Read this first
An air-to-air heat pump does not heat water. It cannot feed radiators and it cannot supply hot water to your taps. If you want a single system that replaces a boiler for both heating and hot water, you are looking for an air-to-water system instead.
What air-to-air means
The heat pump takes heat from outdoor air, as any air source system does. The difference is what it does with it. Rather than transferring the heat into water, it transfers it into air and blows that air directly into the room through an indoor unit mounted on a wall or ceiling.
There is no water circuit, no radiators and no cylinder. Each room or space you want heated has its own indoor unit, fed by refrigerant pipework from a single outdoor unit.
How it differs from an air-to-water heat pump
The outdoor unit looks much the same either way. What differs is what happens to the heat once it is inside, and that is what decides whether the system can do what you need.
Air-to-air
Heat goes straight into the room as warm air, through an indoor unit on the wall. There is no water circuit — so no radiators, and no hot water for your taps. Every room you want heated needs its own indoor unit.
Air-to-water
Heat goes into water, which circulates through radiators or underfloor heating and fills a cylinder for the taps. It does the same jobs a boiler did, which is why it is the type most UK homes install. See air source heat pumps.
| Air-to-air | Air-to-water | |
|---|---|---|
| What it heats | Air, delivered into rooms | Water, for radiators and a cylinder |
| Hot water for taps | No — a separate system is needed | Yes, via a hot water cylinder |
| Works with radiators | No | Yes |
| Indoor equipment | A wall or ceiling unit in each room served | Heat pump unit and cylinder |
| Cooling in summer | Commonly available on the same unit | Only where designed in, and uncommon |
| Grant value | £2,500, residential only | £7,500 |
The industry frequently uses "air source heat pump" to mean the air-to-water type, which is why the two get confused. Both are air source. Only one of them will run your radiators.
Heating and cooling
Because the system moves heat through refrigerant and delivers it as air, many units can run in reverse and cool the room in summer. That is a real advantage over a wet system, where cooling is unusual and has to be designed in from the start.
It is also the reason these units are often sold as air conditioning. The same box, used in the other direction, is a heater — and a far more efficient one than an electric radiator, because it is moving heat rather than making it.
Where it suits a home
Air-to-air makes most sense where there is no wet central heating system to work with, or where the heating problem is confined to particular spaces:
- properties heated by electric panel heaters or storage heaters, with no radiator circuit;
- extensions, loft conversions, garden rooms and home offices;
- flats and small properties where a cylinder cannot be accommodated;
- spaces where summer cooling is as much of a priority as winter heating.
It suits a home less well where there is an existing wet system in good order, or where hot water is part of the problem you are trying to solve. In those cases you would be adding a second system rather than replacing one.
What to plan for
- Hot water. Decide how it will be provided before anything else. This is the question that most often changes the answer.
- Visible indoor units. One per space served, in a position that suits the airflow rather than the décor. Some people mind this; it is worth knowing in advance.
- The outdoor unit. Same considerations as any air source system — clear airflow, sensible siting, room to service it.
- Air movement. These systems heat by blowing warm air, which feels different from radiators. Unit position affects comfort noticeably.
The air-to-air heat pump grant
In England and Wales, an eligible air-to-air heat pump installation currently attracts £2,500 under the Boiler Upgrade Scheme. Two limits are worth stating plainly: the value is lower than the £7,500 for air-to-water and ground source systems, and air-to-air is eligible in residential properties only — it is not eligible in non-residential buildings.
Who qualifies
There is no income test. These are the scheme’s main criteria as they apply to an air-to-air installation:
- The Boiler Upgrade Scheme covers England and Wales only.
- You own the home. Homeowners and landlords can apply; air-to-air is not eligible in business premises.
- The scheme supports replacing fossil fuel heating — gas, oil or LPG — or electric heating, but not an existing heat pump.
- Most new-build properties are excluded, with narrow exceptions for some self-builds.
- Every scheme installation must be carried out by an MCS-certified installer.
- A property is usually not eligible if public funding has already paid for a low-carbon heating system at the same address.
The existing fossil fuel heating system has to be fully replaced: it cannot stay in place alongside the new system. The full criteria, with a short checker, are on the eligibility page.
How the grant is paid
You do not apply yourself. Your MCS-certified installer makes the application and takes the grant off your quote, which by law has to show the total cost before the grant, the value of the grant, what you pay after the grant is deducted. Once a voucher is issued, the installation has to be completed within 3 months.
What the grant does not cover
- The enhanced off-gas value. The £9,000 available to some off-gas oil and LPG homes until 31 March 2027 is for air-to-water and ground source heat pumps. It does not apply to air-to-air.
- Non-residential buildings. Shops, offices and other business premises can use the scheme for other heat pumps, but not for air-to-air.
- Hot water. An air-to-air system does not heat water, so how you will get hot water needs settling before you commit.
The Warm Homes: Local Grant in England does not currently fund air-to-air heat pumps. Installed heat pumps in homes do carry 0% VAT until 31 March 2027. Every grant value is set out side by side on our heat pump grants page.