HVAC Systems · Operation
Defrost Cycle
A periodic mode on air-source heat pumps in heating mode where the unit briefly reverses to cooling, sending hot refrigerant to the outdoor coil to melt accumulated frost. Triggered by time, temperature, or pressure sensors. The indoor blower may stop or blow cool air during a 5–15 minute defrost — this is designed behavior, not a fault. Frequent defrosts (>1 per hour) indicate a real problem.
Common symptoms
- Outdoor unit fan stops with steam rising
- Brief blast of cool air from supply registers in heat mode
- Reversing valve clicks audibly
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