Thermosiphon

A thermosiphon occurs when a gas or liquid in a closed system is heated by some external means. The applied heat causes the lighter heated gas or liquid to rise due to it's lower mass than the colder gas or liquid. The process can be used to move solar heated air or water through a collection system without externally powered pumps or fans. There are 3 requirements for a thermosiphon to function.

1)We need a heat collector and heat source.

2)We need a sealed system.

3) We need the cold inlet at the lower end with the hot outlet above the cold outlet on the storage tank or area to be heated with the air.

The heat would be applied in a collector between the cold inlet and the hot outlet. The collector must allow for a free flow upwards of whatever it contains. This collector can be as simple as a single vertical tube or as complex as multiple vertical tubes with headers top and bottom but the main criteria is a free flow path in an upward direction and, of course, a way to gather heat from whatever source you choose.

Please note that this process can work in reverse. This means you should include some sort of low spot in your piping or ductwork between the cold outlet and the collector to trap cold air or liquid in a "gooseneck" or P trap so that your system does not rob heat when the heat source goes away such as at night with a solar heating panel. Otherwise your system will pump heat to the cold collector untill the heat source returns.

In an air based thermosiphoning solar heater the room it feeds into acts as the sealed "tank".