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Thread: Adding unvented cylinder to combi?

  1. #1
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    Default Adding unvented cylinder to combi?

    Hi all,

    We moved into a 1999-built house a few months ago. Currently using a 28kW combi, which isn't great for 3 bathrooms, 2 with showers.
    From looking at the plumbing in the upstairs airing cupboard, there has been a cylinder fitted in the past, which was then changed to a combi system in 2015.

    Would it be relatively straightforward to have an unvented cylinder added here? Would it simply be plumbed in as part of the central heating circuit, with a zone valve controlling flow to the cylinder coil? The hot water part of the combi could simply be redundant (or could possibly feed nearby dishwasher or washing machine?).

    I have added a photo just to show the existing pipework. To my untrained eye, it seems like everything's there that's needed - There’s two pipes for CH flow, which have been joined together. What I presume is the cylinder return has been capped off. There’s a tundish and a 28mm vent pipe which goes outside. On the water side of things, there’s a 22mm cold feed from under kitchen sink, and a 22mm hot feed which will at one point have been leaving the cylinder. There’s also what I think is a 15mm balanced cold feed, which seems to feed everywhere but kitchen. Hope that makes sense.

    We have Evohome, so would be looking to add the DHW to that too. One plumber I spoke to by phone said I would still need a cable going from the tank to the boiler, so the zone valve can control boiler. He was insistent that there was no other way, and that evohome couldn’t do it wirelessly - is he right?

    Cheers,

    Gareth

    C14133FD-2779-47DC-9E30-5B775B18DB56.jpg

  2. #2
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    The plumber is wrong, Evohome can wirelessly activate more than one BDR91 relay on DHW operation.

  3. #3
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    Fab, thanks. Will a plumber who isn't clued up on Evohome be ok with that? Will they think it's safe enough?

  4. #4
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    Quote Originally Posted by garmcqui View Post
    Fab, thanks. Will a plumber who isn't clued up on Evohome be ok with that? Will they think it's safe enough?
    If you have a boiler control BDR91 you don’t need the Zone valve to fire the boiler.

    If you need to switch the boiler to a DHW mode for instance by connecting a cylinder NTC, you can use another BDR91.

    If there was an RF issue, the DHW zone valve might not close, or the boiler might operate at a lower flow temperature.

  5. #5
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    Yes, I’m using boiler relay. So would just need another BDR91 to control DHW zone valve.


    Just thinking... even if in worst case scenario the DHW zone valve fails to close, surely the maximum the water in the cylinder can be heated to would be the boiler flow temperature, same temperature as radiators?

  6. #6
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    Yes, that’s correct

  7. #7
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    If you have solar panels there is another option. It involves putting the Unvent Cylinder before the combi. But this only works with some combis and only makes sense, if you have solar panels.

  8. #8
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    Nope, no solar (I wish!).

  9. #9
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    I had no idea that some combis could even work with a 60C inlet feed. But apparently Intergas etc are happy with that.

  10. #10
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    I'm confused. Why would the inlet feed be 60C? What would it be now, with just the radiators?

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