Premixing apparatus

A premixing apparatus, in which a fuel gas is mixed with air and an air-fuel mixture is supplied to a burner through a fan, and a gas supply passage for supplying the fuel gas, with a flow-amount regulating valve interposed therein, has a downstream end thereof connected to a gas suction part provided with an air supply passage on an upstream side of the fan, comprising:

• an air-resistance switching device for switching, between large and small, a ventilation resistance at a portion of the air supply passage, which is located on an upstream side of the gas suction part;
• and
• a gas-resistance switching device for switching, between large and small, a-ventilation resistance at a portion of the gas supply passage, which is located on a downstream side of the flow-amount regulating valve,
• wherein the air-resistance switching device is provided at a portion of the air supply passage located on the upstream side of the gas suction part, and is formed of a butterfly valve that is pivoted about a rotational shaft as a center to an opened posture parallel to a longitudinal direction of the air supply passage, and to a closed posture orthogonal to the longitudinal direction of the air supply passage, wherein the gas-resistance switching device is formed of a switching valve that is provided in the gas supply passage and is capable of opening and closing, wherein an interlocking mechanism, which is connected to the rotational shaft of the butterfly valve so as to perform an opening operation or a closing operation of the switching valve in association with a pivotal movement of the butterfly valve to the opened posture or the closed posture, is disposed in a mechanism chamber, which is partitioned by a partitioning part relative to a portion of the air supply passage where the butterfly valve is provided, and which communicates with the downstream end of the gas supply passage, and wherein, in the partitioning part, a pivotally supporting hole, through which the rotational shaft passes, is formed,
• wherein:
• a seal member is provided at a portion of the pivotally supporting hole near the air supply passage, which seals a clearance between that portion and the rotational shaft.