What is the purpose of a crankcase heater in an air-to-air heat pump?

The purpose of a crankcase heater in an air-to-air heat pump is to boil liquid refrigerant out of the compressor oil. When the system is not operating, refrigerant can condense and settle in the compressor crankcase, mixing with the oil. This can lead to liquid refrigerant entering the compressor during startup, which can potentially cause damage due to the lack of lubrication and the risk of liquid slugging.

The crankcase heater warms the compressor oil and refrigerant, which helps vaporize any liquid refrigerant that may have accumulated. By ensuring that only vapor enters the compressor, the crankcase heater protects the compressor's components and maintains its efficiency and lifespan. This is particularly important in cold weather when the chances of refrigerant pooling in the crankcase are higher.

The other choices do not accurately capture the primary function of the crankcase heater in a heat pump system.