A boiler with atmospheric gas burner is provided with a fitting which has a device for adjusting the gas throughput to the burner. This device has an infinitely variable pressure-regulating valve, the control element of which is connected via a coupling device to the actuator of a thermostat which operates as a function of the temperature of the return of the heating system and is provided with an infinitely variable control member for its response threshold. The coupling device may be designed as a lever mechanism or comprise control means actuable by means of pressure medium. The thermostat is a commercially available radiator thermostat valve head.
