Probably nothing, as long as the ice is just a thin layer. In order to understand why the ice is forming, let’s start with the basic principle of how a heat pump works. It does not create heat; instead, it collects and moves it. In the summer, a heat pump absorbs heat from inside the home and moves it outside--thereby lowering the inside temperature. Then, in winter-time, it does the reverse: absorbing heat from outside and moving it inside.

   A heat pump air conditioner operates in three modes: cooling, heating, and defrost. When the system is in heating mode, the coils are in what is called “evaporator” status. They get quite cold and, under some combinations of outdoor temperature and humidity, frost will form on the surface and begin to build up. A sensor at the coils recognizes when this occurs and switches the system into defrost mode, which reverses the flow of the refrigerant for just a minute or two to warm the coils and melt the accumulation of frost/ice. The fan at center of the unit shuts off while the defrost is happening. You may see some water vapor/steam rising off the coils too. When the temperature of the coils reaches about 57º F, the defrost cycle is complete and the system returns to heating. A defrost cycle will occur about every half-hour to hour during very cold weather.

   If the frost buildup on the coils is so thick that it looks like they are encased in a block of solid ice, then the system is not turning on the defrost cycle as necessary. This will decrease the efficiency of the heat pump and possibly damage the equipment, and means the unit requires service. At the other extreme, if the system is switching to defrost mode every few minutes, it would also indicate that the system needs repair.

   But the accumulation of a thin layer of frost/ice, and the subsequent defrost cycle, is a normal for a heat pump during the winter months--especially during the several nights each year when the temperature in Gainesville dips down to around 20º F and we have a “hard freeze.”

    Also, see our blog post The coolant line to the outside unit of my air conditioner is frozen. What's wrong?

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.