When Frosting Is Normal
It is completely normal for a thin layer of frost or ice to form on the outdoor unit of a heat pump during cold, humid weather. Heat pumps extract heat from the air, which causes moisture to condense and freeze on the evaporator coil. Modern heat pumps handle this automatically using a defrost cycle, which reverses the refrigerant flow briefly to melt the ice. During defrosting, you may notice steam coming from the unit and a slight reduction in heating output — this is completely normal.
When Frosting Is a Problem
A fault is indicated when:
- Ice has built up heavily over the entire outdoor unit, covering the fan and blocking airflow
- The unit has been frozen for more than 2–3 hours without defrosting
- Ice extends to the refrigerant pipework or base of the unit
- The heat pump is running but producing no heating despite the outdoor temperature being above -10°C
Common Causes of Excessive Icing
- Defrost control or sensor failure
- Low refrigerant charge (leak)
- Blocked evaporator coil (needs professional cleaning)
- Fan motor failure reducing airflow
- Unit sited in a location prone to water dripping from above and refreezing
What to Do
Do not attempt to chip or pour boiling water on the ice — this can damage the unit. Switch the system off at the controller and call a qualified heat pump engineer. The problem must be diagnosed before the unit is restarted to avoid compressor damage.
Sources: MCS standards and Health and Safety Executive.
