Abstract: A fuel burning system comprises a burner in communication with a burner supply pipe having an automatic two-stage regulator valve for controlling the pressure of fuel gas delivered to the burner. The burner supply pipe is normally fed by a primary supply pipe delivering collected digester or landfill gas through an automatic three-way valve, however a secondary supply pipe carrying natural gas may be selected by the three-way valve for communication with the burner supply pipe. The regulator valve and three-way valve are electrically connected to a standard ignition module by a pressure-sensitive switch to detect the presence of digester or landfill gas in the primary supply pipe. When sufficient gas pressure exists in the primary supply pipe, the three-way valve is operated to permit delivery of the digester or landfill gas to the burner supply pipe and the regulator valve is held in its high stage setting for burning this primary fuel.