Air-to-air vs air-to-water heat pumps: the honest comparison

Here’s the thing most people don’t realise: that “air conditioning unit” is a heat pump. The real question is which kind of heat pump suits your home.

The two families, plainly

Air-to-air (what most people call air conditioning): moves heat between outside air and the air in your rooms, via wall-mounted indoor units. It cools in summer and heats in winter, responds in minutes, and delivers roughly 3.5–5 units of heat per unit of electricity (SCOP 3.5–5). It does not heat water — your boiler or an immersion still does that. Typical cost: £1,500–3,000 for one room; £3,500–7,000 for a 3–4 room multi-split.

Air-to-water (the "heat pump" of government campaigns): moves heat from outside air into your central-heating water — radiators, underfloor, and your hot-water cylinder. It replaces your boiler entirely, but it doesn’t cool*, works best with larger radiators or underfloor heating, and is a bigger, slower project: typically £8,000–15,000 before support. (*Some can run in a limited cooling mode with the right emitters, but it’s not the same as room air conditioning.)

The grant asymmetry — worth knowing before you decide

The Boiler Upgrade Scheme currently offers £7,500 toward an air-to-water heat pump. Air-to-air systems are excluded — despite being heat pumps by any engineering definition. That’s a genuine policy quirk: if your goal is replacing a gas boiler wholesale, the grant tilts the maths toward air-to-water. If your goal is comfort — cool bedrooms in July and efficient top-up heating — air-to-air does things no air-to-water system can, grant or no grant.

Which one is "right"?

• You want cooling (this is the one air-to-water can’t really answer): air-to-air. It’s the only option that gives you genuinely cold rooms in a heatwave.
• You want to ditch the gas boiler completely, radiators and hot water included: air-to-water, ideally with the £7,500 grant.
• Plenty of homes sensibly do both — or start with air-to-air in the rooms that matter (bedrooms, home office) and make the boiler decision later. Starting with air-to-air doesn’t block anything.

The bit nobody mentions: they both love solar

Either way, you’ve converted heating (and cooling) into an electrical load — which is exactly what rooftop solar plus a battery is good at feeding. If a heat pump of either kind is in your future, it’s worth pricing solar at the same time; the running-cost picture changes substantially. More on that here.