What is the primary purpose of the defrost relay in a heat pump?

The primary purpose of the defrost relay in a heat pump is to control auxiliary heat during the defrost cycle. In heat pumps, especially those operating in colder climates, frost can accumulate on the outdoor coil, which can impede the unit's efficiency. The defrost relay is engaged to initiate a defrost cycle when the coil temperature drops to a certain level. During this cycle, the heat pump temporarily switches into a heating mode to melt the accumulated frost and ice.

The auxiliary heat is often activated during this defrost cycle to maintain a comfortable indoor temperature, as the heat pump’s efficiency may be reduced while it is working to clear the outdoor unit of frost. This ensures that occupants do not experience significant drops in temperature while the heat pump is in the process of melting the frost.

Understanding the function of the defrost relay is crucial for recognizing how heat pumps maintain performance during extreme conditions. The other options involve different operational aspects of heat pumps: controlling airflow, switching modes, and managing compressor cycles, but they do not specifically address the function of the defrost relay.