Air-to-air vs air-to-water heat pumps: which is right for your home?

If you want to heat one or a few rooms, add cooling for summer, and keep the budget under about £5,000, an air-to-air heat pump is the right call. If you want to replace a boiler, keep your radiators, and have a heat pump make your hot water too, you want an air-to-water heat pump, and you should budget £10,000 to £14,000 before the £7,500 grant.

That is the short version of the air to air heat pump vs air to water question. Both take heat from outside air with the same refrigerant cycle. The difference is where the heat ends up: air-to-air blows warm (or cool) air into a room from a wall unit, while air-to-water heats the water in your radiators and hot water cylinder. Almost every "heat pump" you read about in the UK, from the Octopus Cosy to the Daikin Altherma and Mitsubishi Ecodan, is air-to-water. Air-to-air is the wall-mounted split most people would call air conditioning, and it is what our air-to-air heat pumps guide covers.

Prices, grants, and rates correct as of September 2026. The energy price cap changes quarterly and the air-to-air grant scheme is still being set up, so check current figures before budgeting.

Air-to-air vs air-to-water at a glance

What each one actually does

An air-to-air heat pump is a reverse-cycle split system. An outdoor unit extracts heat from the air, refrigerant pipes carry it indoors, and a wall unit with a fan blows warm air into the room. Switch it to cooling in July and the cycle runs backwards. The Energy Saving Trust describes it as "functioning similarly to air conditioning", which is fair, because it is air conditioning.

An air-to-water heat pump does the first step the same way, then hands the heat to water. Daikin describes its Altherma range as generating "space heating (or cooling) and hot water" for radiators, underfloor heating, or fan coil units, with a cylinder in the loop. It is a boiler replacement. Nothing changes in the rooms except, often, bigger radiators.

Two practical differences follow. Air-to-air indoor units are noisier than radiators, because there is a fan in the room with you, and they recirculate rather than ventilate. Whether a wall unit can genuinely heat a British room is covered in our can air conditioning heat your home explainer.

Cost to install

Air-to-air is the cheaper install by a wide margin, mostly because there is no plumbing.

For a single room, UK installers quote £1,500 to £3,000 supplied and fitted for a standard wall split, and £2,500 to £4,500 for a premium unit such as a Daikin Perfera. The Energy Saving Trust's figure is about £1,900. A multi-split for two to four rooms runs £3,500 to £7,000 or more; Clever Energy Boilers, a Yorkshire firm that fits both air conditioning and heat pumps, quotes £3,200 to £6,500 for two or three rooms and £5,000 to £9,000+ for four or more. Nesta's February 2026 study put a three to five unit multi-split at £8,000 to £10,000. The number of indoor units drives the price, and our air-to-air heat pump cost guide breaks it down.

Air-to-water starts where multi-split air-to-air stops. The Energy Saving Trust says a typical installation is "around £12,000" before the grant. Octopus reports a median of £4,459 after the £7,500 grant, with new radiators extra at roughly £250 each, and Nesta's modelled costs run from £10,275 for a small home to £13,600 for a large one. Compact flat systems can come in under £10,000; large houses needing new radiators throughout can pass £16,000. Both types qualify for 0% VAT on supply and installation until 31 March 2027.

Grants: £2,500 vs £7,500 (and the £9,000 that air-to-air does not get)

The Boiler Upgrade Scheme in England and Wales pays £7,500 towards an air-to-water heat pump, and since 21 July 2026 £9,000 where an off-gas-grid home is replacing oil or LPG. Air-to-air is not in that uplift.

Air-to-air gets £2,500. The grant has been law since 28 April 2026, it applies to residential properties only, and the unit must replace an existing fossil fuel or electric heating system and be fitted by an MCS-certified installer using an MCS-listed product. That last part is the catch. MCS is still completing the air-to-air scheme: in April 2026 it said installer certification bodies were still gaining UKAS accreditation, and on 1 September 2026 Daikin became the first manufacturer with air-to-air products on the MCS Product Directory. ECIU reported in July that around 45 vouchers had been claimed and no installations completed.

So the grant is real, but as of September 2026 very few households, if any, have received it. Do not plan a purchase around it until you can see both your installer and your product on the MCS directories. The full picture is in our air-to-air heat pump grant guide. Air-to-water, by contrast, has a mature scheme with thousands of certified installers.

Running costs and efficiency

Here the two are closer than the marketing suggests. Nesta concluded that air-to-air systems "are likely to be comparable in efficiency" to air-to-water.

On paper, air-to-air wins. A modern R32 wall split carries a heating SCOP (seasonal coefficient of performance, the kWh of heat you get per kWh of electricity over a heating season) of 4.0 to 5.1; the Daikin Perfera 5kW is rated 4.71. Air-to-water has to heat water to radiator temperature, so its real-world numbers sit lower: Octopus quotes a SCOP of around 3.6 across its installs, and Nesta uses 3.0 as its baseline.

