why does my heat pump freeze up in the winter

Hannabery HVAC. All rights reserved. Heat pumps can ice-up during the winter time. And it is normal for the entire coil to be covered in a white frost, even light ice, during certain weather conditions. But it is
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 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. 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. 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. 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. A few more things to consider. Whatever you do, please, never pick the ice off with a sharp object. The refrigerant coils and fins can be damaged very easily. Please keep in mind that the information found on our website is provided free of charge and Hannabery 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. How Comfortable Do You Want To Be? One of the best ways to avoid frozen coils on a heat pump involves maintenance.


Because multiple issues can cause frozen coils, keeping the heat pump in good shape via annual service appointments ensures that the entire system is operational, and limits the effects of wear and tear. Maintenance also helps keep the system free of dirt buildup, one of the primary contributors that causes coils to freeze. In addition to maintenance, follow these tips to keep the system running well: в Don't let ice and snow accumulate on the outdoor unit. After snowstorms and extreme weather, clear the unit of any accumulation. в Make sure gutters above the outdoor unit are secure. Avoid letting water drip out of the gutters and onto the unit to prevent water leaking into the coils and freezing. An outdoor unit encased in snow and ice can also prohibit airflow, which leads to frozen coils. в Level the unit. The outdoor unit must be 100 percent level; any tilting of the unit can restrict airflow and hinder the system from draining moisture properly. Get expert advice about your air-source heat pump, and schedule regular service visits to keep it in tip-top shape. Editor's note: This is an updated version of an article originally posted on Sept. 30, 2015.

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