A Thermosyphon Solar Water Heating System takes advantage of the fact that hot water rises and cool water sinks. As water is heated it expands and so, as gravity pulls down the relatively heavier cool water molecules, the warmer molecules rise up.
During the day water in the solar collector (flat panel or evacuated tube) is heated by the sun. As the water heats up, it rises pushing up into the insulated tank. Cooler water at the bottom of the tank is then pushed down to re-enter the bottom of the solar collector where it is heated further. This cycle continues throughout the day with the water stored in the tank getting hotter and hotter.
A thermosiphon system requires neither pump nor controller. Cold water from the city water line 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 residence.
This type of system is simple and low-maintenance, and uses no energy, but the cylinder must be located above the solar collectors and the pipes must have a continuous rise. Water flow with a thermo-siphon system is relatively slow. This can significantly increase heat losses from the pipes. See installation for more detail.
