Premixing apparatus

A premixing apparatus for mixing air with a fuel gas to supply a burner with an air-fuel mixture through a fan, including:

• a zero governor for regulating a secondary gas pressure to an atmospheric pressure, the zero governor being interposed in a gas supply passage whose downstream end is connected to a gas suction part disposed in an air supply passage on an upstream side of the fan;
• a motor-driven type of variable throttle valve interposed in a portion, on a downstream side of the zero governor, of the gas supply passage;
• an excess air ratio detecting device for detecting an excess air ratio of the air-fuel mixture;
• and a control device to perform control to adjust the opening degree of the air-fuel mixture so that the excess air ratio of the air-fuel mixture to be detected by the excess air ratio detecting device becomes a predetermined value;
• wherein the variable throttle valve is provided with a sensing device for sensing a movement of a valve body of the variable throttle valve, and
• wherein the control device is constructed such that, at a time of ignition at the burner, a motor of the variable throttle valve is rotated sequentially in one and an opposite of a valve-closing direction and a valve-opening direction and that, only when the movement of the valve body of the variable throttle valve by this rotation has been sensed by the sensing device, ignition control is performed.