Guide · Updated 8 November 2026
How does an air to air heat pump work?
An air to air heat pump moves heat rather than making it. A refrigerant in the outdoor unit absorbs heat from the outside air, even when it's cold, a compressor squeezes the refrigerant so its temperature rises, and the indoor unit blows air across the hot coil to warm the room. In summer the cycle runs in reverse to take heat out of the room.
The cycle, step by step
- Absorb. The refrigerant in the outdoor unit's coil is colder than the outside air, so it soaks up heat and turns to gas. This works well below freezing, because "cold" air still contains heat.
- Compress. An electric compressor squeezes the gas, which raises its temperature well above room temperature.
- Release. The hot refrigerant flows through the pipe to the indoor unit. A fan blows room air across the coil, warming it, and the refrigerant cools back into a liquid.
- Expand. An expansion valve drops the pressure, making the refrigerant very cold again, and it returns outside to start over.
The electricity runs the compressor and fans; the heat comes from outside. That's why one kWh of electricity can deliver three to four kWh or more of heat.
Cooling
A reversing valve sends the refrigerant the other way. Now the indoor coil is the cold one, absorbing heat from the room, and the outdoor unit releases it outside. It also takes moisture out of the air, which is why there's a condensate drain.
Defrosting
In cold, damp weather, frost can build up on the outdoor coil. The unit runs a short defrost cycle every so often, pausing heating for a few minutes. You may see steam from the outdoor unit. This is normal.
What you see
- Indoor unit: usually a slim wall-mounted unit high on the wall. Floor-standing, ceiling cassette and hidden ducted units are also available.
- Outdoor unit: a box with a fan, on a bracket or pad outside. One outdoor unit can serve several indoor units (multi-split).
- Controls: a remote, wall controller or app. Each room's unit can be set separately.
Refrigerants
Most current UK units use R32. Some newer models use R290 (propane), which has a much lower global warming impact. Either way, the refrigerant stays sealed inside the pipes, and only an F-gas certified engineer can work on it.
Questions
Do air to air heat pumps work in cold weather?
Yes. They extract heat from outside air well below freezing, though efficiency drops as it gets colder. Check the unit's rated heating capacity at low temperatures when choosing one.
Why is a heat pump more efficient than an electric heater?
An electric heater turns each kWh of electricity into one kWh of heat. A heat pump uses electricity to move heat from outside, so each kWh can deliver three to four kWh or more of heat.
Sources
- DESNZ: Air-to-air heat pumps literature review
- GOV.UK: F gas in refrigeration, air conditioning and fire protection systems
Checked 8 November 2026.