Boiler Relay and DHW Zone vs CH and DHW Zones?

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  • garmcqui
    Automated Home Guru
    • Jan 2015
    • 119

    Boiler Relay and DHW Zone vs CH and DHW Zones?

    I have an S-Plan system (two zone valves, one for DHW and one for central heating, with HR92's on all radiators. I believe there are two options for configuring Evohome on this system:


    1. Configure Evohome to have no boiler demand, instead having 2 zone valves which in turn control the boiler/pump.

    2. Electrically disconnect and manually open the central heating zone valve, and configure Evohome as having a boiler relay and just one DHW zone valve.



    I'm just wondering what the benefits of each option are, especially in terms of pump overrun etc.

    Thanks!
  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    #2
    My system was installed by "professionals" and they went for option 1. It's also the option shown for S-Plan in the installer manual.

    P.

    Comment

    • garmcqui
      Automated Home Guru
      • Jan 2015
      • 119

      #3
      See I have that option now but it seems a waste, as soon as the heat demand stops, the zone valve closes and then the 10min pump overrun is wasted, just pumping it round the bypass.

      Comment

      • paulockenden
        Automated Home Legend
        • Apr 2015
        • 1719

        #4
        Originally posted by garmcqui View Post
        See I have that option now but it seems a waste, as soon as the heat demand stops, the zone valve closes and then the 10min pump overrun is wasted, just pumping it round the bypass.
        But that's still going to happen isn't it? If the TRVs are all closed even if the heating valve is forced open the bypass is the only place for the water to go.

        Plus, if you get the Evohome to do its version of overrun, the boiler will still do its own overrun when the boiler relay switches off, so you'll end up with two overruns.

        Probably

        P.

        Comment

        • garmcqui
          Automated Home Guru
          • Jan 2015
          • 119

          #5
          No, as the HR92's don't just close fully immediately after boiler demand switches off, they seem to close partially, allowing a little water through even after they've reached their set temperature. This allows the pump overrun to circulate water and excess heat from the boiler which would otherwise be wasted.

          Unsure what you mean by Evohome's version of overrun, I thought that was only on hot water?

          Comment

          • paulockenden
            Automated Home Legend
            • Apr 2015
            • 1719

            #6
            Originally posted by garmcqui View Post

            Unsure what you mean by Evohome's version of overrun, I thought that was only on hot water?
            It's in the hot water part of the menu, but it acts on the boiler relay (if fitted) so I assumed it would work on both?

            Comment

            • garmcqui
              Automated Home Guru
              • Jan 2015
              • 119

              #7
              No it just delays the hot water relay being turned off by the defined time period, after the boiler relay is turned off.

              Comment

              • paulockenden
                Automated Home Legend
                • Apr 2015
                • 1719

                #8
                Ah - there was another post which said it was for Heating too.

                P.
                Last edited by paulockenden; 26 April 2015, 07:43 AM.

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