Device for controlling the supply of a combustible gas to a burner, particularly for water heaters

A device for controlling the supply of a combustible gas to a burner, having a draft shut-off device in an exhaust pipe for the combustion fumes, comprising a main gas supply pipe in which are positioned a first and a second servovalve, in cascade with each other, with respect to a direction of flow of the gas, the servovalves including respective valve seats associated with corresponding shut-off members with diaphragm control for opening of the seats in opposition to respective resilient return members,

• the first and second servovalves including respective first and second electromagnetically operated solenoid control valves for controlling opening and closing of the corresponding servovalves, the solenoid valves configured to operate by opening and closing pipes of corresponding servo-assisted control circuits, in such a way that the corresponding shut-off members of the corresponding servovalves are controlled indirectly, by the diaphragm controls, in order to put the main pipe in fluid communication with corresponding control chambers of the servovalves, one side of the diaphragm of each of the diaphragm controls being subjected to a pressure established in the corresponding control chamber,
• a pilot pipe is branched from the main pipe upstream of the servovalves for supplying a pilot burner,
• the main pipe being provided, upstream of the servovalves, with a magnetic thermoelectric safety unit with a manual activating knob, which acts on the pilot pipe to allow gas to flow to the pilot burner during activation of the unit,
• the draft shut-off device comprises a corresponding shut-off member which is movable from and towards a position in which the fume exhaust pipe is shut off, and diaphragm-controlled pneumatic actuator of the shut-off member, the diaphragm forming, at least partially, a corresponding control chamber of the shut-off member which is connected to and in fluid communication with a section of an auxiliary pipe branched from the main pipe, a first portion upstream of the section being provided for the supply of gas to the control chamber and a second portion, which is an extension of the first portion, being provided downstream of the section for the discharge of the gas from the chamber in the proximity of the pilot burner;
• a corresponding valve is provided on the discharge portion of the auxiliary pipe, downstream of the section, for selective control of opening or shut-off of the discharge portion.