a control system including a valve for controlling the flow of fuel to a burner , a resistive heater element connected between first and second terminals and disposed for igniting fuel emanating from the burner , an isolation transformer comprising a heater secondary winding connected to the first and second terminals so as to provide heating current to the heater element , and an electrode spaced from the heater element in a zone occupied by flame emanating from the burner . also included is a start - up circuit for opening the valve to provide fuel to the burner for ignition by the heater element and a flame sensing circuit comprising an ac source , coupling means connecting the ac source means between the electrode and one of the first and second terminals with the secondary winding connected to the first and second terminals , and a detector circuit for detecting the flow between the electrode and the heater element of current conducted by flame in the zone . finally , a valve control circuit maintains the valve open in response to an output signal provided by the detector .