Heat pump iced-up in Winter:

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 refrigerant and the outside air cannot occur. The unit pictured left, was iced-up so badly, it damaged the fan blades, crushed the outdoor coils; causing the refrigerant to leak out and ruined the compressor - a complete loss.

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How does the defrost cycle work?

On a call for defrost, the reversing valve is energized, switching the system into the air

conditioning mode. That is right - Air Conditioning. The outdoor evaporator becomes

the condenser but at the same time the outdoor fan shuts off. This allows the high pressure

refrigerant circulating through the outdoor coil to get very warm, melting the ice.

At the same, the second stage heat (the back-up heat) is energized to offset or temper the

cold air now blowing out the vents. When a sensor or thermostat in the outdoor unit reaches a certain temperature and/or a certain amount of time goes by, the system goes back to normal heating mode. At this time a cloud of water vapor can usually be seen rising out of the outdoor unit and a "whoosh" sound can be heard as the refrigerant reverses direction. The entire process usually takes between 2 to 10 minutes depending on conditions.

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                                                     Different heat pumps have different ways of determining when to go into defrost. Some                                                                  (older models) 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's iced-up or not. When the thermostat heats up to a certain temperature, defrost is                                                   terminated. Hopefully at this time, much of the frost has melted off.

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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 refrigerant temperature in the coil, and run time. This is the most efficient way to defrost.

If a heat pump is severely iced-up during winter use, it's possible that it's not defrosting, but there are many other causes.

Below is a list of possible causes. Items in red usually require a service call. The bottom four causes in blue are commonly found problems and can be addressed, even fixed by the homeowner.

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Red = Professional fix | Blue = Homeowner fix

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

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A few more things to consider...

 

• If the top of the heat pump is covered in ice, turn it off and remove the ice.

• If a gutter is dripping on the heat pump, repair the gutter.

• Keep snow and leaves away from the heat pump including underneath it.

• If the heat pump has settled into the ground, it must be elevated - usually on blocks or special feet. (Should be done by a technician)

• With the outdoor unit off, (you can set your thermostat to emergency heat), ice can be removed with a garden hose. If the heat pump ices-up again, it is time to schedule a service call.

 

Important: Whatever you do, please, never pick the ice off with a sharp object. The refrigerant coils and fins can be damaged very easily.

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Cliff's HVAC does not assume any liability resulting from the information we provide. We hope this information helps, but please note that these are just rough guidelines, and not all possible situations are covered. Your HVAC system should be inspected and repaired by a trained technician.

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