X-Plan

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  • lloyd
    Automated Home Guru
    • Oct 2020
    • 160

    X-Plan

    I've seen a few references in various posts regarding a so called x-plan, but any links associated with the conversations lead to 404 errors. Can anyone point me to an explanation or diagram?
  • kevinsmart
    Automated Home Ninja
    • Sep 2018
    • 257

    #2


    Why Hot Water Priority (PDHW) is the reason S and Y Plan should be banned. Richard Burrows from The INTERGAS Shop looks at why priority hot water systems are the future of efficient gas condensing system boilers.

    Comment

    • kevinsmart
      Automated Home Ninja
      • Sep 2018
      • 257

      #3
      Basically the CH 2-port is Normally Open and the DHW valve Normally Closed.

      When there is a Priority DHW demand the CH valve is closed and DHW opens, so all flow is to the cylinder.

      With Intergas boilers, typically a short circuited DHW NTC sensor is then connected, switching the boiler to a DHW lower power and higher flow temperature setting.

      Comment

      • lloyd
        Automated Home Guru
        • Oct 2020
        • 160

        #4
        Thanks for those. Makes for interesting reading.

        One thing I'm still struggling to get my head around is why you need to have the CH 2-port normally open. Is it not more accurate to say that you want it closed whenever the DHW is called for? What am I missing?

        Comment

        • kevinsmart
          Automated Home Ninja
          • Sep 2018
          • 257

          #5
          Because default operation is OpenTherm heating with only HR91/2s controlling flow to radiators.

          When there is a demand to heat the DHW cylinder you don’t want 70C flow in your CH circuit leading to overshoots.

          Comment

          • lloyd
            Automated Home Guru
            • Oct 2020
            • 160

            #6
            ok. I had assumed that if you were using OpenTherm then evoHome would simply open the CH / DHW valves as appropriate, and if you had enabled DHW priority that it would then always close the CH valve when heating the DHW. Obviously this is not the case.

            Comment

            • kevinsmart
              Automated Home Ninja
              • Sep 2018
              • 257

              #7
              Evohome support of DHW priority has only just been added to its firmware. Previously if you had CH and DHW valves you would wire them to the DHW BDR91 such CH closes when DHW opens.

              I’m not sure how the new support works with OpenTherm regarding any CH valve.

              Comment

              • lloyd
                Automated Home Guru
                • Oct 2020
                • 160

                #8
                Presumably the temperature sensor connected to the boiler is not needed if you have evohome controlling the DHW.

                Comment

                • kevinsmart
                  Automated Home Ninja
                  • Sep 2018
                  • 257

                  #9
                  Sure, if you are happy with the same max flow temperature and boiler settings for both DHW and CH.

                  In my weather compensation setup, I disconnect the outside temperature sensor on DHW heating, detect it via OpenTherm Gateway firmware and boost the flow temperature to 70C, whereas for UFH CH, max flow temperature is 50C - Outside Temperature, reduced further by Evohome.

                  Comment

                  • lloyd
                    Automated Home Guru
                    • Oct 2020
                    • 160

                    #10
                    Ok so this is where my complete lack of knowledge of OpenTherm (and perhaps naivety about evohome) shows. I had assumed, probably wrongly, that if you were using OT with evohome, then evohome would boost the max flow temperature if DHW was being call for.

                    Comment

                    • kevinsmart
                      Automated Home Ninja
                      • Sep 2018
                      • 257

                      #11
                      Originally posted by lloyd View Post
                      Ok so this is where my complete lack of knowledge of OpenTherm (and perhaps naivety about evohome) shows. I had assumed, probably wrongly, that if you were using OT with evohome, then evohome would boost the max flow temperature if DHW was being call for.
                      Ideally Evohome would allow you to set different max CH and DHW OpenTherm temperatures, but unfortunately it doesn’t. And with some boilers the default 80C or 90C is too high.

                      Comment

                      • sandyman
                        Automated Home Sr Member
                        • Mar 2016
                        • 85

                        #12
                        I use this x-plan setup. My heat-only viessman boiler will not produce the high flow temps needed to heat a DHW cylinder unless its "DHW demand" terminal has voltage applied. without that , the flow temp is in the 50's.
                        So, the evohome HW BDR is wired to open the DHW zone valve. The end-terminal of the DHW zone valve is wired to close the (normally open) CH zone valve and activate the DHW demand mode on the boiler giving 79C flow. Result = 79C water going only to the cylinder. downside is heating circuit has no flow for the reheat time so timing is key.

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