Moving heat in both directions
Refrigerant absorbs heat when it evaporates and releases it when it is compressed. A valve in the outdoor unit reverses which coil does which job, so the same equipment can cool the house or heat it.
Even cold outdoor air holds some heat. How much a heat pump can gather as temperatures fall depends on the model, and many systems have backup electric heat or a furnace for the coldest days.
What is normal in winter
- Air from the vents feels warm rather than hot, and the system runs for longer stretches than a furnace
- A light frost on the outdoor coil on cold, damp days
- Periodic defrost cycles, when the outdoor fan stops, the unit may hiss or steam, and water drains from the coil
What is not normal
- The outdoor unit encased in thick ice that does not clear after a defrost cycle
- Cold air from the vents for long periods
- Backup heat running constantly in mild weather
When professional help makes sense
Heavy ice that stays, or a system that cannot keep the house warm, usually points at a defrost control, a fan, a reversing valve or a refrigerant problem. Leave the ice to melt on its own; chipping at it can damage the coil.