The element serves as a back-up heater when solar energy is not available or when hot water demand exceeds the solar heated water supply. A dip tube at the top or upper side of the tank forces the heated water to return below the heating element, ensuring proper hot water stratification. This tank is plumbed to a second tank which contains a heating element or gas burner that can be used when no solar energy is available. Except for large residential or commercial type installations, the most popular tanks are glass lined or stone lined. These tanks have an outer shell of steel that must be protected from galvanic corrosion.
Some tanks use a galvanized coating.
In order to provide a longer lasting coating, glass and stone linings were developed. Since the glass lining of a tank sometimes has gaps and may be damaged during shipping and handling, an aluminum or magnesium anode rod is installed in the tank. These rods are used to protect exposed tank metal from corrosion. Once the first metal is completely eaten away, the second metal will begin to corrode. If the anode is never allowed to completely dissolve, it will continue to protect the tank.