Final answer:
When a heat pump operates in defrost mode, the outdoor fan is typically turned off to concentrate heat and defrost ice from the coils. This process temporarily reverses the typical heating mode operation and ensures efficiency and longevity of the heat pump.
Step-by-step explanation:
When a heat pump operates in defrost mode, the outdoor fan is normally off. During this mode, the purpose is to allow the heat pump to defrost any ice that may have built up on its coils, which is achieved by temporarily reversing the flow of refrigerant. The absence of the fan operation helps to concentrate heat in the outdoor coil thus facilitating the melting of ice. Without airflow from the outdoor fan, the hot gas in the coil can more effectively deliver heat to melt the frost or ice that has accrued.
In the heating mode of a heat pump, where an electrically driven compressor increases the temperature and pressure of the working fluid, which then transfers heat into the room as it condenses in the condenser coils. The working fluid cools down as it flows back through the expansion valve to the outdoor evaporator coils. During defrost mode, the process is temporarily altered to warm the outdoor coil and fend off ice formation.