Abstract: A gas-fired heating boiler without a blower, with partial load controls, comprises a combustion chamber with heat exchanger elements located above the combustion chamber, a Venturi tube with a nozzle and an air damper arranged in the wall of the combustion chamber. Partial amounts of the air required for combustion are suctioned into the combustion chamber through the Venturi tube and air damper, respectively. The combustion chamber is open toward the heat exchanger elements at its further walls. The air damper is sealed tightly at its edges against the undesirable entry of air into the combustion chamber. The Venturi tube has a cross section which limit the flow of combustion air to a constant partial amount of the air required for full load operation. An opening is provided in the air damper, calibrated for the flow of the difference air volume required in addition to the air flowing through the Venturi tube for the optimum efficiency of the burner in the partial load operation desired.