Abstract: The apparatus has a pneumatic controller, which controls the pressure in a gas conduit (1) in dependence on the pressure in an air conduit (2). Provided downstream of the regulator is a mixing device (4), which has a venturi-like mixing chamber (7). Extending into the inlet opening of the mixing chamber (7) is a nozzle (8), which is connected to the air conduit (2). Defined between the nozzle (8) and the edge (9) of the inlet opening of the mixing chamber (7) is an annular gap (10), which is connected to the gas conduit (1). Also operating in the air conduit (2) is a conical member (11) which is actuated by a linear motor (12). The mixture is withdrawn from the mixing chamber (7) by a fan (5), whose speed is controllable, and fed to the combustion device. The adjusting motor (12) for the conical member (11) operates in dependence on a signal from a sensor, which detects changes in the gas quality, the air pressure and/or the combustion air temperature.