Premixing apparatus

A premixing apparatus that mixes a fuel gas with air and supplies an air-fuel mixture to a burner through a fan, comprising,

• the fan;
• an air supply passage on an upstream side of the fan;
• a gas supply passage having a downstream end communicating with a gas suction part that is disposed in the air supply passage;
• a zero governor that is interposed in the gas supply passage, the zero governor regulating a secondary gas pressure to an atmospheric pressure;
• a variable throttle valve that is interposed in a portion, on an upstream side of the zero governor, of the gas supply passage;
• an excess air ratio detecting device configured to detect an excess air ratio of the air-fuel mixture, the excess air ratio detecting device comprising at least one component selected from
• a flame rod that is arranged to face a combustion surface of the burner and that detects the excess air ratio based on a flame current flowing in the flame rod, and
• a temperature sensor that senses a rear-surface temperature of the combustion surface of the burner, the temperature varying with the excess air ratio;
• and
• a controller that is configured to carry out a first control that varies 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 ratio, which is detected by the excess air ratio detecting device, becomes a predetermined value corresponding to suppression of poor combustion,
• wherein:
• the controller is configured to carry out a third control that:
• when the rotational speed of the fan is increased depending on the required combustion amount and in a case where the excess air ratio of the air-fuel mixture, which is detected by the excess air ratio detecting device, increases beyond the predetermined value, and the opening degree of the variable throttle valve is controlled to increase beyond a predetermined standard opening degree so that the excess air ratio becomes the predetermined value, calculates and memorizes a lower limit of the rotational speed of the fan, at which an increase of the opening degree of the variable throttle valve becomes necessary, as a first threshold;
• and when the rotational speed of the fan increases to the first threshold or faster next time, immediately changes the opening degree of the variable throttle valve to an increased opening degree, which is larger than the predetermined standard opening degree and is obtained by multiplying a deviation of the rotational speed of the fan from the first threshold by a predetermined coefficient.