Air-to-air vs air-to-water: which one cools your home?

If your goal is to cool an overheating home, this comparison has a short answer, and it surprises people: only one of these two heat pumps can cool at all. Both are called "heat pumps", both can be grant-supported, but they solve different problems - so the choice is less about preference and more about what you actually need.

The one difference that settles it

An air-to-water heat pump moves heat into water - it feeds your radiators, underfloor heating and hot-water cylinder. It is an excellent low-carbon replacement for a gas boiler. But it has no cooling mode for your rooms: it cannot bring a hot top-floor bedroom down on a July afternoon.

An air-to-air heat pump moves heat directly into (or out of) the air in your rooms through wall or ceiling units. That reversible airflow is what lets it cool in summer as well as heat in winter. If comfort in a heatwave is the reason you are reading this, air-to-air - or a straightforward air conditioning unit - is the relevant family. Air-to-water is not.

What that means for the grant

Because the systems do different jobs, they sit on different grant routes, and it is worth being precise about the cooling one.

The £2,500 air-to-air pathway of the Boiler Upgrade Scheme is the cooling-relevant grant. You are eligible for the £2,500 grant when an air-to-air heat pump replaces your fossil-fuel or electric heating and is fitted by an MCS-certified installer - it then handles your heating and your cooling. If you keep your boiler and simply add cooling, that add-on is not eligible for the £2,500 grant, and it becomes a private-pay air conditioning job (a single-split, say) carried out by an F-Gas / REFCOM-registered engineer - not MCS, which applies only to the grant-eligible heat-pump route. (Grant rules as of August 2026 and subject to change.)

Air-to-water qualifies for a separate, heating-focused Boiler Upgrade Scheme grant - but since it cannot cool your home, it is outside a cooling decision, and we do not route it as a cooling lead.

Which should you choose?

• You want to cool an overheating home: air-to-air (grant-eligible if it replaces your heating via MCS) or a private-pay AC unit (F-Gas / REFCOM). Around 71% of inner-London homes are flats, where a room-by-room air-to-air unit usually fits far better than any wet system.
• You want to decarbonise your heating and have no cooling need: air-to-water is the better fit - a different question, and a different scheme.

Check what fits your home

Our free cooling checker takes your address, scores your overheating risk and tells you which cooling route fits - the right system, your real grant eligibility, the planning position and a net cost - on screen, with no phone call and no cold-calling.

Check your home in 60 seconds ▸