Below roughly 40 degrees F outdoors a heat pump's outdoor coil runs below freezing and gathers frost. The control periodically switches to cooling mode with the outdoor fan off, sending hot gas through the coil, and brings on strip heat so the house doesn't get a blast of cold air. Walk-in coolers and freezers defrost with off-cycle time, electric heaters or hot gas. Steam rolling off an outdoor unit in winter is usually just defrost.
Outdoor unit's a block of ice, it's not going into defrost, check the sensor on the coil and the defrost board.
Also heard as
- defrost cycle
- defrost mode
Related terms
Heat pump
A refrigeration system that can reverse its flow to move heat indoors in winter as well as outdoors in summer.
Reversing valve
The four-way valve in a heat pump that swaps which coil gets hot discharge gas, switching between heating and cooling.
Iced up
Describes an evaporator coil or suction line coated in ice because it is running below freezing.