Premixing Apparatus

A premixing apparatus for mixing fuel gas with air to supply thus obtained air-fuel mixture, through a fan, to a burner,

• in which a downstream end of a gas supply passage having interposed therein a flow control valve for supplying fuel gas is connected to a gas suction section disposed in an air supply passage on an upstream side of the fan, the premixing apparatus comprising:
• an air resistance changeover means for changing over, between high and low, a ventilation resistance in that section of the air supply passage which is on an upstream side of the gas suction section;
• and
• a gas resistance changeover means for changing over, between high and low, a ventilation resistance in that section of the gas supply passage which is on the downstream side of the flow control valve,
• wherein the air resistance changeover means is constituted by a butterfly valve disposed in that section of the air supply passage which is on an upstream side of the gas suction section so as to be rotatable into an opened posture parallel with a longitudinal direction of the air supply passage or a closed posture at right angles to the longitudinal direction of the air supply passage, and
• wherein the gas resistance changeover means is constituted by a changeover valve disposed in the gas supply passage in an openable and closable manner, and has an interlocking mechanism to open or close the changeover valve in interlocking with the rotation of the butterfly valve into the opened posture or into the closed posture,
• characterized in:
• that the interlocking mechanism maintains the changeover valve in a fully-closed state when the butterfly valve lies in a predetermined first rotation angle range inclusive of the closed posture, and maintains the changeover valve in a fully-opened state when the butterfly valve lies in a predetermined second rotation angle range inclusive of the opened posture;
• that the rotation angle of the butterfly valve at which the excess air ratio of the air-fuel mixture becomes a predetermined appropriate value within the first rotation angle range is set to be a closing-side stop angle of the butterfly valve;
• that the rotation angle of the butterfly valve at which the excess air ratio of the air-fuel mixture becomes a predetermined appropriate value within the second rotation angle range is set to be an open-side stop angle of the butterfly valve;
• and
• that the butterfly valve is controlled in rotation between the closing-side stop angle and the open-side stop angle.