Why would my heat pump freeze up in the winter?

A heat pump can freeze up in the winter due to several mechanical or environmental factors that prevent proper defrosting or heat absorption. The most common cause is restricted airflow across the outdoor coil caused by heavy snow buildup, fallen leaves, or debris blocking the unit, 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 accumulates 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.