The solar storage tank has either one or two internal coils and a backup electric heating element in the single coil units. A heating medium is passed through the solar collectors and internal coil as long as there is an adequate temperature difference between the heating medium and stored water in the tank. The internal coil is located as close to the bottom to facilitate the heat transfer even at low solar collector temperatures.
During water flowing through the water tank, hot water is drawn from the top of the tank while cold water comes into the bottom (by a dip tube or bottom inlet). On single coil tanks, if the hot water demand should exceed the solar heat input or there is an insufficient temperature difference between the heating medium and stored water, the heating element thermostat will activate the electrical heating element for backup heat.
On double coil tanks, the upper coil is connected to the boiler for backup heat. Solar heat output from the internal coil will vary depending on outside conditions and the temperature of the stored water.