Heat Pump Defrost Cycle Explained — What Vancouver Homeowners Need to Know
Published: July 20, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC
What Is the Defrost Cycle?
The defrost cycle is a feature of every air-source heat pump that prevents ice from accumulating on the outdoor coil during winter operation. As the heat pump extracts heat from cold outdoor air, moisture in the air condenses and freezes on the coil surface. This frost buildup acts as insulation, reducing the heat pump's ability to absorb heat. The defrost cycle periodically reverses the refrigerant flow to send hot gas through the outdoor coil, melting the accumulated frost and restoring efficient operation. During defrost, the outdoor fan stops, the indoor fan may slow or stop, and the auxiliary heat may activate to maintain indoor comfort while the heat pump is temporarily in defrost mode rather than heating mode.
What You Will See and Hear During Defrost
During a normal defrost cycle, you may notice: steam or vapour rising from the outdoor unit as the frost melts — this is normal and can be dramatic on cold days; water dripping from the outdoor unit, which may form small icicles on the frame but should not cover the coil; the outdoor fan stops spinning while the defrost runs; the indoor temperature may drop slightly (1-2 degrees), especially if the auxiliary heat is minimal; and the system may make a whooshing sound as the refrigerant flow reverses and pressure equalizes. The defrost cycle typically lasts 5-15 minutes and occurs every 60-90 minutes during normal winter operation, though frequency depends on outdoor humidity and temperature conditions.
Common Defrost Problems
Several issues can affect defrost performance: defrost sensor failure — if the sensor that detects frost on the coil fails, the defrost cycle may not activate, allowing ice to accumulate and block airflow; defrost thermostat failure — if the thermostat that terminates the defrost cycle fails, the defrost may run too long or not at all; low refrigerant charge — a system low on refrigerant cannot generate enough heat for effective defrost; outdoor fan motor failure — without the fan running during normal operation, the coil cannot absorb heat and frost accumulates faster than the defrost can manage; and control board failure — the control board that manages the defrost timing and initiation may malfunction. If ice remains on the outdoor unit for more than 2 hours after outdoor temperatures rise above freezing, call for professional service.
Normal Frost vs Problematic Ice
Distinguishing normal frost from problematic ice buildup is important. Normal frost appears as a thin, even white coating across the coil surface that melts completely during each defrost cycle. Problematic ice forms as thick, uneven ice that may appear blue or clear, concentrates at the bottom of the coil, or does not melt after the defrost cycle ends. Ice forming on the refrigerant lines connecting the outdoor unit indicates low refrigerant. Ice bridging between coil fins or forming a solid block indicates a defrost system malfunction. If you see water pooling under the outdoor unit in freezing weather, the defrost condensation may be freezing rapidly rather than draining, indicating a potential issue with the defrost termination or drainage path.