Plain-language explanation: This page is educational and does not replace testing the actual equipment.
Normal frost
Cold, damp weather can create frost on the outdoor coil. The heat pump should periodically enter defrost to remove it.
Abnormal icing
A coil encased in ice, ice lasting for hours, or ice spreading into the fan area indicates a problem.
Possible causes
Defrost controls, sensors, low refrigerant, outdoor-fan problems, drainage, and airflow conditions can contribute.
What to do
Do not chip the ice. Turn the system off if the fan is striking ice or performance is poor, and request service.
Frequently asked questions
Is steam during defrost normal?
Yes. Melting frost can create a cloud of steam.
Can I pour hot water on the unit?
It is better to avoid improvised methods that may damage the coil or create refreezing.
Will emergency heat help?
Emergency heat may provide temporary indoor heat, but it does not correct the outdoor-unit problem.