Water heaters have a temperature and pressure relief valve to allow water to escape if the water pressure or temperature exceeds the limits of the tank. Tanks normally operate up to 150 psi. The T & P valve is designed to release water from the tank when the pressure exceeds 150 psi. The valve will also open if the water temperature reaches 210 degrees (F). The T & P valve should have pipes connected to safely carry away the water without flooding or exposing people or pets to scalding hot water.
In the center of the water heater is an anode, sometimes called a sacrificial anode. The purpose of this piece of metal is to corrode instead of the tank corroding. The corrosive action of the hot water attacks the anode, extending the life of the tank. Furthermore, the tank has a bonded layer of glass to further protect it from corrosion.
Water heaters build up sediment as it settles out of the water. This sediment should be periodically drained from the tank to increase the life of the appliance and to improve heating efficiency. A drain valve is located near the bottom of the tank and should be used to drain off the sediment or anytime water must be drained from the tank.
Details Unique to an Electric Water Heater
On an electric heater the thermostats are in contact with the tank, and operate mechanically. The thermostat is like a switch. The switch is on by default, but when the temperature of the tank rises to a certain point, it expands and pushes a small metal rod against a dimpled piece of metal. That movement breaks the contact in the thermostat and stops current from flowing to the heating elements. The upper thermostat also functions as a reset switch and is sometimes referred to as the ECO (Energy Cut Off). Pressing the ECO restores to power to the water heater.