Heat pump defrost sequence: what each part does before, during and after defrost

A timeline of a typical heat pump defrost. In heating, the reversing valve is in heat, the outdoor fan runs and backup heat is off. When the board starts defrost, the reversing valve shifts to cooling, the outdoor fan stops and backup heat comes on to temper the air. Defrost ends when the coil sensor warms up or a maximum time passes, then everything returns to heating.

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