In the Thermosiphon system, the water inside the collector flow-tubes is heated when the sun shines on the collector. As the water heats up, it expands slightly and becomes lighter than the cold water in the storage tank mounted above the collector. Cold water is pulled down into the collector inlet by virtue of gravitational force. The cold water pushes the heated water through the collector outlet and into the top of the tank, thus heating the water in the tank.
A Thermosiphon system requires neither a pump nor a controller thus eliminating the need for grid power. Cold water from Utility or storage tank flows directly to the tank on the roof. Solar heated water flows from the rooftop tank to the auxiliary tank installed at ground level whenever water is used within the building.