The problem with gas boosted solar hot water. And the evacuated tube solution.


Gas boosted solar hot water systems are increasingly popular in these days of increasing energy efficiency in our homes and businesses. These systems rely on flat panel solar hot water collectors or evacuated tubes capturing power from the sun to heat up water which is then stored in an insulated storage tank.

With the gas-boosted systems, an instantaneous gas booster sits between the storage tank and the hot water feed into the house and turns on if the hot water coming from the storage tank is not at the required temperature. As its name suggests, the gas booster turns on automatically and burns gas at a rate sufficient enough to bring the water up to the desired temperature at the same rate that it is being used in the house.

The two main reasons why the water in the storage tank might be below the desired temperature are that (a) there has not been enough sunlight to heat the water sufficiently, or (b) the water usage in the house has been high and has emptied the storage tank of hot water.


The Problem

Gas-boosted solar hot water is often marketed as the most efficient way to heat water and as the most cost effective solution, and admittedly it is a simple, logical and efficient way to meet your hot water needs, however, there is problem with this system: The gas boosters are usually factory set to 70 deg C which is well above the temperature requirements of domestic hot water. This means that if hot water that leaves your storage tank at 50 degrees C or 60 degrees C for example, the instantaneous gas booster will turn on and heat the water to 70 degrees C, this water will then pass through a tempering valve which will add the required amount of cold water in order to bring the temperature of the water down to a safe 50 degrees C for use in the house.

In other words, if your solar hot water system is working well and is producing hot water for free at an acceptable temperature for use in the house (50 – 69 degrees C) most instantaneous gas boosters will still turn on boost the water up to 70 degrees C so that it can then be cooled back down to 50 degrees C.



Hopefully you are wondering why most gas boosted solar hot water systems work this way, because it does not sound very smart.

The reason they are set to 70 Deg C is to Kill Legionella which can otherwise cause Legionellosis and illnesses such as Legionnaires' disease which can ultimately lead to death(see for more information about Legionalla and hot water in Australia, or for general information about Legionella).

And the reason for the tempering down to 50 degrees C is to ensure water safety ( An unregulated flow of water at a temperature of 60 degrees could cause a major burn in less than 5 seconds.  


What’s the solution?

The solution to the problem is surprisingly simple – our close coupled low pressure roof mounted tanks do not store the water that you use in your house so there is no risk of Legionella entering your water supply. The Red Circle Solar systems hold and heat up hundreds of Litres of water which is then used as a heat bank to heat up your water when you need it. When you turn on your hot tap, cold water is run through a 30 metre copper coil submerged inside the tank, and heat is transferred from the water in the tank to the water in the copper coil. This hot water is then tempered down a safe to 55 Degrees C using a solar tempering valve.

The best scenario is a Red Circle Solar close coupled systems used in conjunction with a 50 degree C gas booster, which can be controlled using a temperature relay (to turn the booster on or off as needed using the temperature of the water leaving the roof mounted tank as a trigger). This significantly increases the efficiency of the system as a whole, and reduces the gas consumption of the home by ensuring that the gas booster only turns on when the water leaving your close coupled solar hot water systems is below 50 degrees C.


More Information

For more information, have a look through our website:, where you can read more about how the systems work. You can also see our evacuated tube split systems, flue kits for use with wood fires and wetback stoves (it is easy to thermosiphon our close coupled systems without the need for a pump).


For more information on our reccomended gas booster please visit

Any questions regarding any of the above, please do not hesitate to contact us (