[HayZee518]

A gas furnace ignition uses either a flame rod or a millivolt generator to detect the presence of an ignition flame. when it is called upon to light, a high voltage spark is generated across a set of contacts and the pilot valve is opened. If a pilot is not lit during a certain time delay, the burner relay locks out. There may be other timers involved in the circuit for purging the burner chamber. When the sensor detects the continuous flame, the main valve will open.