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.
The diagram below shows how this works in a real world thermosysphon solar water heating system. Such a system offers the advantage that no pump or controller is required to run everything and so it is very simple and less liable to failure.
