Short answer: A modern split air conditioner is a reversible air-to-air heat pump: it can move heat out of a room to cool it, or into a room to heat it. The difference from an 'air-to-air heat pump' is mostly how it is sized and used. An air-to-water heat pump is different again, heating water for radiators and taps rather than blowing warm air.
Air-to-air
Moves heat between outdoor air and indoor air. Heats and cools. Does not heat hot water. Fast to fit, no radiators needed.
Air-to-water
Moves heat from outdoor air into a wet heating system and hot water cylinder. Usually replaces a boiler. Typically no cooling.
Which suits you
Homes with radiators and a gas or oil boiler often look at air-to-water. Homes with electric heating, flats, and anyone who wants summer cooling often find air-to-air simpler and cheaper to install.
Related services
More guides
- How much does air conditioning cost to install in the UK?
- How much does air conditioning cost to run?
- What size air conditioner do I need?
- Fixed split or portable air conditioning?
- Do you need planning permission for air conditioning?
- The £2,500 grant for air-to-air heat pumps explained
- F-gas rules: what homeowners need to know
- R32, R410A and R22: refrigerants explained
- What happens on air conditioning installation day
- Air conditioning noise and your neighbours
- Looking after your air conditioning: an owner's checklist
- Can you fit air conditioning in a listed building?
- Air conditioning in a flat: leases, freeholders and planning
- Keeping a home cool in a heatwave
- Multi-split or separate single systems?
Reviewed October 2026. Rules and prices change; check the date and ask us if anything is unclear.