Can I use solar water heating with a combi boiler?

A solar water heating system does need to supply a cylinder, as the hot water is generated gradually through the day. Because of this, it is difficult to add solar water heating to a heating system that doesn’t include a hot water cylinder - such as a 'combi' boiler that heats water on demand when you turn on a hot water tap or have a shower.

Make sure you differentiate between the terms 'combi' and 'condensing'. All modern boilers are condensing boilers - some will be 'combi' boilers (hot water on demand) and some will be 'system' boilers (feeding a hot water cylinder).

Combi boilers save space by not needing a cylinder and are often mounted in the kitchen (near the hot tap). They are generally fitted in small houses/flats (one or two people) with low hot water demand, whereas a 'system' boiler & hot water cylinder is a better choice for a larger home.

A few combi boilers are designed to accept pre-heated water, so it might be an option to feed pre-heated water from a solar-heated cylinder into such a boiler. If water in the cylinder is hot enough from solar alone, the control unit can feed it directly to taps without going through the boiler. Whether or not this arrangement will actually be more efficient then having solar water heating alongside a system boiler is hard to know - it will depend on several factors.

However, it is very important to note that most combi boilers will not be designed to cope with a hot water input. Feeding in hot water could damage components and cause the boiler to malfunction, leading to expensive repairs or even replacement - and you may well have invalidated the warranty. it is vital to check with the manufacturer before designing a system that feeds hot water into any combi boiler.

You will need to discuss the options with an installer and see what will work in your situation. See our page on how to find a solar water heating installer.

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