Heat pump frozen or iced over: defrost problems

In heating mode the outdoor coil runs colder than the outside air, so moisture in the air freezes onto it. A thin white coating of frost on a cold, damp day is expected. The heat pump deals with it by running a defrost cycle: it briefly switches to cooling mode, sends hot refrigerant through the outdoor coil, stops the outdoor fan and melts the frost. You may see steam and hear a whoosh when the valve shifts. Indoors, the backup heat usually comes on so the air does not turn cold.

A problem looks different. Ice is thick and clear, it bridges the fins, it covers the top or the fan, or it builds up from the base. Frost that never clears between cycles also counts. An iced coil cannot absorb heat, so the house gets colder and the backup heat runs nonstop.

1. Blocked airflow. Snow drifts, leaves, a clogged coil or plants crowding the unit.
2. Water falling on the unit. A leaking gutter or roof edge above it, or meltwater that cannot drain away from the base and refreezes.
3. Defrost sensor or control board failure. The unit never gets the signal to defrost, or it ends the cycle too early.
4. Outdoor fan motor failure. No air moves across the coil and ice forms fast.
5. Low refrigerant from a leak. The coil runs colder than it should and frosts heavily.
6. Reversing valve stuck. The unit cannot switch into defrost.

You can fix the first two yourself. The others sit behind panels or inside the sealed refrigerant system, and you cannot confirm them without test equipment.

Switch the thermostat to emergency heat. That shuts off the outdoor unit and heats the house with the backup source while the ice melts. Clear what you can reach by hand without touching the fins. Let the ice melt on its own. If the unit ices up again after it thaws and you restart it, the cause is mechanical and you need a service call.

Running a heat pump that is frozen solid risks damage to the fan blades and the compressor, so do not leave it grinding away. The page on a heat pump not heating covers what else to check if the house is losing temperature.

The tech will thaw the unit safely, then force a defrost cycle and watch whether the valve shifts, the fan stops and the coil warms. Expect tests on the defrost sensor and control board, the outdoor fan motor and capacitor, and the refrigerant pressures. A failed sensor or board is a moderate repair. A leak or a reversing valve is a bigger job. Ask what failed and why before you approve the quote.

Ice on the indoor coil or the copper lines during cooling season points to low airflow or low refrigerant, the same as on a central AC. Turn the system off, run the fan only to thaw the coil, replace the filter and call for service if it freezes again. The heat pump overview links to the repair page.