Defrost mode is normal
In cold, damp weather, frost builds up on the outdoor coil. The heat pump periodically reverses for a few minutes to melt it. You may see steam, hear a whoosh as the reversing valve shifts and notice the outdoor fan stop. Backup heat usually runs during defrost so you don't feel cold air.
When ice is a problem
- The whole unit is encased in ice for hours.
- Ice builds on the top or the fan.
- The unit never seems to go into defrost.
Causes include a failed defrost control or sensor, low refrigerant, a gutter dripping onto the unit or the unit sitting too low in snow. Switch to emergency heat and call a technician.
Aux heat vs. emergency heat
Auxiliary heat turns on automatically when the heat pump needs help. Emergency heat is a manual setting that turns the heat pump off and uses only the backup heat, which is useful if the outdoor unit fails but expensive to run.
Winter tips
- Avoid big setbacks: raising the temperature several degrees at once can trigger expensive backup heat.
- Keep the unit clear of snow, ice and drifting leaves.
- Change filters regularly; heat pumps are sensitive to airflow.
