Heat Pump Frozen Outside

The defrost board initiates regular defrost cycles based on time and temperature. When it fails, the unit doesn't defrost on schedule and ice accumulates.

Set thermostat to EMERGENCY HEAT and wait 2-4 hours for natural thaw. Then resume normal HEAT and observe — does a defrost cycle ever initiate? (Outdoor fan stops, you may hear refrigerant flow changes.) No defrost cycle in 90+ minutes = board failure.

The sensor attached to the outdoor coil tells the defrost board when conditions need defrost. Bad sensor = board never gets the signal.

Locate the sensor (usually a small probe clipped to the outdoor coil). Visual: damaged wiring, loose connection, sensor displaced from coil. Multimeter test for resistance per manufacturer specs (varies with temperature).

Insufficient refrigerant lowers coil temperature below normal operating range. Coil gets cold enough to freeze ambient moisture faster than defrost can keep up.

Symptoms: weaker heat output even when defrosted, longer run times, hissing near connections. Confirmed by tech with gauges.

Reversing valve can't switch into defrost (cooling) mode briefly, so ice never melts. May get stuck mechanically or have solenoid failure.

Set to COOL. Feel the small copper line at outdoor unit — should be cold. If never cold, valve is stuck. May be tappable free with a rubber mallet.

If the outdoor fan isn't spinning at all or is spinning slowly, the coil can't shed heat properly and accumulates ice. Fan also helps shed melted water during defrost.

Observe fan during operation. Should spin at full speed steadily. Not running, intermittent, or slow = motor problem (or capacitor).

Snow drifts, falling icicles, or ground-level ice has buried the unit. Airflow blocked = can't shed frost = ice accumulates more.

Visual. Unit should have 2 feet of clearance on all sides and 5 feet above.