The flue gas thermostat is used to monitor the exhaust gas temperature in the flue pipe of heat-generating systems. The sensor of the ATW consists of a metal tube and a metal rod arranged within it, which have different coefficients of expansion, resulting in a length difference when the temperature changes. This difference acts on a precision microswitch, which closes the control circuit for the KDS above a fixed set limit temperature (here 40 degrees). After the temperature at the sensor drops by about 15 K, the microswitch resets, and the control circuit becomes high-resistance again. Specifically designed for the exhaust air controls KDS 110 and KDS 116. The normally closed contact is factory bridged with a 10k resistor.