thermosiphon

thumb|Thermosyphon circulation in a simple solar water heater (not a working model; there is no water supply to replenish the tank when the tap is used)

A thermosiphon (or thermosyphon) is a device that employs a method of passive heat exchange based on natural convection, which circulates a fluid without the necessity of a mechanical pump. Thermosiphoning is used for circulation of liquids and volatile gases in heating and cooling applications such as heat pumps, water heaters, boilers and furnaces. Thermosiphoning also occurs across air temperature gradients such as those occurring in a wood-fire chimney or solar chimney.