How Does a Thermosiphon Solar Water Heater Work?

What are Phermosiphon Solar Geysers?

Solar water heating systems is not a new concept. It has been in use for more than a century. This ad was placed in 1902 and was for a phermosiphon water heating system.


Let us first have some fun and look at what happens with cold and warm water. This video demonstrates how hot water rises and cold water sinks.

This video about convection also illustrates the behavior of cold and hot water.

Another video which clearly shows how hot water rises.

Let us also have a look at this fun video to proof that hot water rises and cold water sinks.

Now that we are convinced that hot water rises we can look at how a thermosiphon solar geyser functions.

In case of a thermosiphon system the geyser must be at least 300mm higher than the top of the solar collector.


Water in the solar collector is heated by the sun and as we noticed will rise to the top of the collector.

Due to the closed loop piping of the solar collector and the water heater (geyser), the warm water will rise to the geyser and cold water from the geyser will sink towards the bottom of the solar collector.

This means water is starting to circulate (siphon) through the solar collector and geyser as long as the sun is heating the water in the collector.


  1. Hot water to household
  2. Geyser
  3. Hot water rises to geyser from the solar collector
  4. The sun is heating the water in the collector causing hot water to rise
  5. Cold (colder) water is siphoned from the geyser to the collector

A phermosiphon solar water heating system is the most reliable system due to its simplicity.

The solar geyser systems that can be observed on top of roofs with the geyser are phermosiphon systems.


A popular and economical option is to convert an existing geyser into a solar system if the geyser is still in a good condition (ideally not older than 3 years). A retrofit is done by adding a solar collector and is a split system – the collector is on the roof and the geyser either in the ceiling or elsewhere i.e. the garage.

If the construction of the roof (and pitch) allows it the geyser can be raised to implement a phermosiphon system. If this is not possible a pump system is necessary. The operation of a pump system will be discussed in the next article.

Many home owners prefer a split solar system for aesthetic reasons – as well as flat plate solar collectors. Flat plate collectors in recent years are highly efficient and compare well with the performance of EVT (evacuated tube) collectors.


There is no doubt that by installing or having a solar geyser great savings is established on electricity costs.

The ROI for a solar water heating system is ± 5 years and a very good investment – from then onwards savings increase exponentially.

We have serviced solar collectors that have been installed for at least 15 years and are still in good working order. If geysers are serviced regularly they can also last surprisingly long.

Helderberg Solar Energy has done many new solar geyser installations and retrofits. A big concern is the quality of installations and plumbing observed in many cases.

A solar water heating system requires the installation of various safety features. We also experienced many instances where these features were ignored (not installed) which cause damage to equipment – geysers and collectors. This in turn can cause damage to property.

We urge home owners if they consider installing a solar water geyser to make use of a reputable solar installer.

We proud ourselves on the quality of our installations and after sales service. Most definitely we honour our warranties.


1 thought on “How Does a Thermosiphon Solar Water Heater Work?”

  1. I have a solar water heating system on my roof and need it checked. Also heat pump – need it serviced. Need someone who knows how the controller works.

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