The technology inside the air source heat pump is similar to any domestic refrigerator, which uses a vapour compression cycle. The main components in the air source heat pump are the compressor, the expansion valve and two heat exchangers (an evaporator and a condenser).

1. Refrigerant in the evaporator is colder than the heat source. This causes the heat to move from the heat source (in this case the outside air) to the refrigerant, which then evaporates.
2. This vapour moves to the compressor and reaches a higher temperature and pressure.
3. The hot vapour now enters the condenser and gives off heat as it condenses.
4. The refrigerant then moves to the expansion valve; drops in temperature and pressure; and then returns to the evaporator.