When acted on by a heat source, such as the sun or underground heat, fluids often rise up. For example, when the sun warms up a body of water the water molecules within the water expand, making them more Buoyant, causing these heated water molecules to rise. On the other hand, cooler or cold water causes the water molecules to contract and become less buoyant (denser) and sink.

This principle of natural convection of fluid motion is used to full effect within a solar water heating system. This effect is known as thermosyphon (alt.thermosiphon) and refers to the passive heat exchange, based on natural convection, which circulates liquid without the necessity of a mechanical pump.

Reverse Thermosyphon

It must be noted that in a solar water heating system, just as the sun heats the water in the collector, the night air can cool a collector, causing reverse flow. Think about it: water in a collector is cooled by night time stagnation. Cold water is heavier and sinks, pushing the entire loop into reverse flow, moving warmer water from the tank to the collector, which is in turn cooled. This causes some of that valuable hot water to cool rapidly. To prevent this, the storage tank must be positioned at least 300mm above the solar collectors or alternatively an anti-reverse circulation valve must be installed.
thermosyphon.