in fig3 and 4 which depict respectively heating and cooling curves for the mercury filled bulb 45 and capillary tube 43 when ignition occurs and the bulb 45 is heated it reaches temperature t 1 at time t 1 at this temperature the cold position contacts 21 and 42 of flame switch 44 open the bulb 45 continues to heat and when it reaches temperature t 2 at time t 2 the hot contacts 21 and 66 close further heating to a stabilization temperature t 3 occurs and the resilient switch arm provides an override for further heating of bulb 45 when the flame is extinguished the bulb cools from temperature t 3 at temperature t 4 fig4 contacts 21 and 66 open after a time interval t 4 to t 5 the bulb cools sufficiently to temperature t 5 to permit contacts 21 and 42 to close a thermocouple flame sensor has similar operating characteristics in that its voltage declines as the thermocouple cools following flame extinguishment switch actuation voltages can be similarly selected to provide a time interval t 5 t 4 between breaking of the hot contact and making of the cold contact of the flame switch for use in the invention in operation the control system senses a heating demand by the closure of the switch contacts 34 of thermostat 32 supplying electrical potential through the cold contact 42 of flame switch 44 to the igniter 36 simultaneously the potential is applied through conductor 62 to the coil of relay.