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Does solar hot water work with a combi-boiler?

Posted by Paul Hutchens on 8 June 2012 at 9:05 am

Q: We are about to install a new combi boiler. We also want to install a solar water and CH system. Would like to use an unvented solar store with heat exchanger coil to CH flow on combi (with temperature controlled diverter valve) and the mains pressure cold feed through a heat exchanger in solar store then into the combi hot water circuit. We want to get this installed soon. Any information would be appreciated.

A: First of all - don't install a combi-boiler!

Combis are designed to heat water instantaneously from the mains and do not require hot water storage. They are designed for relatively low hot water usage households and are convenient as they provide hot water on demand and save space on a hot water storage cylinder.

Solar thermal heating systems require a storage cylinder as the energy generated from the sun in the form of heat is sporadic and is produced in proportion with daylight and sunshine which means that, as it cannot be guaranteed to be available when you need it, it must be stored in a hot water cylinder.

Existing combis can be modified to fit a hot water storage cylinder and a solar heating system by fitting a cylinder and splitting the heating circuit of the combi to heat it - this is what you appear to describe.

However, if you are fitting a new boiler, fit the correct one for solar heating in the first place. I suggest a good quality system boiler with an unvented stainless steel hot water cylinder along with your solar heating system.

I strongly suggest that you contact a reputable MCS certified solar installer who is a member of the Solar Trade Association. They should be able to provide good advice and quality service.

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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5 comments - read them below or add one


artioxComment left on: 14 March 2019 at 1:55 pm


"If the OP wants to combine a combi with solar then fitting one of the few that do take hot DHW input, and not using the combisol-diverter would make a more efficient system"

Why would it make it more efficient? Is it not better to drain Solar tank as much as you can and feed gas boiler cold/warm water from Solar tank (which will be warmer than water from tap)?

Say if you feed gas boiler with 50 degrees water and heat it up by another 15 all what that will do is increase the temperature in your solar tank and you will incur more loses.

I think combisol-diverter would be more efficient because of above.

Am I missing something?

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 26 June 2012 at 7:57 pm

Hi Wookey

Thank you for your contribution. You are correct in your analysis of the costs; collectors can be bought for £300 - £400 each but the price of pump stations (or the constituent parts), cylinders, pipework, expansion vessels etc all add to the cost. 

I don't agree that I misunderstood the question. The question says "we are about to install a new combi-boiler" implying that they had a choice and could install a system boiler instead. The question therefore related to a workaround that may not be required.

I also agree with the precept that combis are best suited to low DHW usage households and system boilers for high usage ones. I would go further and say that solar thermal heating may not be suitable where DHW usage is low.

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wookeyComment left on: 26 June 2012 at 2:33 pm

Banjax: Solar thermal collectors aren't particularly expensive are they? £300 (47mm 20 tube) to £400 (58mm 20tube) +VAT each which is enough for most installations.  That's from eco-nomical, which is where I bought from and have been very pleased with performance - there are other suppliers that don't charge a fortune - Navitron,  The cost in an installation is mostly in the tank and the installation work itself, not particularly the panels. My entire 40-tube installation cost £800 in equipment and has been great.

WRT to combis and solar, it is an awkward mix, but Paul has misunderstood what the questioner proposed. The questioner was talking about fitting a solar combi diverter (e.g Grant combisol) to use the combi when tank water was cool, divert it straight to taps when not.

Paul is talking about turning the combi back into a sytem boiler by ignoring the instant DHW part. Either of these approaches can work.

I find it amusing that this embarassing confusion is displayed right alongside exhortations to use a registered MCS/STA member. I'm just a member of the public and I appear to have more clue.

It does seem to me that solar+instant water heating is a greatly underused technology. Given the high standing losses from hot water cyclinders/thermal stores it _ought_ to be more efficient to just do the top-up heating with an instant heater (i.e combi). But most combis don't take hot water input for DHW (or if they do the manufacturers don't admit it).

If the OP wants to combine a combi with solar then fitting one of the few that do take hot DHW input, and not using the combisol-diverter would make a more efficient system. Atmos do such a boiler, for example (HE32) - there are a few others out there. The Navitron forum or Green Building forum are great places for advice on this sort of thing.

There are (often ignored) losses in a combi too, to do with heat-up time and cooling energy losses every time a tap is turned.  Some proper research to quantify these in comparison to cylinder standing losses would be really useful. I found very little info on the subject when deciding which way to jump 3 years ago. In a low-usage household, the combi set-up is likely to be better than the system-boiler setup, and the opposite in a high-usage household. But where does the dividing line lie?

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banjaxComment left on: 8 June 2012 at 4:20 pm

From an installers point of view. Why are solar thermal collectors so expensive?

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graham_fComment left on: 8 June 2012 at 10:28 am

We've got solar-thermal panels and a combi boiler. The panels pre-heat the water in an unvented cylinder, and the "warm" water from the cylinder then goes to the combi to be brought up to the correct temperature. If the water is heated sufficiently by the panels an automatic diverter valve sends it direct from the cylinder into the hot water circuit, bypassing the boiler. Central heating is handled solely by the combi.

 We ended up with that set-up because the house had the combi boiler when we bought it and we weren't ready to change it at that stage. We were fortunate that it was a model that could accept pre-heated water - apparently not all can.

On the advice of our installers we fitted a cylinder that has a second heating coil (which we're currently not using) so that if / when we replace the boiler we can get something other than a combi (or possibly look at air-source heat pump) to heat the water in the cylinder.

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