What Is the Defrost Cycle?

The defrost cycle is a heat pump's routine for clearing frost from the outdoor coil in heating mode. The system briefly reverses to send warm refrigerant through the outdoor coil, melting accumulated frost, while backup heat usually covers the house for those few minutes.

Also called: defrost mode

Why it matters to you

Damp coastal winter air frosts outdoor coils readily even at temperatures well above freezing, so defrost cycles are a normal part of winter operation here. The steam that rises off the unit during defrost is melting frost, not a fault.

What to do about it

A cycle lasting a few minutes and ending in a return to normal heating is working correctly. Coil ice that never clears, or defrost cycles running constantly, points to a sensor, charge or drainage problem worth having checked.