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Combi Boiler and Conventional Boiler in series

Postby sky_blue_kev » Tue May 19, 2009 6:41 pm

Hi there..... times are hard as we all know..... and so a theoretical question if I may please. I have an oil fired combi boiler that is struggling to heat the whole of my house since it has been extended. I have aquired a second hand oil fired conventional boiler for free and wondered if it would be possible to plumb the conventional boiler in line with the combi ? (output of the combi into the return of the conventional..... etc) In theory the combined output of the 2 would more than satisfy the requirements ? Any issues anyone can see in principle ?

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Postby plumbbob » Wed May 27, 2009 5:43 pm

To be honest, I would say that you can't link the boilers together as you suggest. In fact, I am sure you can't.

Firstly, when hot tap water is called for, the combi diverts the heating circuit to circulate the boiler water around a secondary heat exchanger. This means the water in the second boiler cannot circulate so may boil.

There is also going to be a control issue here too. How can you control both boilers to heat the water together? You will find one boiler has shut down thinking it has reached temperature while the other is thumping away trying to increase the temp.

The combi is not going to be happy either. There are known issues with trying to connect them in circuits which utilise solar energy. Condensing boilers use computer controls so damage could be caused if used outside the manufactures expectations.

The old boiler is probably designed to work only on a low pressure vented system so there will almost certainly be issues here too.

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