The contacts of the overheating thermostat of the gas boiler are normally closed and included in the circuit with the control board. When a certain temperature is reached (usually 100-105 ° C), the contacts open, thereby giving a signal to the electronic board about the excess heating of the coolant - and it turns off the boiler, giving an error message on the display.
The flue gas thermostat is installed in the upper part of the combustion chamber of boilers with natural smoke exhaust. In boilers with forced draft, its functions are performed by a pressure switch. The draft sensor has the same principle of operation as the safety thermostat for overheating protection, but the opening of its contacts occurs at a lower temperature (usually up to 65-85 °C).