An air source heat pump is a way to save money and the environment. An air source heat pump has three cycles, namely the heating cycle, the cooling cycle and the defrost cycle. Let us look at the process of each cycle individually for a better understanding on how an air source heat pump works.
The Heating Cycle
During this cycle, heat is taken from outdoor air and ‘pumped’ indoors.
The process begins with the liquid refrigerant which passes through the expansion device. As is passes through the expansion it changes to a low pressure liquid/vapor mixture. This mixture then goes to the outdoor coil acting as an evaporator coil. The liquid refrigerant absorbs heat from the outdoor air causing it to boils. Once boiled it become a low temperature vapor.
This vapor then passes through the reversing valve to the accumulator, collecting any remaining liquid on its journey. Once the vapor enters the compressor it is compressed, reducing its volume. This causes the vapor to heat up.
The reversing valve now sends the now hot gas to the indoor coil, or condenser. This hot gas transfers heat to the indoor air, causing the refrigerant to condense into a liquid. This liquid then returns to the expansion device and the process is then repeated.
The Cooling Cycle
In the summer months, this cycle is reversed to cool your house down. This time the unit takes heat out of the indoor air and rejects it outside.
Similar to the heating cycle, the liquid refrigerant passes through the expansion device, which again changes it to a low pressure liquid/vapor mixture. It then continues to the indoor coil. The liquid refrigerant absorbs heat from the indoor air causing it to boil. It once again becomes a low temperature vapor.
This vapor then passes through the reversing valve to the accumulator. During this process the vapor collects any remaining liquid before it enters the compressor. The vapor is then compressed, causing it to heat up.
The now hot gas is then passed through the reversing valve to the outdoor coil. The heat from the gas is then transferred to the outdoor air, causing the refrigerant to condense into a liquid. The liquid returns to the expansion device, repeating this cycle.
The Defrost Cycle
If the outdoor temperature falls to near or below freezing and the heat pump is in the heating mode, moisture in the air passing over the outside coil condenses and freezes it. The amount of frost depends on the temperature outside and the amount of moisture in the air.
However, this frost buildup needs to be removed at some point as it decreases the efficiency of the coil. To remove frost buildup the heat pump will switch into the defrost mode.
Firstly, the reversing valve switches the device to the cooling mode. Hot gas is sent to the outdoor coil to melt the frost. While this is happening, the outdoor fan distributes warm air through your house.
At the same time the heat pump is cooling the air in the ductwork. Under normal circumstances the heat pump would warm this air as it is distributed throughout the house.
Baxi wrote this article about how Air Source heat pumps work.