The Passive Greenhouse “Inside Collector” is a thermosiphoning or convective loop system.  It circulates water between the collector plates and the storage tank by taking advantage of water’s tendency to rise when hot and sink when cold.

When the system is installed as shown above, with the tank higher than the collector, neither pumps nor thermostats are required. 

Solar-heated water rises by convection from the top of the collector to the upper part of the tank, where it is ready to be drawn off for use.  The household water supply line introduces cold water through a dip tube to the bottom of the tank.  From there it flows to the bottom of the collector and is drawn upwards by the heating of the collector fins.  Circulation continues as long as the fins are transferring heat to the pipes but stops after the sun disappears and the fins become cool.  Insulation around the tank keeps the water in the tank warm until the sun appears again.