The thermo-siphon system: Without an exchanger and a circulating pump, the movement of the water between the reservoir and the collector is based on inertie and the difference of temperature of the heated water.

Concerning the thermo-siphon hybrid system, the back-up may be electric, refer to the figure in which a heating resistance enables to heat water for a larger water availability. We supply back-ups also with a diesel or gas burner.

In farming environment where the availability of common water poses problem, a simple reservoir put above the height of the water heater is sufficient to create the useful working pressure.