Why do heat pumps freeze up in winter?

Heat pumps frequently freeze up in winter when frost or ice accumulation on the outdoor condensing unit exceeds the unit's ability to melt it away during standard defrost cycles. A primary cause is restricted airflow across the outdoor coil caused by heavy snow buildup, fallen leaves, or debris blocking the cabinet, which prevents the coil from absorbing ambient heat from the air. Another frequent culprit is a malfunctioning defrost control board or a faulty defrost sensor that fails to initiate the automatic defrost cycle, leaving the unit trapped in heating mode while ice builds up continuously. Additionally, low refrigerant levels from a leak reduce operating pressures, causing temperatures on the coil to drop well below freezing and turning normal moisture into a solid block of ice. You can verify the cause by checking outdoor airflow and observing whether the defrost cycle engages properly every thirty to ninety minutes.