Why is my heat pump freezing up in winter?

Heat pumps naturally accumulate frost in cold weather. The defrost cycle reverses the system to melt this frost periodically. A cycle lasts 5-15 minutes.

If ice isn't melting between defrost cycles, the defrost control board, sensor, or reversing valve may be faulty. This requires professional diagnosis.

Poor airflow from a dirty filter or blocked outdoor coils can also cause ice buildup. Check and clean both indoor filter and outdoor unit.