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Can I have a solar thermal system if I have a combi-boiler?

Posted by Gabriel Wondrausch on 1 July 2009 at 9:55 am

NB: For more up to date information on this subject see this newer blog (28/1/15)

There are two ways of installing a solar thermal system with a combination boiler. In both cases you will need a new hot water cylinder. This can often be the biggest problem as in most cases where a combi-boiler is installed, there is no room for a cylinder

The design of the system will depend on the make and model of combi-boiler. Some combi-boilers accept pre-heated water and some will not. This depends on whether or not the manufacturer has used temperature rated components on the incoming side of the boiler. About 45% of combi-boilers do accept pre-heated water, and others accept water up to certain temperatures. This information is available from the manufacturer.

If a combi-boiler accepts pre-heated water we design the system as a pre-heat system, and connect the solar cylinder on to the mains feed to the boiler.  We use a pressurized cylinder as combi-boilers are designed to work at mains pressure. When the water temperature in the cylinder is below 50˚C the solar system feeds the combi with pre-heated water which means it has to work less hard to bring the water in the cylinder up to the desired temperature. Once the cylinder temperature gets above 50˚C a diverter valve is used to bypass the combi and send the solar heated water directly to the hot water outlets.

If a combi boiler does not accept pre-heated water we can essentially convert the combi back to a system boiler. We do this by installing a twin coil cylinder and taking a loop off the heating circuit of the boiler and connecting it to the higher coil in the cylinder. This heating circuit is then controlled via a 3 port valve and a timer. The combi-boiler will heat the cylinder at the times set on the timer. We have to leave one tap connected to the combi side but then all the other hot water will be connected to the new cylinder.

In both cases, you can have a solar thermal system which is compatible with your combi- boiler.  It just depends on what type of boiler you have and how much space there is available.

Photo by 1Happysnapper

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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. 

More information about solar thermal and combi boilers from YouGen

Is your condensing boiler actually condensing?

How to avoid bugs in your solar hot water

Solar thermal infomation page

Heating and hot water information page


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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