Adding solar thermal to existing bosch combi idea.

[Jonnyo]

Hello,
So I want to add a solar thermal system to my existing Worcester bosch greenstar but don't want to lose the functional benefits of the combi boiler.

I'm proposing to install the solar collector on the side of the house in a position that will enable gravity flow to a 200litre tank just above on the inside. Then a vent tank and overheat radiator in the roof to prevent boiling the system. Control valve would open the radiator in power failure.

Then the bit that I'm not sure about. I have a bosch greenstar boiler. I want to pass the incoming cold feed through a heat exchanger with water from the tank to pre heat the cold before entering the boiler.. The cold is very cold here even in summer. The greenstar is apperently modulating so could it accept the water being lifted up to say 35-40c or would this be a problem? If it is I could put the heat exchanger after the boiler and lift the hot water temp up higher and use TRV on all my taps.

I could also do a similar heat exchanger tapped into the radiator circuit return with pipe sensors to detect if the cylinder temp is above my rad temp and if so it would activate and heat the boiler return.

Reason why I'm looking at doing it all this way is I feel if causes the least disruption to the existing pipework and seems pretty low cost. Diagram to follow

 

[Jonnyo]

 

[oz-plumber]

All looks fine except for the fact that it looks like a perfectly designed breeding system for Legionella.

Even though the solar system is separate from the potable water side, via a heat exchanger, you would have to dose the solar side of the system with some ( potent ) chemicals.

There are other ways you can set the system up to achieve what you want to achieve, but that would rely on the combi heating the water ti 60 C to kill bacteria.

Some Combi's also go to fault mode if the incoming water temp is higher than a certain temp.

 

[Jonnyo]

All looks fine except for the fact that it looks like a perfectly designed breeding system for Legionella.

Even though the solar system is separate from the potable water side, via a heat exchanger, you would have to dose the solar side of the system with some ( potent ) chemicals.

There are other ways you can set the system up to achieve what you want to achieve, but that would rely on the combi heating the water ti 60 C to kill bacteria.

Some Combi's also go to fault mode if the incoming water temp is higher than a certain temp.

Thanks for looking at this. I did look at the legionella risk assessment and (unless I'm missing something, please do tell) there shouldn't be any risk to people because they aren't accessing the water on the solar side. I look at it in the same way I had a vented woodburner circuit in my old house that went sent heat to my store via a heat exchanger. Woodburner heat up and go cold with transient temps in the risk range.....

This design, the thermal store side is a closed loop. The pump from the thermal store would only come on when someone opens a tap (flow switch)
If legionella is a risk could easily add a immersion immersion in the tank but again I personally didn't see it as a risk. Again please highlight where you see the risk

 

[Jonnyo]

With regard to the legionella and boiler issues I could put the exchanger on the output side of the boiler. Allow my small thermal store to increase in temp to say 70-80c.l plus have an immersion on a program. This would sterilise the tank however I would have to put a trv to prevent scalding. They main aim of this is to come up with a simple way to supplement my boiler (and reduce gas) not get rid of it.

 

[ShaunCorbs]

Have you had a look at a thermal store as it includes most of your kit you require in one unit

 

[Jonnyo]

Have you had a look at a thermal store as it includes most of your kit you require in one unit

Yeah, gledhill do a small one with the heat exchanged. I had quite a complicated system in my last house.. Had a jaspi 500 litre thermal store electric immersions and a woodburner going 24/7 but I have zero space in this New house. We have downsized. I could get away with 150-200litre cylinder on bracket welded and bolted through the wall Anything bigger I will be needing steels. I'm hoping by keeping the cylinder small it will heat up quicker and be responsive and just cut down the gas usage.... Not replace it. I just have no real experience with gas boilers and don't know whether having the exchanger before will be a problem.

 

[ShaunCorbs]

No won’t have a problem before as they already have a system look at a flue gas heat recovery unit

 

[Desktop987456]

Yeah, gledhill do a small one with the heat exchanged. I had quite a complicated system in my last house.. Had a jaspi 500 litre thermal store electric immersions and a woodburner going 24/7 but I have zero space in this New house. We have downsized. I could get away with 150-200litre cylinder on bracket welded and bolted through the wall Anything bigger I will be needing steels. I'm hoping by keeping the cylinder small it will heat up quicker and be responsive and just cut down the gas usage.... Not replace it. I just have no real experience with gas boilers and don't know whether having the exchanger before will be a problem.

Did you ever install the heat exchanger before the boiler and share your results, I am trying to do something similar to cut down my gas usage but couldn't.