The coils on my heat pump are coated with ice on cold mornings. What's wrong with it?
Saturday, October 13, 2018
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:
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