A device for heating a fluid, for example, in a central heating boiler, comprising a heat exchanger (7) in the fluid conduit, a liquid or gaseous fuel burner (2) associated with said heat exchanger (1) and a housing (5) enveloping the heat exchanger (1) and the burner (2) to form a combustion channel having an inlet (6) for the combustion air and an outlet (7) for the exhaust gases, as well as a blowing apparatus (8) for producing a forced transport of combustion gases through the channel, wherein a pressure-difference switch (9) communicates on the one hand through a conduit (14) with a measuring point (P l ) of the channel near the blowing apparatus and on the other hand throuwh a further conduit (17) with a second measuring point (P 2 ) further remote from the blowing apparatus (8) in the channel, said switch (9) controlling a valve (4) in the fuel supply and in the further conduit a second valve (18) controlled by the pressure-difference switch (9) is included, said second valve (18) establishing a communication between the other side of the pressure-difference switch (9) with the atmosphere when the fuel valve (4) is closed in order to providing a sufficiently high pressure difference for overcoming the resistances in the various valves.