Homeowner Guides

Why heat pumps frost in winter

In heating mode, the outdoor coil is cold. Moisture in outside air can condense and freeze on it, especially near freezing temperatures with high humidity. The system reverses operation for a short defrost cycle so the coil can warm and shed frost. Auxiliary heat may help maintain indoor comfort during that cycle, depending on the system.

When buildup points to a problem

Persistent icing can have several causes that look similar from outside:

• a defrost sensor, control, or board is not initiating or completing defrost;
• the outdoor coil is obstructed by debris or snow;
• condensate or meltwater cannot drain and refreezes;
• the outdoor fan is not moving air correctly;
• refrigerant-system performance is abnormal;
• the equipment is being asked to operate outside its intended conditions.

Those categories require different tests. Ice alone does not prove a refrigerant leak, and adding refrigerant without diagnosing the system is not a responsible repair.