File:Thermosiphon water heating system.SVG

Thermosiphon (alt. thermosyphon) refers to a method of passive heat exchange based on natural convection which circulates liquid without the necessity of a mechanical pump. This circulation can either be open-loop, as when liquid in a holding tank is passed in one direction via a heated transfer tube mounted at the bottom of the tank to a distribution point - even one mounted above the originating tank - or it can be a vertical closed-loop circuit with return to the original vessel. Its intended purpose is to simplify the pumping of liquid and/or heat transfer, by avoiding the cost and complexity of a conventional liquid pump.

Simple thermosiphonEdit

Convective movement of the liquid starts when liquid in the loop is heated, causing it to expand and become less dense, and thus more buoyant than the cooler water in the bottom of the loop. Convection moves heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity. Ideally, the liquid flows easily because a good thermosiphon should have very little hydraulic resistance.

Heat PipesEdit

In some situations the flow of liquid may be reduced further, or stopped, perhaps because the loop is not entirely full of liquid. In this case, the system no longer convects, so it is not a "thermosiphon."

Heat can still be transferred in this system by the evaporation and condensation of vapor; however, the system is properly classified as a heat pipe. If the system also contains other fluids, such as air, then the heat flux density will be less than in a real heat pipe, which only contains a single fluid.

The thermosiphon has been sometimes incorrectly described as a 'gravity return heat pipe' [1]. Heat pipes usually have a wick to return the condensate to the evaporator via capillary action. A wick is not needed in a thermosiphon because gravity moves the liquid [2]. The wick allows heat pipes to transfer heat when there is no gravity, which is useful in space. A thermosiphon is "simpler" than a heat pipe [3]. (Single-phase) thermosiphons can only transfer heat "upward", or away from the acceleration vector. Thus, orientation is much more important for thermosiphons than for heatpipes. Also, thermosiphons can fail because a bubble in the loop, and require a circulating loop of pipes.

Reboilers and CalandriaEdit

If the piping of a thermosiphon resists flow, or excessive heat is applied, the liquid may boil. Since the gas is more buoyant than the liquid, the convective pressure is greater. This is a well known invention called a reboiler. A group of reboilers attached to a pair of plena is called a calandria.

The term "phase change thermosiphon" is a misnomer and should be avoided. When phase change occurs in a thermosiphon, it means that the system either does not have enough fluid, or it is too small to transfer all of the heat by convection alone. To improve the performance, either more fluid is needed (possibly in a larger thermosiphon), or all other fluids (including air) should be pumped out of the loop.

Solar energyEdit

Solar heater dsc00632

Solar heating system featuring a thermosiphon


Solar hot water supply system with thermosiphon (schematically):
1: water tap
2: insulated container
3: warm water inlet
4: solar thermal collector
5: fresh water supply

Thermosiphon solar water heating system

A thermosiphon solar system

Thermosiphon solar water heating system 2

A thermosiphon solar hybrid system

Thermosiphons are used in some liquid-based solar heating systems to heat a liquid such as water. The water is heated passively by solar energy and relies on heat energy being transferred from the sun to a solar collector. The heat from the collector can be transferred to water in two ways: directly where water circulates through the collector, or indirectly where an anti-freeze solution carries the heat from the collector and transfers it to water in the tank via a heat exchanger. Convection allows for the movement of the heated liquid out of the solar collector to be replaced by colder liquid which is in turn heated. Due to this principle, it is necessary for the water to be stored in a tank above the collector.


Thermosiphons are used in computing to describe a system for watercooling the internal computer components, most commonly referring to the processor. While any suitable liquid can be used, water is the easiest liquid to use in thermosiphon systems. Unlike traditional watercooling systems, thermosiphon systems do not rely on a water pump (or a pump for other liquids) but rely on convection for the movement of heated water (which may become vapour) from the components upwards to a heat exchanger. There the water is cooled and is ready to be recirculated. The most commonly used heat exchanger is a radiator where air is blown actively through a fan system to condense the vapour to a liquid. The liquid is recirculated through the system, thus repeating the process. No pump is required - the vaporization and condensation cycle is self sustaining.


Modern processors get relatively hot. Even with a common heat sink and fan cooling the processor, operating temperatures may still reach up to 70 °C (160 °F). A thermosiphon can handle heat output at a much wider temperature range than any heat sink and fan, and can maintain the processor 10–20 °C cooler. In some cases a thermosiphon may also be less bulky than a normal heat sink and fan.


Thermosiphons must be mounted such that vapor rises up and liquid flows down to the boiler with no bends in the tubing for liquid to pool. Also, the thermosiphon’s fan that cools the gas needs cool air to operate.

Ground coolingEdit

Heat pipes are used at locations in higher latitudes like northern Alaska and Canada to prevent ice-rich permafrost from melting below buildings and other infrastructure such as schools, air hangars, community water tanks, and even some stretches of highway. Heat pipes are also a common feature along the length of the Trans-Alaska Pipeline System. In these applications the solution in the pipes is often carbon dioxide or ammonia. At the bottom of the heat pipe, heat from the ground warms the liquid and converts it to a vapor. Cooling from the heat sink fins above ground releases this heat to the atmosphere and causes the vapor to condense on the outer pipe wall, which then drains back into the liquid pool at the bottom of the heat pump.

See alsoEdit


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