A furnace limit switch is a component of a forced-air furnace that is responsible for turning on the furnace blower once the desired heat has been reached. Limit Switches regulate when your home is heated by gauging the temperature of the surrounding air. If the temperature inside the furnace gets too hot, the limit will open up, resulting in the burners shutting off. This limit opens at 140° and closes at 120°. It uses an automatic reset along with Fixed Programming.