29 March 2017
A furnace's fan pushes warm air through the duct system and into a home. However, if the fan runs continuously, it will end up pushing not-yet-heated air into the home. To keep this from happening, the fan is designed to come on only after the air in the furnace plenum has reached a certain temperature. A fan limit control switch is what helps to ensure that the fan only runs when there is enough warm air to blow into a building.