Switch on the water heater, and if the temperature of the water is in
the range where the thermostat will call for it to be heated, then the
heater control board will send juice to the igniter–a device that
produces a spark in front of the heater gas burner. Once the igniter is
sparking, then a valve opens, allowing gas to flow into the burner,
where the spark ignites it. At the same time, the control board is
sensing for a flame. When the burner flame is sensed, then the igniter
is switched off.