A fan limit switch on your furnace is a vital part of the controls, as it controls what happens if there are any malfunctions within the furnace. It will turn off the burners if it detects a malfunction. Not only that, but it controls when the blowers operated as well. How it essentially works is by turning the blower on and off by regulating the air temperature within in the air chamber. You can adjust the dial to preset temperatures that determine when the fan turns on and off. Once the thermostat reaches that present temperature, the burners will turn off, but the fan will continue to run until the air chamber cools down, at which point it will switch the dial off.