What does the outdoor fan do during defrost on a heat pump and why?

During the defrost cycle of a heat pump, the outdoor fan stops to minimize defrost time. This is crucial because the primary goal during defrost is to effectively melt any accumulated frost or ice on the outdoor coil.

When the fan stops, it allows for the heat generated by the reversing valve and the indoor coil to build up around the outdoor coil, promoting faster and more efficient melting of the frost. By minimizing airflow over the coil, the warmer air can concentrate on the frost, thus reducing the time required to clear the ice and allowing the heat pump to return to heating mode more quickly.

While continuous fan operation might seem beneficial for maximizing airflow or improving efficiency, it would actually prolong the defrosting process by cooling the coil with ambient air and diluting the warm air necessary for effective defrosting. Therefore, stopping the outdoor fan during this period is an intentional and effective design choice in heat pump operation.