Solar Thermal


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Assembly Guide

Installation Guide




Other Parts

Installation Guide

LED Lighting

Heat Pumps

Rainwater Harvesting

Solar Overview












Can I use a thermosyphon system?

A conventional solar system uses a pump to circulate the solar fluid when the collector temperature exceeds the hot water temperature.  An alternative approach is to use a thermosyphoning system.  This relies on the fact that the solar fluid is less dense when hot than when cold.  Colder fluid in a hot water cylinder will thus displace warmer fluid in the collector automatically, doing away with the need for a pump and controller.  The disadvantage of the system is that the collector must be lower than the hot water cylinder, which rules out the typical layout of collectors on the roof and tank in the airing cupboard.

Making a thermosyphon system work is a bit of a black art, but there are some rules of thumb to follow.  Use larger diameter pipe, minimum 22mm, and make sure all of the pipe work slopes in the right direction as steeply as possible (no slope reversals to clear obstacles etc).  Ensure that there is a slope of at least 1:2 throughout the circuit, ie for every one metre drop there are no more than two metres of horizontal travel. It is a case of the steeper the better.  More vertical pipe runs in one place do not compensate for more horizontal runs elsewhere.  Placing the collector so that one end of the manifold is a little higher than the other is a good thing.

Pay particular attention to removing all air in the solar fluid (good practice in any case) and monitor the system carefully when first commissioned.  Thermosyphoning has a habit of occurring when unwanted, but failing inexplicably when planned!

Be aware also that a heat dump circuit becomes more difficult to implement with a thermosyphoning system.


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Last modified: 30-06-10