Gas water heater is mainly composed of valve body assembly, main burner, small fire burner, heat exchanger, safety device, etc. It also includes flue type water heater flue, forced exhaust device of forced exhaust water heater. The valve body assembly controls the working procedures of the entire water heater, which includes water valve, air valve, micro switch and igniter, etc. When installing the water heater, valves should be installed on the inlet pipe, outlet pipe, and gas pipe.