Water heaters are used everywhere to supply hot water in residential and commercial buildings. A water heater is a closed vessel used to supply potable hot water. This water is heated by the combustion of fuels, electricity or other sources and withdrawn for external use at pressures less than 160 psig or a heat input of 200,000 Btu/hour. The water heater must include all controls and devices necessary to prevent water temperatures from exceeding 210 degrees F.