Heat pump iced-up in winter:

Heat pumps do ice-up in the winter time. It is normal for the entire coil to be covered in a white frost, even light ice, during certain weather conditions. It is not normal however, for the entire unit to be encased in ice, including the top of the unit and the insides of the coil for an extended period of time. This indicates a problem and should be addressed quickly to save energy and avoid serious damage to the equipment.

Heat pumps will naturally ice-up in the winter but will periodically go into a defrost cycle to de-ice the coils. This keeps the unit running efficiently. If the coils are blocked by ice, proper heat transfer between the freon and the outside air cannot occur.

Different heat pumps have different ways of determining when to go into defrost. Some use mechanical timers in conjunction with a defrost thermostat. If the thermostat is cold enough and enough time goes by, the unit will go into the defrost mode whether it is iced-up or not. When the thermostat heats up to a certain temperature, defrost is terminated.

Most of the newer equipment today uses solid-state control modules with temperature sensors. Even more sophisticated is the Demand Defrost system which makes calculations based on the outside air, the freon temperature in the coil and run time. This is the most efficient way to defrost.

If a heat pump is severely iced-up in the winter it is possible that it isn't defrosting but there are many other causes. Below is a list of possible causes.

Bad defrost control or timer
Bad defrost thermostat or sensor
Bad defrost relay
Sticking reversing valve
Bad reversing valve solenoid coil
Bad outdoor fan motor
Low refrigerant charge
Restriction
Outdoor coil blocked - possibly with leaves or snow drift
Unit sunk in ground - nowhere for ice to melt and drain off
Leaking gutter dripping water onto top of unit
Freezing rain - causes top of unit to freeze over - once this happens the rest of the unit will also freeze over.

The bottom four causes are common problems and can be addressed by the homeowner. If the top of the unit is covered in ice, turn it off and remove the ice. If a gutter is dripping, repair the gutter. Keep snow and leaves away from unit including underneath it. If the unit has settled in the ground, it must be elevated. With the unit off, ice can be removed with a garden hose. If the unit ices-up again, it is time to schedule a service call.