When the gas water heater, the flame on the heat exchanger heating the same time, fire platoon fire hole near the thermocouple head heated side heating, thermoelectric power generation, to pass through the circuit solenoid valve coil generates a magnetic force, keeping the solenoid valve open (At this point the solenoid valve has been opened by external force). When unexpected flameout, the thermocouple head heated side gradually cooled, electromotive force disappears, the solenoid valve magnetic disappeared under the action of the spring reset.