Opentherm and S Plan with Evohome HW priority

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  • mtmcgavock
    Automated Home Legend
    • Mar 2017
    • 507

    #16
    Originally posted by bruce_miranda View Post
    Yes I use the boiler's pump output to run both the pump as well as open the CH zone valve in my S plan. I don't use a CH BDR.
    Doesn't a system boiler have an auxiliary pump output. What if you need to run a secondary pump in tandem?
    No it doesn't. Some of the larger models do, but if you wanted to use an additional pump you'd just usually wire it to the S/L from the motorised valves.

    It manually open should be fine for now. There usually isn't a large demand in one go and it seems to be working fine. Just need a solution to give us HW priority back.

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    • MajorDanby
      Automated Home Jr Member
      • Jan 2019
      • 44

      #17
      Pretty sure it does, I’ll double check tomorrow for you

      Comment

      • MajorDanby
        Automated Home Jr Member
        • Jan 2019
        • 44

        #18
        So for the last hour my system has been sitting at 11% demand. In that time the boiler has been alternating between being off and modulating down to a flow temperature of 37 degrees. Regarding the CH valve this has been both open and closed during that period, although it has certainly been closed and cycling the water through the bypass longer than it has been open

        Comment

        • bruce_miranda
          Automated Home Legend
          • Jul 2014
          • 2307

          #19
          When you say Boiler has been OFF with 11% heat demand? Do you mean the boiler isn't firing? Because with OpenTherm the boiler should be ON with absolutely any heat demand, however small.

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          • MajorDanby
            Automated Home Jr Member
            • Jan 2019
            • 44

            #20
            Poor choice of language Bruce, firing up to 37 degrees and then just running the pump. Boiler remains on. I would say that over the hour period the BDR was opening the CH valve for maybe 15-20 mins

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            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              #21
              I still think that a CH valve on an OT system is unnecessary except for when the DHW switches on. That stops the sudden in-rush of HW into the CH circuit. Otherwise I think the TPI on the CH valve defeats the benefits of the OT modulation. This is another use case where I disagree with Honeywell's design decision to force TPI into every BDR. There should have been a simple On Off relay as a device. UFH, CH+OT would benefit from that.

              Comment

              • mtmcgavock
                Automated Home Legend
                • Mar 2017
                • 507

                #22
                Originally posted by MajorDanby View Post
                So for the last hour my system has been sitting at 11% demand. In that time the boiler has been alternating between being off and modulating down to a flow temperature of 37 degrees. Regarding the CH valve this has been both open and closed during that period, although it has certainly been closed and cycling the water through the bypass longer than it has been open
                Thanks, as I suspected. Glad it isn't just mine, just defeating the object of OT and HW priority features.

                Originally posted by bruce_miranda View Post
                I still think that a CH valve on an OT system is unnecessary except for when the DHW switches on. That stops the sudden in-rush of HW into the CH circuit. Otherwise I think the TPI on the CH valve defeats the benefits of the OT modulation. This is another use case where I disagree with Honeywell's design decision to force TPI into every BDR. There should have been a simple On Off relay as a device. UFH, CH+OT would benefit from that.
                Yes it does indeed. My boiler is firing with a low flow temp and with nowhere for it to go. I don't understand though, surely software based could just tell the CH BDR91 to stay open like the HW relay does. All they needed to do was to tell it a 100% demand whenever there is a OT demand like the HW BDR91 works.

                Comment

                • bruce_miranda
                  Automated Home Legend
                  • Jul 2014
                  • 2307

                  #23
                  That was the hack that I had programmed into Domoticz for when I was using a BDR91 for my UFH. For any Heat Demand greater than 0%, I was sending a 100% Heat Demsnd to the UFH BDR. That stopped it from doing TPI. Now I have moved to a HM80 and while that fixed some issues it highlighted others!

                  This is where pairing a BDR91 to multiple devices really comes handy. So I basically paired my UFH BDR twice I.e. Once with the controller as a Zone Valve for the zone and then with my HGI80 for my hack

                  Comment

                  • mtmcgavock
                    Automated Home Legend
                    • Mar 2017
                    • 507

                    #24
                    Ended up fitting a NO Motorised valve to sort the issue. Shame it couldn't be sorted software based, as in my eyes it's a relatively straight forward fix.

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      #25
                      I am trying to see if I can force the newer BDR91T to bind as a Heat Pump relay. Because that does a TPI cycle only once per hour.

                      Comment

                      • mtmcgavock
                        Automated Home Legend
                        • Mar 2017
                        • 507

                        #26
                        Originally posted by bruce_miranda View Post
                        I am trying to see if I can force the newer BDR91T to bind as a Heat Pump relay. Because that does a TPI cycle only once per hour.
                        Let me know how you go on.

                        In terms of savings, I think i've saved 30% in gas over the last week. Doing readings over a longer period to see how it goes.

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