A premixing apparatus that mixes a fuel gas with air and supplies an air-fuel mixture to a burner through a fan, comprising:
- an air supply passage on an upstream side of the fan;
- a gas supply passage of which a downstream end is connected to a gas suction part that is arranged in the air supply passage;
- a zero governor that is interposed in the gas supply passage and regulates a secondary gas pressure to an atmospheric pressure;
- a variable throttle valve that is interposed in a portion, on a downstream side of the zero governor, of the gas supply passage and is driven by a stepping motor;
- an excess air ratio detecting device that detects an excess air ratio of the air-fuel mixture;
- and
- a control device,
- wherein the control device is configured to carry out a first control that is able to change a rotational speed of the fan depending on a required combustion amount and a second control that regulates an opening degree of the variable throttle valve so that the excess air ratio of the air-fuel mixture, which is detected by the excess air ratio detecting device, becomes a predetermined value,
- wherein:
- on a premise that a rotational direction of the stepping motor, which causes the opening degree of the variable throttle valve to change to one of an increase and a decrease, is defined as a normal-rotation direction and a rotational direction of the stepping motor, which causes the opening degree of the variable throttle valve to change to the other of the increase and the decrease, is defined as a reversal direction,
- the control device is configured to carry out:
- a third control as the second control, which, in a case where the excess air ratio of the air-fuel mixture, which is detected by the excess air ratio detecting device, has been a value that is out of a predetermined acceptable-range which includes the appropriate value and will become the appropriate value by causing the opening degree of the variable throttle valve to change to the above-mentioned one, at a state where the stepping motor has been caused to rotate by a predetermined unit-angle in the normal-rotation direction, repeatedly distinguishes whether the excess air ratio of the air fuel mixture, which is detected by the excess air ratio detecting device, has become the appropriate value until the excess air ratio of the air-fuel mixture, which is detected by the excess air ratio detecting device, becomes the appropriate value;
- and a fourth control as the second control, which, in a case where the excess air ratio of the air-fuel mixture, which is detected by the excess air ratio detecting device, has been a value that is out of the acceptable-range and will become the appropriate value by causing the opening degree of the variable throttle valve to change to the above-mentioned other, causes the stepping motor to rotate in the reversal direction till a predetermined first standard rotational-position that is set based on a change characteristic line of the reversal direction, which expresses a relationship between a rotational position of the stepping motor and opening degree of the variable throttle valve at time when the stepping motor is caused to rotate in the reversal direction, and is anticipated that the excess air ratio of the air-fuel mixture will become a regulation standard-value which deviates from the acceptable range by a fixed quantity in a changed direction at time when the opening degree of the variable throttle valve is caused to change to the above-mentioned other, next, causes the stepping motor to rotate from the first standard rotational-position in the normal-rotation direction till a second standard rotational-position that is calculated based on a change characteristic line of the normal-rotation direction, which expresses a relationship between the rotational position of the stepping motor and the opening degree of the variable throttle valve at time when the stepping motor is caused to rotate in the normal-rotation direction, and reaches a short, by a predetermined angle in the normal-rotation direction, of a target rotational-position which is anticipated that the excess ratio of the air-fuel mixture will become the appropriate value, and subsequently, at the state where the stepping motor is caused to rotate by the predetermined unit-angle in the normal-rotation direction, repeatedly distinguishes whether the excess air ratio of the air-fuel mixture, which is detected by the detecting device, has become the appropriate value until the excess air ratio of the air-fuel mixture, which is detected by the excess air ratio detecting device, becomes the appropriate.