Short answer: Air to water heat pumps heat water for radiators and taps, and suit London houses with wet central heating and room for a cylinder. Air to air heat pumps warm and cool the room air directly through indoor units, suit flats and homes heated by electric heaters, but usually do not heat water. Grants are £7,500 and £2,500 respectively.
Same idea, different delivery
Both types take heat from the outside air, even on cold days, and move it into your home using a refrigerant circuit and a compressor. The difference is what they heat.
An air to water heat pump heats water. That water goes round radiators or underfloor heating and into a hot water cylinder, replacing a gas boiler’s job.
An air to air heat pump heats air. It is a reversible split system: one outdoor unit, one or more indoor units on walls or ceilings, with refrigerant pipes between them. It warms each room directly and, because it is reversible, it can cool too.
| Air to air | Air to water | |
|---|---|---|
| Delivers heat through | wall, ceiling or ducted indoor units | radiators or underfloor heating |
| Hot water | usually not | yes, via a cylinder |
| Cooling | yes | rarely in UK homes |
| Needs radiators and pipework | no | yes, often larger radiators |
| Needs a cylinder | no | yes |
| Outdoor unit | smaller, can be wall mounted | often larger |
| BUS grant | £2,500 | £7,500 |
| Permitted development limit on flats | 0.6 cubic metres outdoor unit | same |
Which London homes suit air to water
Air to water makes sense where there is already a wet central heating system, space for a hot water cylinder and a good spot for a larger outdoor unit. That describes many houses in outer London: 1930s semis in Barnet and Ealing, Edwardian houses in Richmond, detached homes with gardens. It replaces the gas boiler for both heating and hot water. It does not cool, and radiators may need upgrading to work at lower water temperatures.
Which London homes suit air to air
Air to air fits a lot of London that air to water struggles with:
- flats heated by electric panel or storage heaters, with no wet system and no room for a cylinder upgrade
- small flats and maisonettes where a large outdoor unit will not fit
- top floor flats and lofts that need cooling in summer as much as heat in winter
- homes where adding radiators and pipework would mean ripping up period floors
The Energy Saving Trust says air to air heat pumps can be a good choice for smaller homes and flats, and that outdoor units are typically smaller than air to water ones. More than half of London households live in a flat according to the 2021 Census, so this matters here more than anywhere in the UK.
The hot water question
The biggest practical difference is hot water. Most air to air systems do not provide it. If you are replacing a combi boiler with air to air, you need another plan, such as:
- a hot water cylinder with an electric immersion heater
- an electric shower plus small point of use heaters at sinks
- one of the few air to air models that can also heat a cylinder
If you plan to keep a gas boiler running alongside, the BUS grant is unlikely to apply, because Ofgem requires the new system to replace the existing fossil fuel or electric heating, be sized for the home’s full space heating needs, and it does not fund hybrid systems. Check the exact air to air rules with your installer before you commit.
Grants and timing
The government’s Boiler Upgrade Scheme page lists £7,500 towards an air source (air to water) heat pump and £2,500 towards an air to air heat pump. Ofgem’s rules say the installer must be MCS certified, air to air grants are for homes rather than non residential buildings, and new builds and social housing are excluded. As of 20 October 2026, the Energy Saving Trust says there are no MCS certified installers for air to air yet, with certification expected before the end of 2026. Our guide to the air to air heat pump grant covers this in detail.
Planning in a borough like Camden
Both types are air source heat pumps for planning purposes. On a house in England, one can be permitted development if it meets the size, siting and 37 dB(A) noise rules, and is not used solely for cooling. A block of flats gets one in total, and a single flat has no right of its own. Camden has 40 conservation areas covering about half the borough, according to Camden Council, and many of its homes are flats in converted houses or mansion blocks, so applications are common for both types. See our Camden area page.
A simple way to decide
Ask yourself three questions:
- Do you have radiators and space for a cylinder? If yes, air to water is worth pricing.
- Do you want cooling as well as heating? If yes, air to air is the only one that does both well.
- Is your home a flat with electric heating? If yes, air to air is usually the practical choice.
Our air to air heat pump page explains how we size a system for winter, not just summer.
Quick questions
Is an air to air heat pump the same as air conditioning?
It uses the same technology as a reversible split air conditioner. It is designed and sized to heat the home through winter, and it can cool in summer.
Can an air to air heat pump heat my hot water?
Most cannot. You need a separate hot water source such as a cylinder with an immersion heater, an electric shower or point of use heaters.
Which heat pump gets the bigger grant?
The Boiler Upgrade Scheme lists £7,500 for air to water and £2,500 for air to air in eligible homes in England and Wales, both through an MCS certified installer.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.