What three actions typically occur during defrost in an air-to-air heat pump?

During the defrost cycle in an air-to-air heat pump, the primary goal is to remove frost accumulation from the outdoor coil. This is necessary because frost can impair the heat pump's efficiency and capacity by blocking the airflow and insulation properties of the coil.

When the heat pump goes into defrost mode, it typically switches from heating mode to a temporary cooling mode to facilitate the melting of the frost. In this process, the outdoor fan usually shuts off to prevent the cold outdoor air from further freezing the melting ice on the coil. Instead, the heat generated from the compressor is redirected, and auxiliary heat often activates to maintain indoor comfort while the frost is being cleared away. This ensures that the system continues to provide adequate heating to the building during this defrosting period.

Understanding this cycle helps clarify why the correct response emphasizes the system switching to cooling mode, the shutting off of the outdoor fan, and the activation of auxiliary heat. Each of these actions is essential for effective defrosting while minimizing heat loss indoors.