Why Your Heat Pump Is Freezing Up — and How to Fix It

If you have looked out at your heat pump on a cold Maine morning and seen frost or even a layer of ice on the outdoor unit, you are not alone — and a small amount of frost is completely normal. The problem is when that frost turns into a solid block of ice that does not melt away.

Heat pumps run a defrost cycle automatically. Several times a day in cold weather, the system briefly reverses to melt frost off the outdoor coil, then returns to heating. You might notice steam rising off the unit during this cycle — that is the system working exactly as designed.

Trouble starts when the defrost cycle fails or the unit cannot keep up. The most common causes are a faulty defrost sensor or control board, low refrigerant from a leak, a failing reversing valve, or a fan motor that has stopped turning. When any of these go wrong, ice builds faster than the system can clear it.

There are also simple physical causes. Snow drifting against the unit, ice from a roof drip line falling onto the top of it, or leaves and debris blocking airflow can all trap moisture and cause ice to accumulate. Keeping the area around the unit clear and making sure it is raised off the ground helps a lot.

If your heat pump is fully iced over, switch it to emergency or backup heat if you have it, and avoid running the heat pump until it is inspected — running an iced unit can damage the compressor, which is the most expensive part to replace. Never pour hot water on it or chip the ice off; you can puncture the coil.

A heat pump that keeps freezing up almost always needs a professional diagnosis. Aces Highlands services and repairs all major heat pump brands across Southern Maine. We will find the real cause — sensor, refrigerant, or airflow — and get your system defrosting and heating properly again.