During the winter months, a light coating of ice forming on the outside unit of a heat pump system is completely normal, but if the unit is covered in thick ice that doesn’t melt during the defrost mode, that may be a sign of trouble
A heat pump works by extracting heat from the outside air. When that outside air gets colder, its relative humidity rises. As a result, water vapor from the air condenses on the heat pump’s coils. If the outside temperatures are cold enough, the vapor freezes on the coils, showing up as a light frost or thin layer of ice. As part of your heat pump’s normal operation, it senses this ice forming and switches temporarily into defrost mode. When defrosting, the heat pump essentially runs in reverse, radiating heat from the coils to melt the ice. Defrost mode generally takes about two to ten minutes to clear the ice. Once the ice is gone, the heat pump automatically switches back to normal heating mode.
If this is what your heat pump is doing—forming a thin layer of ice, then melting it in defrost mode—you can rest assured that it’s working normally. You can help to keep the amount of ice down by improving the airflow to your heat pump. Easy ways to do this include changing the air intake filter regularly and clearing away any snow or debris that builds up around your outside unit.
On the other hand, if your heat pump has a thick coating of ice that doesn’t melt quickly in defrost mode then you likely have a problem that requires an HVAC technician to fix. Issues that can cause this sort of freezing include low refrigerant levels, broken defrost controls or sensors, a malfunctioning fan motor, or a dirty coil that needs cleaning. You should switch to the emergency heat setting on your thermostat (which will shut off the heat pump and turn on the electric heat strips) and call a licensed HVAC technician for service.