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Solar thermal panels are possible with a combi boiler

Posted by Gabriel Wondrausch on 27 August 2010 at 9:56 am

You will find more up to date information on this subject in this blog (28/1/15).

 Solar hot water panels require a cylinder, but it is possible to use them with a combi boiler. This video explains how it works. And for those who prefer to read the information, here's a summary:

If your combi boiler accepts pre-heated water (some do, some don't, it's important to make sure yours is the former), you can have a pressurised cylinder fitted on the incoming side of the boiler to preheat the water. This means the boiler doesn't have to work so hard.

It's very important to make sure the combi boiler is compatible before attempting this. It has to have temperature-rated components on the incoming side and it has to be fully modulating to ensure the water isn't overheated as it goes through.

Once the solar heated water gets hot enough that it doesn't need further heating, it can be set up to bypass the combi boiler, and go direct to the taps.

More information about solar thermal from YouGen

Solar hot water: 7 things to check before you install

Solar hot water without changing your cylinder

Solar thermal information page

Can I use solar thermal for space heating?

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

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

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3 comments - read them below or add one

Cathy Debenham

Cathy DebenhamComment left on: 8 October 2012 at 2:57 pm

Hi Nick

Sorry about the slow response - I was away all last week. You can have solar thermal and a combi boiler, but it isn't generally recommended. I suggest that you read this blog, and the comments below it, which I think may answer your question - If not, do come back and ask again.

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Nick Hanna

Nick HannaComment left on: 5 October 2012 at 12:15 pm

Hi there. We have a solar thermal system with a thermal store (and woodburner). Our Vaillant boiler is very old and we were thinking of replacing it with a combi boiler. Two heating engineers have visited to quote and neither has asked to look at the thermal store. British Gas just visited and told us that Bosch (their only boiler supplier) do not recommend using a combi boiler with a thermal store, EG with a pre-heated return it will not condense properly. Any thoughts please? Also, if we have to install a traditional boiler who much will that save?

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Jayhawk International Ltd

Jayhawk International LtdComment left on: 27 August 2010 at 5:51 pm

As a further comment Gabrial relating to the pre-heating of freezing water that passes into a plate heat exchanger of a Combi boiler.


Ignoring for now what gas boiler manufactures tell you, all Combi Boilers can except pre-heated water as long as you first understand that from the first frost on the ground until May, the incoming water is freezing, meaning 4c in Scotland and 6c in the rest of England, even down here in Dorset.


The performance of a Combi begins to improve on the domestic hot water side around May when the sun is over head for 4-5 hrs a day which heats up the ground where the water pipes are buried as it heats up the swimming pool and sea.


The incoming water temperature in summer months rises to around 15c, that’s cold but not freezing, so in effect the water has been preheated indirectly from the sun.


The old Combi boilers had a Diaphragm which can take a maximum temperature of 23c where the new Combi is installed with a Mechanical Valve, that can take any temperature.

Its a proven fact that most older Combi boilers are rubbish during winter months when you need high flow and good temperatures over the winter when it comes to the delivery of hot water to where its needed, but perform very well once you get to summer and a warmer cold water.

So supplying solar pre-heated water through what we designed as a Combi Fortic thermal store cylinder to keep it simple with a 20 tube heat pipe collector we designed as freeze free to heat that fortic direct, not through a coil and everything else sold with closed loop systems, the system is affordable at £2,500 installed.

If the Combi is due to be changed, my advice would be, not to replace it, instead go back to a traditonal boiler with a thermal store heated by solar that can provide hot water and space heating


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