I’ve spoken to him and he’s happy to take a look. How do I message you privately with his details?

I went through the same thought process as our house was poorly insulated and only had electricity connected. One cold April after moving in was all it took to relaise storage heaters had to go. Lots of research and we went for air-to-air heating but an industrial rather than residential unit. Cost less, more powerful and just a little bit bigger. 6 years later it still works well in conjunction with a 5kw woodburner and lots of other insulation measures. We have a 4Kw PV solar array so at the time FIT payments were good so we still earn more than we pay out in electricity and have a cosy or cool house house all year round. You can find out more here http://www.superhomes.org.uk/superhomes/essex-colchester-alresford-station-road/

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Thanks for this @bhammick. If you want to share your experience of putting up solar panels as well there’s a thread here.

Welcome to the community and thanks for a great first post!

Just so others are aware there is another thread relevant to this discussion from Graham Baker:

Guide to Domestic Scale Renewable Energy

You can get underfloor heating as a pretty thin Matt a few mm thick now. The guy at Sunamp was telling me. Worth giving them a call to go through the requirements. You probably need around 22-28kwh or thermal storage to heat a whole house assuming 3 bedroom. This is a rough guestimate.

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Hi @Richard_Phillips, thanks for the info. Not sure how this fits in with an air-to-air heat pump though? Are you proposing to heat a sunamp-style thermal battery with a hear pump? If so this is not air-to-air. The reason that I was interested in air-to-air was the difficulty of retro-fitting a wet system (either conventional radiators or underfloor heating) where there is none already.

Having a wet central heating system really does give additional options for heating though as there is a much larger thermal mass to heat up and therefore your heating requirements can be more constant. It opens up viable options for heat pumps, solar thermal, thermal batteries etc.

Without a wet system, your options are basically just electric radiators or air-to-air heat pumps. I could be missing something here though :thinking:

Thanks @Richard_Phillips. I think you are absolutely right and that the conventional wisdom seems to be that you should insulate before investing in any fancy heating technology. It is generally cheaper and as you said, any heat pump will be fighting a loosing battle unless the property is properly insulated.

I’ve actually parked the heat pump idea for now and am working on insulation instead. We have already had the double glazing (wooden frames) worked on as they were very draughty. I’m also working on adding another layer of insulation to the upper floor.

I have already asked the community What is the best insulation to use? and Are consumer-grade thermal imaging cameras any good for spotting heat leaks in a house? and the responses have been really helpful.

Thanks again for your input.

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Hi @russell Just had a thought about your insulation challenge. Have you looked at cork-based products? You might be able to find something that has thermal mass sufficient to store heat, insulate and float! And low embedded carbon! As I say, just a thought!

Thanks @Locky. Will take a look