What is the most common method of defrosting the outdoor coil on an air source heat pump?

The most common method of defrosting the outdoor coil on an air source heat pump is reverse cycle defrost. This method utilizes the heat pump's existing refrigeration system to reverse the flow of refrigerant, allowing warm refrigerant gas to circulate through the outdoor coil. This temporarily heats the coil, melting any ice or frost that has accumulated.

Reverse cycle defrost is favored because it efficiently uses the heat already generated by the heat pump, minimizing energy consumption compared to other methods. Furthermore, since the heat pump is already in operation, this method integrates seamlessly into its normal functioning without the need for additional energy sources or equipment, such as electric resistance or hot gas.

Other defrost methods exist, but they are typically less common or used in specific cases. For example, electric resistance defrost can be energy-intensive and is generally not preferred for regular use. Hot gas defrost is also effective but is used less often in standard residential systems due to complexity and cost. Water defrost is not typically used because it requires additional plumbing and water access, which is not practical in many outdoor installations.