Abstract: The invention is with respect to a fail-safe system for stopping damage to water boilers, whose thermostats are not in working order.In an automatic combustion control system of the sort having an elastic thermostat bellows element placed within an outer shell within the water space of a boiler and joined by a chain or the like with the air door for controlling the air inlet rate at some point in the connection between the thermostat element and the door, there is a temperature-sensitive part, for example in the form of a plug of fusible metal or collapsing material, or in the form of a soldered join using a special fusible solder.In one form of the invention, this temperature-sensitive part may take the form of a ring of temperature-sensitive material placed at one end of a spring which is forced together on the temperature increasing.