What is the Function of the Hot Air Furnace Fan Limit Switch?
A fan limit switch (shown at above right on an oil-fired warm air furnace and in our sketch at left) where we can see about 3/4 of the silver colored dial where the fan limit switch settings are made. The fan limit control switch, whose cover has been removed, is about in the center of the photo, and is partially hidden by a low voltage transformer and a metal electrical junction box which are mounted at the right side of the furnace cabinet. The soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with back pressure in the combustion chamber. The photo at above left shows the same type of combination furnace control in use on a gas-fired heating furnace.
