Is a wire-wound variable resistor that is placed near the bimetal element of a thermostat to provide extra sources of heat to help prevent the thermostat from allowing the system to overheat the room. Connected in series with the R-W circuit. Causes the spiral bi-metal element to open before room set-point is reached.

A device that anticipates the need for cutting off the heating system prematurely so the fan can cool the furnace.

a small resistor integrated into the bi-metal coil