Thermal Solar Panels With A Combi Boiler

ComicGeek

Alex1983 wrote: »

I wouldn't say there definitely not made like that but I can't see it, for that system to work surely the cold water pipework would have to go through the solar panel or some sort of heat exchanger to transfer the heat.

No, the solar thermal panels are a closed loop system heating the hot water cylinder through the internal coil. Standard cylinder set-up, just replacing the system boiler with the solar thermal panels.

lstar337 wrote: »

Yeah, the panels are a closed system and only heat the water in the tank through a heat exchanger. The combi needs mains pressure water to function though, and high pressure water tanks are a PITA at the best of times.

Not sure how a combi would react to a warm feed on the inlet either.

A mains pressurised unvented cylinder is used which maintains the inlet pressure to the combi. It's not compatible with all combis due to the issue with incoming water temperature - some combis aren't suitable as they could overheat the supply water as they're expecting mains water temperature at the inlet.

Not saying it's a good idea, lots of reasons not to do it this way. But I have come across a number of these systems - some in court with house owners trying to sue rogue installers!! In fact, I had an entertaining couple of hours (sad, I know!) with a salesman trying to persuade me this was a good idea for my previous house.

ComicGeek

lstar337 wrote: »

The post I quoted was describing a system where the cylinder is heated via panels (during sunny periods), and then fed into the input of the combi, sort of like a solar pre-heat type arrangement.

That I have never seen!

It was a system being sold by a number of companies about 10 years ago, trying to flog expensive solar thermal systems on a wider scale to people with combis. I've been involved as an expert witness in a number of court cases where these systems should never been installed, and customers are told that they will get a return for their £12k in less than 10 years - when in reality they are only saving about £60 a year.....

Not a common system thank God, but it does unfortunately exist.

Alex1983

If a unvented cyclinder is used then why would it feed its water to the combi? I can't see the sense in that and can't think of any combi that would not overheat if your put pre heated water into it and then asking it to raise the temperature by 35-45c, it wouldn't modulate down low enough and would cut out.

I vaguely remember seeing a system like that a few years ago and I think it was a greenstar and a small cyclinder in a garage and they had some sort of blending valve so when the unvented was up to temperature it took its heat from there and if it wasn't then it came from the combi but that unvented never feed the combi but that was the only time I've seen it in 15years.

Like I said I've seen a few piped as system boilers with the combi dhw just supplying a kitchen.

ComicGeek

Alex1983 wrote: »

If a unvented cyclinder is used then why would it feed its water to the combi? I can't see the sense in that and can't think of any combi that would not overheat if your put pre heated water into it and then asking it to raise the temperature by 35-45c, it wouldn't modulate down low enough and would cut out.

I vaguely remember seeing a system like that a few years ago and I think it was a greenstar and a small cyclinder in a garage and they had some sort of blending valve so when the unvented was up to temperature it took its heat from there and if it wasn't then it came from the combi but that unvented never feed the combi but that was the only time I've seen it in 15years.

Like I said I've seen a few piped as system boilers with the combi dhw just supplying a kitchen.

The OP has stated that they have solar thermal panels, hot water cylinder and a combi boiler. While it is very rare, all I'm saying is that I have seen a number of these systems as I described.

I agree that it's a stupid idea, which is why most of these have been court cases. But they do exist regardless. Unless the OP returns with more information it's only guesses - just offering an alternative possibility to those stating that it must be a system boiler not a combi.

Alex1983

I not saying your wrong, I'm just interested in how it was supposed to work so if I ever see one I would know what it is. Just because I've never seen it doesn't mean it doesn't exist. So are you saying they don't work and we're missold?

ComicGeek

The idea behind it was for companies to sell more solar panels at inflated prices. With a preheated supply into the combi boiler from the solar cylinder, it was sold as a way of saving money.

There were some systems we saw where it never worked because the combi couldn't modulate the output temperature. In these cases the installers actually installed a diverter, so the owners thought that it was working but the heat just sat in the cylinder until the solar panels vented when they got too hot.

In other cases the combis were able to modulate the output, but the storage temperature of the cylinders were set to about 30 degC from memory, so the actual benefit from the system was pretty small.

In all cases the owners were promised big annual savings, in some cases larger than their actual energy bills.... So yes, they didn't work and were missold.

However, there are still reputable companies today selling these systems, and a lot of modern combis are able to deal with higher inlet temperatures to enable this - have a look at this website http://centralheating.co.uk/combi-preheat/ and there are a lot of common Vaillant, Worcester, Glow-worm combis with maximum inlet temperatures of 60 degC. So it can work in practice, but the annual energy savings are so small that it's not worth doing for the costs involved.

Alex1983

Thanks for the information. Can't see any sense to it. If there is a way of automatically switching between the cyclinder and the combi then I could understand but as you explained it seems pointless. If your going to spend that money you may as well stick in a system boiler but then the outlay will not meet the saving.

I have solar at my house, it was here when we moved in and I find it quite good, it's attached to a system boiler and gets the tank up to 70 on good days.

wookie008

Thanks for the input people! I only just found the pole to get into the loft this morning (we've just moved in so the house is bit of a mess) so I'm going to pop in there this evening to check out the boiler.

There's a very good chance it isn't a combi, the seller had said it was but on reflection they could of been making the assumption themselves.

I_have_spoken

Ironically the supposed efficiency of condensing combis will be largely negated if the incoming water feed is already warm.

Moss5

There is an alternative use of solar water heating combined with a combi boiler.

Your wet-solar system could be keeping an unvented hot-tank hot. Opening a hot tap causes the combi to fire and (after a delay) feed heated water to the hot tank.
This drives-out the stored hot water to the tap without delay.
The unvented tank could have an electric immersion heater to maintain temperature.
You will save water by not waiting for hot water to reach the tap.

Some combi boilers use a small tank of hot water to provide hot water at the tap without delay.