The gas water heater is mainly composed of a valve body assembly, a main burner, a small fire burner, a heat exchanger, a safety device, etc. It also includes the flue of a flue water heater and the forced exhaust device of a forced exhaust water heater. The valve body assembly controls the entire working program of the water heater, including water valves, gas valves, microswitches, and igniters. When installing a water heater, valves should be installed on the inlet, outlet, and gas pipes.