How do I plumb a solar water heating system into a combination boiler?

Here are two solar plumbing diagrams and a few words of advice to solar plumbers considering fitting solar a solar water heating system to a combi boiler.

I have put two solar combi plumbing diagrams below, because there are two types of combi boilers, when considered in terms of how control their flame size in response to incoming water temperatures. The combi boilers which adjust the flame size gradually are called modulating combi boilers. Those combi boilers which don’t adjust are non-modulating combi boilers: these need more complex control valves.

It is essential to call your boiler provider before pre-fitting solar thermal to a combi-boiler and get then to OK your proposed specification of fitting solar to a combi. Please also call us for more information and advice.

Solar is about storing heat as and when the sun shines and the using it as and when you want to. Therefore you will need a hot water store. Please use an appropriately sized thermal store (and find some space to locate it near the combi boiler!) as part of the solar/combi installation.

It is vitally important to store the solar heat correctly and safely. Never use a normal full volume type hot water cylinder. Using normal solar cylinders (whether low pressure vented or high pressure unvented) as pre-feeds to combi-boilers do not provide adequate Legionella safety. Please only use a thermal store since for safety reasons it has a very low stored domestic hot water volume (a tenth or even less of the volume of a normal cylinder) plus it has a rapid water turnover (typically at least every 12 hours instead of 24-36 hours). Make sure you carry out a legionella Risk Assessment of all solar thermal installations: further safety controls may be required, even in these solar pre-feeds to combi boiler installations.

Solartwin combi boiler solar preheating.
Historically, it was difficult to integrate solar water heating with a combi boiler, as by definition combi’s heat water instantaneously therefore do not require hot water cylinders. As solar panels heat water across a day, and not necessarily when there is hot water demand, the need is still there to store solar heated water until you are ready to use it.
Combi boilers generally fall into two categories; “Solar ready” fully modulating which can accept preheated water up to 60°C or non-modulating which can typically accept preheated water no more than 25-30°C.

All new boilers are condensing where older models may not be, however this does mean the boiler requires replacement as long as it boiler is in good working condition.
The Solartwin combi preheat system offers a non – intrusive solution by use of a low maintenance vented thermal store, dimensions 571 x 571 x 750mm, that will fit in a kitchen unit below or to the side of the combi boiler.
The solar panel pipes will feed down from the roof down to the location of the thermal store. The mains water supply to the combi, and hot water pipes are connected to the combi valve(s). In some cases the bottom of the kitchen unit may need to be cut out so the thermal store will will fit (floor standing).
If space is not an issue we can also offer a slightly larger thermal store, dimensions 450 x 900mm, which also requires a 300mm in height feed and expansion tank sat on top of it.
As Solartwin combi preheat systems are indirect, scale can be controlled with just a single dose of scale inhibitor rather than ion-exchange water softener. We do recommend that a magnetic or electronic scale inhibitor device is installed on the mains cold water supply to prevent scale build up on the inside of the hot water heat exchanger. (Please note that this type of scale control is NOT suitable for direct Solartwin systems.)
The solar pump will be mounted at the water level in the thermal store / feed and expansion tank fixed onto an anti-vibration mount. Due to the mechanics of the pump you may hear a gentle whirring of the pump when operating in peak sun conditions.
Where possible we position the solar controller outside of the cupboard/unit so that you can see how your system is performing.
Once installed there is no scheduled maintenance required for the Solartwin combi preheat system. We do recommend a number of simple visual checks, and that the water in the thermal stores is drained down, and replaced, every 7 years in order to remove any debris left naturally by the incoming mains water supply and to top up the scale inhibitor.

More details are in our installation method statement. I hope you find this information is useful…