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back boiler to combi boiler dhw connections

Postby zebb » Sat Jul 26, 2008 9:22 am

hi ,i'm about to buy a multifule stove with a back boiler for central heating and domestic hot water,i have a gas combi boiler at present and want to keep this system as is , but i want to connect the new back boiler to this system so that the gas system is only used when needed, the question is what connections and parts do i need to make this all work in harmony, any feedback is appreciated thanks.
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Postby plumbbob » Sat Jul 26, 2008 8:35 pm

It is unlikely the two systems are compatible. You will need to check with the combi manufacturer about suitability.

Generally the only connection is by using a system boiler with a hot water storage facility and separate heating pump.
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Postby Devallon » Sun Jul 27, 2008 2:20 pm

You're going to have a problem here. A combi boiler generates hot water when you want it, but a back boiler system will only heat water when the fire is lit - and this then needs to be stored. Your combi boiler won't have a hot water cylinder to store the hot water as it doesn't need one.
You'll have to fit a hot water cylinder and change your boiler to a conventional boiler that heats a cylinder or thermal store.
Good luck.
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Postby zebb » Thu Jul 31, 2008 10:09 pm

Apparently there is a Reflux flow increase device with circulating pump , thermal regulators that should make the back boiler work with my central heating system , does anyone know what this is and how it works ? and is there a supplier in the uk with such devices ?
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Postby htg engineer » Sat Aug 02, 2008 10:46 am

Sounds like too much hassle to me.

A combi boiler works off mains cold water, therefore the hot water is at mains pressure. When a hot tap is opened the water operates microswitchs in the boiler, to bring the boiler on, how would you stop the combi coming on when the hot water tap is opened ?

What efficiency rating does the new boiler have ? condensing ? is it legal to fit this boiler ?

The easiest way I can think of, is to fit the boiler so that it only feeds the kitchen taps, but if you haven't had the stove on you wont have hot water.

Or give it up as a bad job - and stick with the combi. What you're wanting to do - is go back in time. People got rid of these and installed combi's as they are alot more efficient.

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Postby RobinGB » Tue Oct 21, 2008 1:31 pm

Hi Zebb,

This may be to late for you but I thought I would post anyway incase someone else reads this and wants to do the same.

What you ask is possible but if you want the wood burner to provide hot water you would need to install a hot "water cylinder/heat sump" for the reasons previously mentioned. This negates one of the big benefits of a combi - no cylinder.

I ma not sure what type of woodburner you are thinking of, i.e. perhaps a large output model that would live in the garage and could completely negate the needs for the combi for anything other than when you run out of wood, or like many one for the sitting room with a relatively small output, i.e. 5kw or less to the back boiler.

If it is the latter then it is much simpler to just connect the wood burner to the rad system of the combi, so effectively the rad system becomes the heatsump. There are two models I know where this can be done without much trouble, the WHAM Monet and the HETA Scanline 580 aqua. both need a mains cold water supply as part of the safety system but apart from that they are relatively simple and pain free to install (compared to trying to get hot water as well). See the manufacturers website for installation details.

Good luck
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Postby zebb » Tue Oct 21, 2008 9:25 pm

Hi Robin thanks very much for the reply , i think i will go with the central heating link and either forget about the domestic hot water link or a separate domestic hot water link as it seems too much work or too complicated, the back boiler i have oredered is the Arrow eco boiler seb 20 , it will be in my draughty open plan computer room . :D
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