Green Building Forum - Air-to-air heat pumps

Air-to-air heat pumps

djh — Tue, 18 Nov 2025 17:34:04 +0000

So, the government has included air-air heat pumps in the BUS (see https://www.gov.uk/government/news/discounts-for-families-to-keep-warm-in-winter-and-cool-in-summer ), which has set me thinking about them again. Does anybody have any ideas about whether this will decrease or increase prices, and when. Or how to find a competent and trustworthy installer? (in Suffolk if it matters) And about any specific products?

PS They've also included subsidies for 'heat batteries', but since heat batteries are now more expensive than electrical batteries for the same capacity, it seems like another poorly thought-out scheme!

Air-to-air heat pumps

Doubting_Thomas — Fri, 21 Nov 2025 13:45:04 +0000

Posted By: djhheat batteries are now more expensive than electrical batteries for the same capacity

I hadn't thought about it like this, but I guess you're right. We got our 6kWh & 3kWh heat batteries for £3500 installed, when it was the best way to store excess PV for DHW, and Tesla powerwalls were pretty much the only mainstream electric battery options for home use.

The market has changed a lot since 2018! Looks like around £4k might get you 10kWh of home electric battery these days. So certainly reaching cost parity.

The next question is how the water is heated. When I was looking back in the day you needed a three-phase supply to do decent instantaneous heating, and the heat battery offered a better alternative to storing hot water in a cylinder. (by better I mean smaller footprint and, crucially for passivhaus, lower standing losses).

Nowadays for DHW I guess you'd pair the battery with a heat pump to get more out of it?

Air-to-air heat pumps

djh — Fri, 21 Nov 2025 19:30:24 +0000

Posted By: Doubting_ThomasNowadays for DHW I guess you'd pair the battery with a heat pump to get more out of it?

If I was going for an "ASHP" then yes, but if I'm going for air-air I'm tempted to leave the thermal store and immersion heaters in place; at least for now.

We put some extra insulation around the thermal store, and being next to an MVHR extract vent with the MVHR having automatic summer bypass, the heat loss hasn't been a problem for us.

Air-to-air heat pumps

owlman — Fri, 21 Nov 2025 20:03:11 +0000

How is your thermal store currently heated.

Air-to-air heat pumps

djh — Fri, 21 Nov 2025 20:15:58 +0000

Posted By: owlmanHow is your thermal store currently heated.

Two immersion heaters. One at the bottom powered by a solar diverter. One half-way up controlled by a timeswitch. The lower one is set to a higher temperature.

During the summer and most of spring and autumn, solar gives us all our hot water. Off-peak mains tops it up at other times of year.

Air-to-air heat pumps

owlman — Sat, 22 Nov 2025 10:21:20 +0000

That sounds fairly similar to my accumulator tank although mine has a spare internal coil too.
Does yours supply both space heating and DHW? Does it have an internal coil?

Air-to-air heat pumps

djh — Sat, 22 Nov 2025 11:46:06 +0000

Posted By: owlmanThat sounds fairly similar to my accumulator tank although mine has a spare internal coil too.
Does yours supply both space heating and DHW? Does it have an internal coil?

Our space heating is presently all direct electric. No wet heating at all. There're no internal coils AFAIK. DHW is produced by an external PHE.

Air-to-air heat pumps

owlman — Sat, 22 Nov 2025 13:12:37 +0000

Re, the internal coil. The reason I asked was I'm pretty sure you used to be able to get air/air split units where one of the splits feeds the small bore refrigerant pipes to a PHE which subsequently could feed an internal "store" water coil, or maybe even the store itself, I'm guessing if it's all potable. If that is possible it sounds a fairly straightforward plumbing job to feed the new PHE heated water direct into the incoming store feed.
I know that Toshiba do something of that ilk whereby the air/air unit could also supply DHW which may satisfy any caveats the BUS scheme may have. I think its a complete internal unit which in your case may require you to swap out the existing set up. Whether you're up for that or not I don't know but it may be an inroad into the BUS scheme, where I feel the BUS may have such DHW supply caveats, I haven't read the small print.

Air-to-air heat pumps

WillInAberdeen — Sat, 22 Nov 2025 16:20:35 +0000

IIRC the BUS grant eligibility for air-water heatpumps is they must provide 100% of the space heating load of the home (as calculated by the dubious MCS method) AND they must provide 100% of the hot water. This was to stop people claiming the grant, installing a token-sized heatpump, then retaining/using a non heatpump system in parallel.

The consultation for adding air-air heatpumps to BUS had a line where they noted that air-air heatpumps generally can't do hot water and so this requirement would need to be changed. They wanted suggestions how to change it.

