When a heat pump is operating in heating mode, the outdoor exchanger becomes very cold. If its surface temperature drops below the dew point and below zero, moisture in the air can freeze onto the fins.
When does ice form
The phenomenon is most pronounced when the outdoor air has low temperature and high relative humidity.
Why is ice a problem
Ice obstructs airflow and reduces heat transfer from the outdoor air to the refrigerant circuit.
Normal or a malfunction?
A thin layer of ice that is periodically removed via defrost is normal. Continuous or very heavy icing requires inspection.
What to check
Coil cleanliness, unobstructed airflow, sensors, fan, and proper operation of the defrost cycle.
