Plenum Mount Duct Limit Controls may be used in low voltage or line voltage circuits and are usually wired to open circuit to both heat source and blower when duct air temperature exceeds the temperature setting. The bi-metallic probe is inserted into the warm plenum of the heating furnace. The probe expands as the duct warms, such as the event of a fire, turning a gear which turns the fan limit control dial. As the fan limit switch control dial rotates, electrical contacts inside the switch turn the fan on or off. The switch can also be used to activate a set of louvers to close off air flow or turn on an alarm. Human intervention is needed to reset the Firestat’s switch.