A clever compressor heater (without a heating element) for split-system operation in cold weather.
When using a split system for heating in sub-zero outside temperatures, forced heating of the compressor crankcase is necessary to protect it from failure due to oil thickening and the possible presence of liquid freon. Special heating elements are usually used for this purpose, but one is also possible. A cheap W1209 thermostat is used for control. A split system starter is used for switching, but two five-pin 12V relays from an auto parts store are better.