Adding unvented cylinder to combi?

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  • garmcqui
    Automated Home Guru
    • Jan 2015
    • 119

    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
  • kevinsmart
    Automated Home Ninja
    • Sep 2018
    • 257

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

    Comment

    • garmcqui
      Automated Home Guru
      • Jan 2015
      • 119

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

      Comment

      • kevinsmart
        Automated Home Ninja
        • Sep 2018
        • 257

        #4
        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.

        Comment

        • garmcqui
          Automated Home Guru
          • Jan 2015
          • 119

          #5
          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?

          Comment

          • kevinsmart
            Automated Home Ninja
            • Sep 2018
            • 257

            #6
            Yes, that’s correct

            Comment

            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              #7
              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.

              Comment

              • garmcqui
                Automated Home Guru
                • Jan 2015
                • 119

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

                Comment

                • bruce_miranda
                  Automated Home Legend
                  • Jul 2014
                  • 2307

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

                  Comment

                  • garmcqui
                    Automated Home Guru
                    • Jan 2015
                    • 119

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

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      #11
                      I have confused your thread sorry. You were asking if an unvented cylinder could be placed after the boiler. This is the normal configuration. And Evohome will work for you.

                      What I was saying is that, if you had solar, wanted a combi but felt the combi was not enough for simultaneous showers then an unvented could be placed before the combi. This unvented would be preheated by solar.
                      The combi then potentially could get a 60C inlet, and has to do very little work to top up the heat. So could still be sufficient

                      Comment

                      • StuartP
                        Moderator
                        • Feb 2004
                        • 81

                        #12
                        Originally posted by bruce_miranda View Post
                        I had no idea that some combis could even work with a 60C inlet feed. But apparently Intergas etc are happy with that.
                        Intergas certainly opens up options, I've toyed with a cylinder as you described, but also with a coil taking the CH return into the boiler to further increase the change of the boiler condensing, whilst pre-heating the DHW.

                        There's the installation manual for pre heated water by solar.

                        Comment

                        • StuartP
                          Moderator
                          • Feb 2004
                          • 81

                          #13
                          Details of per-heated solar DHW on an intergas boiler
                          Screenshot 2021-03-23 at 18.56.56.jpg

                          Comment

                          • mtmcgavock
                            Automated Home Legend
                            • Mar 2017
                            • 507

                            #14
                            If you have an Unvented cylinder you will require a cable back to the zone valve. You cannot just use the Hot Water Kit to control the zone valve.

                            The Cylinder High Limit stat is still required to bring it up to regulations, a fail safe device. To utilise this you'll need a cable back to cut the power the zone valve should an overheat situation occur.

                            You must use the high limit stat, so your installer is correct.

                            Comment

                            • kevinsmart
                              Automated Home Ninja
                              • Sep 2018
                              • 257

                              #15
                              Originally posted by mtmcgavock View Post
                              If you have an Unvented cylinder you will require a cable back to the zone valve. You cannot just use the Hot Water Kit to control the zone valve.

                              The Cylinder High Limit stat is still required to bring it up to regulations, a fail safe device. To utilise this you'll need a cable back to cut the power the zone valve should an overheat situation occur.

                              You must use the high limit stat, so your installer is correct.
                              The installer was indicating a cable needed to run from the zone valve to control the boiler.

                              What you describe doesn’t require a cable back to the boiler, I think just wiring the high limit stat in series between the DHW BDR91 output and the DHW zone valve.

                              Comment

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