They also said they won't fund dedicated air-to-DHW pumps, such as the integrated heatpump cylinders that come up occasionally.

Edit: the consultation has closed and the government response is
"To be eligible for BUS funding, AAHPs will
need to be installed to provide space heating. They will also need to be MCS certified
products, as well as meeting ecodesign requirements for heating and cooling. We will allow
AAHPs to be installed alongside other electric appliances that provide supplementary space or
water heating.
To ensure AAHP installations maximise potential carbon emission savings, AAHPs installed
alongside existing fossil fuel heating appliances will not be eligible for funding"

Air-to-air heat pumps

djh — Sat, 22 Nov 2025 18:01:19 +0000

Posted By: WillInAberdeenEdit: the consultation has closed and the government response is
"To be eligible for BUS funding, AAHPs will
need to be installed to provide space heating. They will also need to be MCS certified
products, as well as meeting ecodesign requirements for heating and cooling. We will allow
AAHPs to be installed alongside other electric appliances that provide supplementary space or
water heating.
To ensure AAHP installations maximise potential carbon emission savings, AAHPs installed
alongside existing fossil fuel heating appliances will not be eligible for funding"

Yes, that's my understanding and I believe my house will qualify, which is why I'm asking the questions.

Air-to-air heat pumps

stonecold — Wed, 26 Nov 2025 21:12:39 +0000

Home batteries are available from a number of different suppliers, ours is Givenergy's All In One but we had quotes with prices for other companies that did not line the pockets of Elon Musk.

Sunamp do a phase-change solid battery thermal store https://acarchitects.biz/self-build-blog/self-build-blog-sunamp-heat-batteries

Air-to-air heat pumps

ChrisGT — Wed, 26 Nov 2025 21:13:26 +0000

I've just had a quote for £1350 for a 2.5kw air to air HP for my passivhaus retrofit. I don't have much room for a cylinder so am tempted just to combine it with instantaneous water heating. Is this sensible?

Air-to-air heat pumps

djh — Wed, 26 Nov 2025 21:19:22 +0000

Posted By: ChrisGTI've just had a quote for £1350 for a 2.5kw air to air HP for my passivhaus retrofit. I don't have much room for a cylinder so am tempted just to combine it with instantaneous water heating. Is this sensible?

I think it might be. It depends on how much hot water you use, and also whether you have solar panels and suchlike.

Who's the quote from, if you don't mind my asking, and which heat pump?

Air-to-air heat pumps

djh — Wed, 26 Nov 2025 21:30:08 +0000

Posted By: stonecoldHome batteries are available from a number of different suppliers

Yes, I'm talking to people about getting a Fox ESS battery via the Solar Together project.

Sunamp do a phase-change solid battery thermal store

That's the kind of thing that I think is now more expensive than an electrical battery. Your link is nine years old when they made more sense.

Air-to-air heat pumps

WillInAberdeen — Wed, 26 Nov 2025 23:13:25 +0000

Interested how fast can these home batteries be discharged? Looked a while ago and a 10kWh battery could discharge at about 0.5C = 5kW, which was useless for instant hot water for a shower or bath.

Seemed then that the advantage of a hot water cylinder over an electric battery and instant heater, was rapid hot water delivery, difficult to fully electrify homes without this. Larger sunamps are also good for instant hot water at shower flow rates.

In principle an electric battery could discharge at high enough kW rate, like in an electric car, but that might need a bigger battery and/or battery coolant systems.

What's the current state of art?

(Sorry if this is sidetracked thread)

Air-to-air heat pumps

djh — Thu, 27 Nov 2025 12:13:33 +0000

Posted By: WillInAberdeenInterested how fast can these home batteries be discharged?

The one I'm looking at - EP11 - has a stated max discharge current of 27A, which is a bit more than 5 kW as you say, but not a whole lot. I assume that's at mains volts, not the internal 380V! You can put units in parallel if you want more power.

edit to add: I see that may be 2 x EP5 would be better from a max dischrage rate POV.

Air-to-air heat pumps

ChrisGT — Thu, 27 Nov 2025 17:19:43 +0000

Hi DJH its a Haier AS25RBAHRA4
2.7kwh R32 wall mounted unit.
We use about 1100kwh/pa for hw.
For heating we use about 550kwh pa. We don't mind if the temp drops a bit in winter and find the oven will heat the house.
.

Air-to-air heat pumps

owlman — Thu, 27 Nov 2025 19:43:22 +0000

Posted By: djh
Posted By: WillInAberdeenInterested how fast can these home batteries be discharged?
The one I'm looking at - EP11 - has a stated max discharge current of 27A, which is a bit more than 5 kW as you say, but not a whole lot. I assume that's at mains volts, not the internal 380V! You can put units in parallel if you want more power.

edit to add: I see that may be 2 x EP5 would be better from a max dischrage rate POV.

