Backup heat and the defrost cycle
Two operating behaviours surprise people in their first cold-climate winter with a heat pump: the system occasionally calls on a secondary heat source, and the outdoor unit periodically pauses to melt frost off its coil. Both are normal, and understanding them removes most of the worry.
Backup heat covers the gap below the balance point
As covered in the performance article, the balance point is the outdoor temperature where the heat pump's output equals the home's heat loss. Below it, a supplementary source makes up the difference. There are a few common arrangements.
Electric resistance backup
Many ducted systems include electric resistance heating elements in the air handler. They are simple and reliable, but their efficiency is fixed at roughly one unit of heat per unit of electricity, far below the heat pump's. The aim is to use them only for the coldest hours, not as a primary source.
Dual-fuel pairing
In a dual-fuel setup, the heat pump handles mild and moderate cold, and an existing fuel furnace takes over below a set changeover temperature. This pairing is common in homes that already had a gas or propane furnace, and it lets each system work where it is most effective.
Practical note: The changeover temperature is a setting, not a fixed property of the equipment. Setting it too high makes the home lean on expensive backup heat sooner than necessary; setting it too low can leave the home cool on extreme nights. It is worth revisiting after the first winter.
The defrost cycle is normal, not a fault
When an outdoor unit runs in cold, humid conditions, moisture from the air freezes onto the outdoor coil. A frosted coil cannot absorb heat efficiently, so the unit periodically reverses its cycle for a few minutes to warm the coil and melt the frost. During this defrost cycle you may notice steam rising from the outdoor unit and a brief pause in heating, sometimes accompanied by backup heat to keep indoor temperatures steady.
Signs that defrost is working as designed include short, occasional cycles followed by a return to normal heating. Persistent heavy icing that never clears, by contrast, is worth having a technician inspect.
| Situation | What the system does |
|---|---|
| Above the balance point | Heat pump carries the full load alone |
| Below the balance point | Backup heat supplements the heat pump |
| Frost building on the coil | Brief defrost cycle melts the frost |
| During defrost | Backup heat may hold indoor temperature |
Keeping it running well
- Keep the outdoor unit clear of snow drifts and ensure meltwater can drain away.
- Maintain clearance around the unit so air flows freely across the coil.
- Review the changeover or backup-heat settings with the installer after the first cold season.
Public references
Continue reading
What happens to capacity below freezing →
Sizing a cold-climate heat pump →