A crankcase heater is an electrical component in a compressor in an air-conditioning system, heat pump system, or a chiller system. The crankcase heater is normally ON all the time even when the unit is not running, though temperature sensors and set points may be used to turn it off when not needed. 

A crankcase heater's sole purpose is to prevent refrigerant migration and mixing with crankcase oil, when the unit is off, and to prevent condensation of refrigerant in the crankcase of a compressor. The crankcase heater is designed to keep the refrigerant at a temperature higher than the coldest part of the system. The resistance of the crankcase heater determines the heat it will produce when voltage is applied.