Why is your heat pump steaming or making a whooshing noise in cold weather? It is almost always defrost mode, and it is normal. When a heat pump heats your home in winter, frost builds up on the outdoor coil. Every so often the system reverses for a few minutes to warm the coil and melt that frost. You may see steam rise from the unit, hear a whoosh as the valve switches, notice the outdoor fan stop, and feel cooler air from the vents for a short time. Then heating picks back up.
The Jersey Shore is a good place to see it. Damp winter air near the ocean and the bays builds frost on outdoor coils faster than dry cold does, so heat pump owners in Wall Township, Brick, Point Pleasant and the beach towns often notice defrost cycles on raw, near-freezing days more than on the coldest clear ones.
What happens during a defrost cycle
In heating mode, the outdoor coil is colder than the outside air, because it is pulling heat out of that air. Moisture in the air freezes on the coil. As frost thickens, it blocks airflow and the heat pump loses efficiency.
When sensors detect frost, the system switches into defrost:
- A reversing valve switches the system so warm refrigerant flows to the outdoor coil
- The outdoor fan stops so the heat stays on the coil
- Frost melts and some of it evaporates, which is the steam you see
- Meltwater drips off the bottom of the unit
- After a few minutes, the valve switches back and normal heating resumes
On a ducted system, backup heat often comes on during defrost so the air from your vents does not feel cold. On a ductless system, the indoor unit may stop blowing until defrost is over. Both are normal.
The U.S. Department of Energy’s page on air-source heat pumps has more on how these systems work in cold weather.
Normal vs. a problem
Defrost is normal. These are not:
- A unit encased in ice that does not clear after defrost cycles
- Defrost running very often, every few minutes, all day
- No defrost at all while frost keeps building on the coil
- Loud grinding or banging rather than the usual whoosh
- Little or no heat inside even when the unit is running
Those point to issues such as a failed defrost sensor or control board, a reversing valve problem, an outdoor fan motor that is not running or a low refrigerant charge. They need a technician.
What you can do
- Keep snow, leaves and debris cleared from around and on top of the outdoor unit
- Make sure the unit sits up off the ground so meltwater can drain away instead of refreezing around the base
- Check that a gutter or roof edge is not dripping directly onto the unit, which can build ice fast
- Do not chip ice off the coil or pour hot water on it, since both can damage the fins and coil
- Leave the thermostat alone during a defrost cycle and give it a few minutes to finish
How cold is too cold for a heat pump?
Modern cold-climate heat pumps keep heating well below freezing, and Jersey Shore winters are milder than most of the state’s. The colder it gets, though, the harder the system works and the more often it may defrost. That is why many ducted heat pumps are paired with backup heat, either electric heat strips or an existing furnace or boiler, which carries the load on the coldest nights. If your backup heat is running constantly on ordinary winter days, that is worth a service call, because it can point to a heat pump that is not doing its share.
Thinking about a heat pump?
If you are considering a heat pump for a shore home and wondering how it handles winter, our post on whether a heat pump is right for your home covers how they compare with furnaces and boilers. A yearly heating maintenance visit checks the defrost controls, refrigerant charge and outdoor fan before the cold sets in.
If your heat pump is iced up or not heating, Rostron Premium Home Services can help. Learn about our heating installation and emergency heating service, or call 732-374-9949.
