Air-to-air vs air-to-water heat pump: which suits your home?

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

An air-to-air heat pump warms the air in your rooms, and an air-to-water heat pump heats the water in your radiators. The choice usually comes down to radiators and hot water against warm air and cooling.

Both take heat from the outside air, so you get more heat out than the electricity you put in. You’ll often see air-to-water called an air source heat pump, and GOV.UK uses both names.

These are general comparisons. We fit air-to-air systems, not air-to-water.

Sources: GOV.UK on the grant, MCS on air-to-air systems, the Energy Saving Trust and Nesta’s report on home heating options (February 2025).

Which heat pump suits your home?

The right heat pump depends on how your home is heated now, and whether you want cooling.

Flats and small homes

Air-to-air usually suits a flat or small home best. The Energy Saving Trust says it can be a good choice where air-to-water won’t work. Its outdoor units are typically smaller, and it needs no radiators.

The Energy Saving Trust also lists compact heat pumps for small, well-insulated modern flats. These need a cupboard of around 80cm by 80cm, near an outside wall.

In a leasehold flat, outdoor work can need your freeholder’s yes, and in some cases planning permission. We’ll check both at the visit. Agreeing it with your freeholder.

Sources: the Energy Saving Trust and the Leasehold Advisory Service.

Homes with electric heating

Air-to-air often suits a home with storage heaters or electric panels best, because there are no radiators to reuse. Air-to-water would mean adding radiators and pipes from scratch. Swapping out storage heaters.

Source: the Energy Saving Trust.

Houses with radiators and a boiler

Air-to-water usually suits a house with radiators that’s replacing its boiler for good. It feeds the radiators you have, though some may need to be larger, and it heats your water too.

Air-to-air suits the same house if you keep the boiler and add a unit or two. That helps rooms that get too hot or too cold. How air con heats a home.

Source: the government’s heat pump explainer (March 2024).

Homes that overheat in summer

Air-to-air is the one to pick if you want cooling as well as heat, because the same indoor units do both jobs. Air-to-water doesn’t usually cool. How to cool a whole home.

Bigger homes

Bigger homes usually lean towards air-to-water, because air-to-air needs more indoor units and its cost climbs quickly. Hot water and bathrooms also need separate heaters, and those cost more to run.

The Energy Saving Trust says air-to-air is still possible in a larger home.

Sources: Nesta’s report on home heating options (February 2025), the Energy Saving Trust and the government’s modelling.

What fitting each one involves

Fitting air-to-air is usually the smaller job, because your radiators and heating pipes aren’t touched. Air-to-water usually costs more to fit, before any grant. What an air-to-air system costs.

In some cases an outdoor unit needs planning permission, whichever type you choose. For an air-to-air system, we’ll check it at the visit. How planning works for air con.

Source: the planning order.

Fitting air-to-air

Fitting air-to-air usually involves:

• an outdoor unit on a wall or the ground
• an indoor unit in each room or zone, usually high on a wall
• thin refrigerant pipes, often in a slim plastic channel near the ceiling
• a check of your electrics, and a new circuit if needed

Allow one to three days for a system covering several rooms. What happens when it’s fitted.

Source: the Energy Saving Trust.

Fitting air-to-water

Fitting air-to-water usually involves:

• an outdoor unit with clear space around it, on a wall or the ground
• a hot water cylinder, which the Energy Saving Trust says usually fits a cupboard of at least 80cm by 80cm
• a check of your radiators, as some may need to be larger
• changes to your heating pipes, where the system needs them

Nesta puts the disruption at 2 to 5 days.

Sources: the Energy Saving Trust, Nesta’s report on home heating options (February 2025) and the government’s heat pump explainer.

Which costs less to run?

Running costs for heating rooms are close, and hot water is what separates the two. The government published modelling in July 2026 that compared them in the same homes.

The sum is 26.32p, Ofgem’s capped electricity price, divided by each figure. MCS’s 3.42 for air-to-air, about 7.7p, sits inside that range. Treat these as a guide:

• The price is Ofgem’s cap for 1 October to 31 December 2026, by Direct Debit, and yours may differ.
• The report says its air-to-air figures are less certain, because the data behind them is limited.
• The modelling used one electricity price all day, so a tariff with cheaper hours could change the result.

Hot water changes the totals. Air-to-water heats your water with the heat pump. Most air-to-air homes use an immersion heater, at about one unit of heat for each unit of electricity.

With hot water counted, and the same cooling added to each system, the modelling found:

• Three larger homes: air-to-water cost less to run in a 3-bed semi and two detached houses.
• A 2-bed terrace: the two came out about level.
• A studio flat: air-to-air cost less than electric panels, and a model that also heated water cost less than gas. Air-to-water wasn’t modelled there.

Work out what it would cost to run.

Sources: the government’s modelling (July 2026), Ofgem’s price cap, MCS’s air-to-air standard and a report for the Committee on Fuel Poverty.

How the grants compare

The grant for air-to-water is £7,500 and can be used today, and the grant for air-to-air is £2,500 and can’t be used yet. MCS says no installer is certified for air-to-air systems. We read that on MCS’s website on 1 November 2026.

Both grants come from the Boiler Upgrade Scheme, and they share these rules:

• The home is in England or Wales.
• The new system replaces the old heating, so a unit added to a room won’t qualify.
• The installer is certified by MCS.
• Each property gets one grant.

The bigger grant narrows the gap in fitting costs. On Nesta’s 2025 estimates, air-to-water comes out lower after its grant in a medium or large home. Those estimates came before the air-to-air grant.

Who the air-to-air grant covers.

Sources: GOV.UK on the grant, who can apply, the 2026 regulations, MCS on air-to-air systems and Nesta (February 2025).

If you want air-to-water

Air-to-water is a job for a heating firm, and MCS’s Find an Installer tool is the public way to look for one. We fit air-to-air systems, so we can’t quote for air-to-water work.

The £7,500 grant needs an installer certified by MCS, so choose air source heat pump in the tool.

If air-to-air could suit your home, we’ll go through it room by room at the survey. We fit these systems in all 32 London boroughs and the City of London. What an air-to-air system involves.