The fan relay is inside the unit. The thermostat switches 24 vac control voltage to the components like the heat strips, the compressor and outside fan as well as the inside fan motor. If the fan works with the heater and air conditioner then 24 vac is present. If not, then the 24vac transformer is bad. Sometimes you can find a fuse for the transformer, others have the fuse built internally and cannot be changed. If the inside fan is not working in the fan only mode then look at the thermostat as the problem.
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The fan relay is inside the unit. The thermostat switches 24 vac control voltage to the components like the heat strips, the compressor and outside fan as well as the inside fan motor. If the fan works with the heater and air conditioner then 24 vac is present. If not, then the 24vac transformer is bad. Sometimes you can find a fuse for the transformer, others have the fuse built internally and cannot be changed. If the inside fan is not working in the fan only mode then look at the thermostat as the problem.