The short answer
An air-to-air heat pump collects heat from outdoor air and delivers it to your rooms as warm air, usually through wall-mounted units. It is the same equipment as air conditioning that can heat, so it also cools in summer, but it can't heat water.
On this page
The name describes the journey the heat takes: from the air outside to the air inside your home. It often causes confusion, because in the UK "air source heat pump" tends to bring to mind a system that runs radiators and a hot water cylinder, which is a different product. Knowing which is which makes quotes, planning rules and product descriptions easier to follow.
What the name means
Heat pumps are named after where the heat comes from and where it goes:
- Air-to-air: from outdoor air to indoor air, through indoor units that blow warm air into rooms.
- Air-to-water: from outdoor air to water, which flows through radiators or underfloor heating and usually heats a hot water cylinder too.
- Ground source: from pipes buried in the ground, usually to water used in the same way.
"Air source heat pump" covers both of the first two. An air conditioner that can heat is an air-to-air heat pump, and there is no difference in the equipment. A cooling-only air conditioner uses the same refrigeration cycle but can't reverse it.
How it compares with other heat pumps
| Air-to-air | Air-to-water | Ground source | |
|---|---|---|---|
| Heat comes from | Outdoor air | Outdoor air | The ground |
| Heat goes into | Room air | Radiators or underfloor | Radiators or underfloor |
| Hot water | No | Yes, usually via a cylinder | Yes, usually via a cylinder |
| Cooling | Yes | Not usually | Not usually |
All three rely on the same basic cycle of a refrigerant boiling, being compressed and condensing. What differs is where the heat is collected and how it reaches you.
Strengths and limits
What an air-to-air system does well:
- Heats and cools from the same indoor units.
- Responds quickly, since warm air comes straight off the indoor coil into the room.
- Lets each room with its own indoor unit be set and switched separately.
- Needs no radiators, water pipework or hot water cylinder.
Where it falls short:
- It only heats rooms that have an indoor unit, or that are served by ducts from one.
- It can't provide hot water, so a boiler, immersion heater or another system still has to.
- Heat arrives as moving air, usually from high on the wall, which feels different from a radiator.
- Its output and efficiency fall as the weather gets colder, and in damp, near-freezing conditions it pauses to defrost. There is more on this in how air conditioning heats a room.
On its own or alongside other heating
An air-to-air system can be the main heating, with an indoor unit in each room that needs one. It can also work alongside an existing boiler, covering some rooms while the boiler looks after the hot water and the rest of the house.
Several rooms can share one outdoor unit through a multi-split system, which helps where there's only room outside for one unit. Planning rules also limit how many heat pumps a house can have, as described below.
Whether heating this way costs more or less to run than gas depends on your tariffs and on how efficiently the system runs in your home. The method for working it out is in heating with air conditioning compared with gas.
Planning and regulations
In England, air source heat pumps, including air-to-air systems, can usually be fitted without planning permission under permitted development, as long as the conditions are met. They include:
- An air-to-air system must not be used solely for cooling.
- A house that isn't detached, such as a semi or a terrace, can have one heat pump; a detached house can have two.
- The outdoor unit, including any housing, must be no more than 1.5 cubic metres.
- The installation must meet the MCS 020(a) noise standard for neighbouring homes.
Other conditions cover roofs, walls facing a road and conservation areas. Listed buildings need listed building consent. Flats, and homes whose permitted development rights have been removed, follow different rules. Scotland, Wales and Northern Ireland have their own rules. See do you need planning permission? for the details.
A split air-to-air system is connected into a sealed refrigerant circuit on site, so it isn't DIY work whatever refrigerant it uses, and anyone installing one that uses an F-gas refrigerant such as R32 needs the right F-gas qualification. Most systems also need a dedicated electrical circuit, and in England and Wales a new circuit is notifiable under Part P of the Building Regulations.
Related guides
- How air conditioning heats a roomTo heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.
- Heating with air conditioning compared with gasWhich costs less depends on your prices and equipment. Work out the cost of a kWh of heat from each: gas unit rate divided by boiler efficiency, and electricity unit rate divided by the air conditioner's COP.
- Do you need planning permission?Usually not in England, where air-to-air heat pumps that aren't used only for cooling count as permitted development if conditions on number, size, position and noise are met. Listed buildings, flats and protected areas follow different rules.
- Multi-split systemsA multi-split system runs two or more indoor units, usually in different rooms, from one outdoor unit. Each room has its own controls, and your home needs only one outdoor unit rather than one for each room.