An air source heat pump uses the outside air to heat or cool a building. When used to heat a building this is achieved by transferring heat inside from the outside air, and when used to cool a building this is achieved by transferring heat from inside to the outside air. To achieve heat transfer in either direction, air source heat pumps use a system that includes a heat exchanger, a compressor and a means to transfer heat from one area to the other, e.g., pipes filled with a refrigerant.
The heating process starts with a cold refrigerant that is moved outside where it becomes heated by a combination of outside air being blown by a fan onto refrigerant coils, and a compressor that further increases the temperature through compressing the refrigerant. The heated refrigerant is then moved indoors where it passes through another set of refrigerant coils (heating coils) where another fan extracts the heat from the coils by blowing air on it. The heated air can then be distributed about the building through air ducts. Lastly the refrigerant is passed through an expansion valve that cools it down to begin the cycle all over again.
The cooling process is virtually the reverse of the heating process, whereby a reversing valve near the compressor changes the direction of the refrigerant flow.