The latest EP12 has a max discharge current of 30A.

Air-to-air heat pumps

WillInAberdeen — Thu, 27 Nov 2025 20:48:22 +0000

If you had enough electric batteries to enable instant water heating instead of hot water storage (let's say 10kW to drive an electric shower), would you need a 10kW G99 cert from the DNO in case it was diverted to export? Or do they come certified to do export limiting?

Air-to-air heat pumps

owlman — Fri, 28 Nov 2025 12:20:25 +0000

Yes some, apparently do come with limiting, but in my case the DNO wouldn't accept it and my fitter had to fit a slightly smaller hybrid inverter.

Air-to-air heat pumps

djh — Fri, 28 Nov 2025 12:32:26 +0000

Posted By: WillInAberdeenIf you had enough electric batteries to enable instant water heating instead of hot water storage (let's say 10kW to drive an electric shower), would you need a 10kW G99 cert from the DNO in case it was diverted to export? Or do they come certified to do export limiting?

Dunno; good question. I'll add it to my list of things to ask the installer who's quoting.

Air-to-air heat pumps

djh — Fri, 28 Nov 2025 12:34:10 +0000

Posted By: owlmanThe latest EP12 has a max discharge current of 30A.

Posted By: owlmanYes some, apparently do come with limiting, but in my case the DNO wouldn't accept it and my fitter had to fit a slightly smaller hybrid inverter.

Thanks for those nuggets I'll see what mine says.

Air-to-air heat pumps

owlman — Sat, 29 Nov 2025 14:09:26 +0000

Consider too battery system siting, as connections between Inverter and CU can get onerous.

Air-to-air heat pumps

GarethC — Mon, 01 Dec 2025 18:29:06 +0000

If you bought, say, 30kWh of batteries, presumably that would easily give you a high enough discharge rate to do hot water.

Could you do something like draw 10kW from the batteries and 10kW from grid to run a 20kW instant water heater?

And the big battery, charged with cheap, green off peak leccy, if it could cover your entire daytime needs, including running your A2A, would deliver super cheap, super green heat?

Air-to-air heat pumps

fostertom — Mon, 01 Dec 2025 18:54:47 +0000

Posted By: GarethCthe big battery, charged with cheap, green off peak leccy ... would deliver super cheap, super green heat?

would cost far more to run than a heat pump.

Air-to-air heat pumps

GarethC — Mon, 01 Dec 2025 19:06:22 +0000

I mean run the heat pump off the battery

Air-to-air heat pumps

fostertom — Mon, 01 Dec 2025 19:20:34 +0000

Ah. Gd thought - what do our boffins say about running a HP on off-peak stored elec? Off-peak's not all that cheap, the batts would cost a lot (but getting cheaper?), maybe spend their cost on fabric retrofit, save more elect consumption that way than the off-peak saving; and/or spend it on ASHP-system optimisation for example UFH if not already, for consumption saving via maximal COP.

Air-to-air heat pumps

djh — Mon, 01 Dec 2025 21:42:44 +0000

All possible but not really relevant in my situation. I already only run my heating overnight on cheap electricity. I'm sure I could run a A-A heat pump off a battery as well; indeed I might well do that if I decide to go for one. No way is wet heating economical, as well as being a huge amount of faff.

Air-to-air heat pumps

fostertom — Mon, 01 Dec 2025 23:37:23 +0000

Posted By: djhif I decide to go for one

Hasn't your house already got its heating system? - or re-doing it just for fun?

Posted By: djhNo way is wet heating economical

Now you tell us - we're listening.

Air-to-air heat pumps

RobL — Tue, 02 Dec 2025 08:36:09 +0000

We run our tiny 2.5kW heatpump off a 16kWh fogstar battery, with a 3.6kW sunsynk inverter. DHW is conventional though, not what is suggested above - there’s a tank that is heated by the heatpump using cheap overnight electric, while the batt charges for the day ahead. It rarely runs out of battery - typically on an overcast day when there’s people visiting and a lot of cooking going on. I find 3.6kW power is enough such that >95% of all household use is covered by it, with only small amounts pulled from the grid whilst the toaster & kettle etc. are on together - we don’t particularly try to stagger stuff. Obviously a bigger inverter would allow more - but I expect it would have larger static loss, and for us would need G100 which I’d rather avoid. -My uncle has the larger 8kW inverter, and the G100 implemented is flaky at best, it depends on the firmware and how it is used, so can permit too much power output.
If we could only heat hot water with direct electric, I’d get a waste water heat exchanger for the shower.