A water heater is an apparatus for heating water and then storing it for later use. A residential water heater typically warms water to a temperature between 120° and 140°F. When a hot-water faucet is opened, hot water flows from the top of the tank toward the faucet, and cold water enters the tank to replace it. In an electric water heater, the thermostat senses a drop in the water's temperature and completes an electrical circuit

To the heating elements, Electric water heaters usually have both an upper and a lower heating element; each is controlled by a separate thermostat. Once water in the tank reaches the set temperature, the thermostats stop the flow of electricity to the heating elements. In addition, an anode (magnesium) rod attracts impurities in the water that would otherwise attack the metal tank. Many homes are located in areas with hard water.