How the Heat Pump Defrost Cycle Works

Why a heat pump needs to defrost at all

In heating mode, the outdoor coil is the part that collects heat from the outside air. To pull heat from air that is already cold, the refrigerant inside the coil has to run even colder. When the coil surface drops below freezing, moisture in the air turns to frost on the fins.

A thin layer of frost is harmless. A thick layer acts like a blanket. It blocks airflow through the coil, so the system collects less heat, runs longer and pushes cooler air into the house. Left alone, frost turns into solid ice. The defrost cycle exists to clear that frost before it becomes a problem.

How the heat pump defrost cycle works, step by step

Timed defrost and demand defrost

Older and simpler systems use a timed defrost. Every set interval of run time, if the coil sensor reads cold enough, the board starts a cycle whether there is much frost or not. Newer systems use demand defrost. They compare readings from sensors and only defrost when the data says frost is building. Demand defrost wastes less energy, but it depends on sensors that can fail or drift. Neither type is better at clearing ice. Each simply decides when to start in a different way.

What a normal defrost looks and sounds like

Most cycles last a few minutes. Control boards typically cap a cycle at around ten to fifteen minutes even if the coil has not fully cleared. How often it runs depends on the weather. On a damp, foggy day near freezing, defrost may come around often. On a very cold, dry Colorado night there is less moisture in the air, so it may run less than you would expect.

Signs the defrost cycle is not working

The real warning sign is ice that stays. If defrost is doing its job, the coil should look mostly clear shortly after each cycle. Watch for these:

Common causes include a failed coil sensor or outdoor temperature sensor, a faulty defrost control board, a reversing valve that sticks, a bad relay, or a low refrigerant charge that makes the coil run colder than it should. Each one needs a technician with gauges and meters to confirm. Our page on heat pump defrost cycle problems covers these faults in more detail.

When the problem is outside the unit

Sometimes defrost works fine and the ice comes from somewhere else. A roof edge or gutter dripping onto the unit can coat it faster than any defrost can clear. Snow drifting against the sides or a unit sitting too low can trap meltwater, which then refreezes around the base and climbs into the coil. These are common around Parker after a heavy spring snow.

What you can safely do

Getting a defrost problem looked at

A heat pump stuck in a frozen state will lean on backup heat, which can push your Parker electric bill up quickly, and it can stress the compressor. When you call, a local heat pump technician can test the sensors, board, valve and charge, then tell you what failed and what the fix involves.