Flame ionization control of a partially premixed gas burner with regulated secondary air

A gas burning furnace comprising

• a gas burner mounted in a combustion chamber, said combustion chamber having an exhaust outlet adapted to be connected to a flue,
• a gas inlet to said burner connected to a gas control adapted to receive gas from a gas source,
• a primary air inlet of said burner connected to an air supply means supplying primary air to said burner to mix with said gas received by the burner to provide a premixed gas go air ratio for burning of said gas,
• ignition means for igniting said premixed gas and air mixture,
• control means responsive to an ionization current of the burner flame, said ionization current produced by a sensor means, wherein said sensor means has a characteristic maximum value for said ionization current at a premixed gas and air mixture of said burner where incomplete combustion due to a shortage of primary air exists, said control means being connected to said gas control to control said gas and said ratio of gas to primary air supply to maintain said ionization current at said maximum value, and
• a secondary air inlet to said combustion chamber connected to said air supply means, said secondary air inlet being sized with respect to said primary air inlet to proportionally maintain an excess air in said combustion chamber for complete combustion of said gas in said combustion chamber.