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How to avoid bugs in your solar hot water

Posted by Paul Hutchens on 21 June 2010 at 9:07 am

The plumbing, heating and solar industry is paranoid about legionella and bacterial infections in hot water systems. This is right and proper as the consequences of legionella infection can be fatal in vulnerable people - but it is actually very rare and I am told there are no recorded cases linked to solar hot water systems.

Legionella (and other related bacterial infections) thrive in water at between 25 and 45C. Legionella is an airbourne risk so it is worse for showering than for, say, hand washing. Legionella, however, is also very easily destroyed by raising domestic hot water temperatures to around 55 - 60C for just a few minutes.

The potential problem with solar thermal systems is that in times of lower sunlight levels, such as winter, spring or autumn, the solar collectors may only raise the water temperatures to 25 - 45C.

So what should be done?

All well designed systems should have provision to back up the solar with another heat source. We recommend that you set your boiler or immersion heater to come on each day to ensure that the water temperature reaches 60C.

When we hand over the solar heating system to the new proud homeowner we advise them to have their boiler come on for an hour or so each evening. This gives the solar a chance to do its best with the radiation levels available during the day before we switch on the fossil fuel source!

The boiler will have a thermostat so if the solar has provided sufficient heat on its own - the boiler will recognise this and not fire up. Your system should never try to heat the same water twice!

This sort of issue highlights the need for quality installations - particularly as legionella risk is easily averted. This is where the Microgeneration Certification Scheme (MCS) comes in to its own as an MCS accredited installer will have to prove that they are minimising this risk.

If you are unsure please call your installer who will be happy to advise. If you cannot contact your installer, please feel free to contact Eco2Solar.

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around the UK.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

5 comments - read them below or add one

Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 26 August 2010 at 3:59 pm

It really depends on the configuration of the system you are trying to create and the boiler in question.

There is usually still the potential issue of legionella unless the cylinder contents are heated to 60C regularly.


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AlyBee

AlyBeeComment left on: 26 August 2010 at 3:55 pm

Thanks Paul. So is it preferable to convert the combi to a standard boiler over using these solar diverter valves ? ive not seen any pros/cons for both options.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 26 August 2010 at 3:48 pm

Generally speaking best practice is to convert the combi to a system boiler; so that the solar system and the combi heat the cylinder which then supplies the hot water. This avoids issues with boilers that will not accept pre-heated water and potential legionella problems.

Unvented and vented are acceptable; unvented is better though because there is no stored water in the header tank.

The Solarvert replaces the immersion heater I believe; which may indeed heat the top of the cylinder. In order to ensuer that the entire tank is heated you could fit a de-stratification pump to circulate the heated water.

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AlyBee

AlyBeeComment left on: 26 August 2010 at 3:37 pm

Could someone clarify what the best practice is when designing systems combi boilers & solar thermal and L8 protection please. I'm seeing several valves appear on the market from Grant (combisol) / Intasol Combi Solar Diverter /  etc to connect combi's to HW STH setups, and is there an increased L8 risk with these setups ?

What's the recommendation for the tank. vented/unvented.

Also if someone has a solarvert fitting into the top of a tank and not the bottom, how do you ensure the bottom of the tank reaches 60 deg ?

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fuelexplorer

fuelexplorerComment left on: 22 June 2010 at 8:59 pm

An alternative Paul would be to use a heat store and only heat water as needed though a heat-exchanger.  Simples! 

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