Combi boilers have become increasingly popular in the last 20 years. They are great for smaller properties or those worried about space, because you don’t have a big old hot water cylinder to make room for.
Solar thermal is a growing industry and we have seen a massive surge in interest in this technology since the introduction of the Renewable Heat Incentive (launched in March of last year). The ability to produce hot water from the sun’s energy is obviously a nice idea since not only are you doing your bit for the planet – you also don’t need to fire up the boiler so much. The size of the solar thermal panels also makes it very popular for city dwellers, whose roofs are often too small to accommodate a full solar PV system. The main issue with solar thermal though, despite the fact the panels are relatively small, is that you need a tank to store the hot water that is produced.
For properties with combi boilers – you can probably see the issue here – there is no tank!
Many people therefore think that if you have a combi boiler then solar thermal system is a no go – but this isn’t entirely true though!
If you have a compatible combi boiler, it may be possible to feed pre-heated water from your solar thermal system into the boiler. That means your boiler won’t have to work as hard to heat the water to the required temperature and so you should end up making considerable savings.
Even if it isn’t compatible with your particular boiler, you can essentially turn the combi back into a heat only boiler, using the new solar thermal tank as a new hot water store.
If the components allow, the preheated water will enter the boiler and be boosted up to the necessary temperature. Once the water hits 50 degrees or more, the water is diverted so the boiler no longer heats the water.
If the boiler isn’t compatible, the answer is to effectively turn the boiler into a system boiler. This is done by installing a twin coil cylinder and taking a loop off the heating circuit, then connecting it to the higher coil in the cylinder. A valve and timer then control the heating circuit.
How much extra does this setup cost?
A typical solar thermal system will set you back around £4,000, but you will need to install a cylinder and extra pipe work with a combi, so you can expect another £1,000 on top to make the solar thermal work with a combi. As long as you have the required space for the cylinder, there’s no reason why you shouldn’t go ahead, as the payback time is still very reasonable, and you can still claim the RHI payment.
Installing Solar Thermal
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We like to replace a Heatline system gas boiler to a new combi gas boiler assisted with soler panel energy if possible.
Can you recommend possible makes and type. We have a smal 11 years old terraced house with three bedrooms upstairs. Is it possible to use existing hot water storage tank, which is next to bath tab. The house is facing to the west.
Hi, a combi boiler won’t have a hot water tank – therefore this set up is not ideal. You would be better off installing a regular (heat-only) boiler – these produce the hot water for your home and also provide heating via radiators. The advantage here is that you could install a dual feed hot water tank – this would be serviced by the solar thermal panel and also the boiler. It is unlikely your existing hot water storage tank would be able to handle both feeds.
It is important not to confuse combi boilers with condensing boilers – combi boilers are a type of water that produce hot water on demand (there is not hot water storage tank). All new boilers are condensing boilers (so combi condensing or heat only condensing) – this simply refers to the fact that they reuse the heat in the exhaust gases so makes the boilers more efficient.
Hello from the (energy-wasting) US! Can you recommend any particular brands of combi-condensing boiler for use with a solar domestic HW preheat? I currently have a working system where the DHW uses a solar preheat (home built) into a conventional propane DHW heater (cylinder in your terms). This is now 20 yrs old and only 62% efficient. An identical hot water heater supplies radiant floor heat. I’d like to replace both hot water heaters with a modern combi unit. This seems to be unheard of in the US, but is done out your way. The brands are the same, so it would help to find out which ones support the application. Thanks in advance!
I am thinking about installing a hot water tank to complement my new 8 kW solar PV (currently there is a combi and no tank). How do I decide between a thermal store and an unvented tank? Also, how to size it? The guide says 40 litres per person, but for solar PV maybe bigger is better?