Using the Ofgem price cap for 1 July to 30 September 2026 (26.11p/kWh electricity, direct debit, Great Britain average; 7.33p/kWh gas), the cost per kWh of heat delivered is:

October's price cap moves electricity to 26.32p/kWh, which changes these figures by less than 0.1p.

One caveat: the Energy Saving Trust says there is "not much data on how air-to-air systems perform in the UK", so those SCOPs are manufacturer ratings, not field results. And air-to-water's efficiency covers your hot water too, which is the biggest gap in the table.

Hot water: the deciding factor for many homes

Air-to-air does not heat water. Take out a gas boiler and fit wall units and you need another way to make hot water: an immersion heater in a cylinder, an electric shower, or point-of-use heaters. An immersion heater works at COP 1.0, which Heat Geek, an air-to-water installer, rightly calls the "efficiency trap": your heating gets four times cheaper and your hot water gets no cheaper at all. For a family running baths and showers every morning, that eats a good share of the saving.

Air-to-water heats the cylinder at roughly COP 3, so hot water costs a third as much. Nesta found many air-to-air adopters simply keep their boiler for hot water, and that heating alone with air-to-air still cuts a home's direct emissions by around 75%.

There is a middle ground. Daikin's Multi+ is a multi-split that also heats a hot water tank of up to 230 litres, and it was among the first products listed with MCS.

If you already heat with electricity and your hot water comes from an immersion or an electric shower, air-to-air changes nothing about your hot water and cuts your heating bill by two thirds or more. That is the easiest heat pump decision in Britain.

Disruption, space, and planning

An air-to-air single split is fitted in half a day to a day: a hole through the wall, a pair of pipes, a cable, and the outdoor unit on a bracket. Nesta reports one to three days for a multi-split with "no need for extensive pipework alterations".

Air-to-water is a heating project. Nesta puts a typical installation at two to five days, and it usually means a new hot water cylinder (the Energy Saving Trust says allow a cupboard of about 80cm x 80cm), often bigger radiators, sometimes new pipework, and no heating while the boiler comes out.

Planning in England is the same for both. Since 29 May 2025, Class G permitted development covers an air source heat pump of up to 1.5m³ per unit, two units on a detached house or bungalow and one on a semi, terrace, or flat. The unit must not be cooling-only (a reverse-cycle air-to-air unit is fine), and it must meet MCS 020-a, which caps noise at 37.0 dB(A) measured one metre from a neighbour's habitable-room window or door. Listed buildings and conservation areas have tighter limits, the devolved nations have their own rules, and you should always check with your local planning authority. Our air conditioning planning permission guide has the detail.

Both types contain refrigerant, so a split of either kind needs an F-Gas certified installer. DIY installation of a split system is a criminal offence.

Which is right for your home?

Air-to-air is the better choice if you

• Live in a flat. Small outdoor unit, no cylinder, one or two indoor units. Daikin itself points air-to-air at apartments and smaller homes.

• Heat with storage heaters or panel heaters. More than 1.7 million UK homes do, on Octopus Energy's figure. You go from 26p per kWh of heat to around 6p and keep your existing hot water. Nesta singles out direct electric homes as the strongest case, and our guide to replacing storage heaters with an air-to-air heat pump walks through it.

• Want to heat one or two rooms, an extension, or a garden room. A wet system cannot be extended cheaply into a garden office; a single split can.

• Want cooling. Nesta found cooling was "the single biggest driver" for people choosing air-to-air. An air-to-water system will not cool your bedroom in a heatwave.

• Want it done this week for under £5,000, with no grant paperwork holding you up.

Air-to-water is the better choice if you

• Are replacing a gas or oil boiler in a house with radiators. The £7,500 grant (or £9,000 off the gas grid) is designed for exactly this, and it brings a £12,000 job down to £4,000 to £5,000.

• Need a heat pump to make your hot water. The immersion-heater penalty on air-to-air is real for a busy household.

• Want whole-house heating with no fans in the rooms. Radiators or underfloor, silent indoors, one system, one thermostat.

• Care about EPC and resale. Assessors and buyers understand an Ecodan or an Altherma; a set of wall units is still a newer story.

Hybrids, and running both

You do not have to pick one. Some homes keep the boiler (or fit a small air-to-water system) for radiators and hot water, then add a single air-to-air split in the living room or bedroom for summer cooling and quick winter top-up heat. Octopus, which fits only air-to-water, refers customers to partners for exactly this. Daikin also notes that "some hybrid heat pumps can connect to air-to-air heat pump (also known as air conditioners)", and its Multi+ is the nearest thing to a single-box answer: multi-split heating and cooling plus a cylinder.

Remember that outdoor units count together for planning. Two units on a semi is one too many for permitted development, so a home running both may need a planning application.

More on heating with air conditioning in our heating guides.