OpenTherm control behaviour

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  • bruce_miranda
    Automated Home Legend
    • Jul 2014
    • 2307

    I am waiting for the Heat demand implementation on the firmware to compare what the OT Bridge is sent. At the moment I dont think Domoticz is tracking the correct value, largely driven by the fact that Dan doesn't have an OT Bridge. My biggest worry is that there doesn't appear to be a great deal of RF chatter between the controller and the OT Bridge. It almost appears that the OT bridge reads all the messages flying around and decides what to do without the controller telling it. But am not entirely sure too.

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    • dty
      Automated Home Ninja
      • Aug 2016
      • 489

      I very much doubt that's happening. Given the relative location of a couple of my HR92s and my OT Bridge, I don't think they'll be able to hear each other.

      When you say "there doesn't appear to be a great deal of RF chatter", how are you gauging that? Remember that the firmware we're running on the radios doesn't parse OT messages very well, so you won't see them at all in the Evohome logs unless you're looking for parsing errors. (Oh, unless you're using an HGI80? I can't remember.)

      Comment

      • bruce_miranda
        Automated Home Legend
        • Jul 2014
        • 2307

        I have an HGI80.

        Comment

        • DBMandrake
          Automated Home Legend
          • Sep 2014
          • 2361

          Originally posted by bruce_miranda View Post
          I have an HGI80.
          If it works like a BDR91 then you'll only see a heat demand message sent to it every 20 minutes, or when the heat demand changes significantly. If your house is at a nice steady equilibrium there may be no need to send changes to the heat demand - in which case you'd see one message every 20 minutes repeating the same or very similar heat demand.

          Is this what you see ?

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          • paulockenden
            Automated Home Legend
            • Apr 2015
            • 1719

            I suspect that a BDR91 boiler relay and an OT Bridge are effectively working off the same signal from the controller.

            The only difference is how they deal with anything less than 100% heat demand. With the BDR91 it's by pulsing the boiler on and off, whilst with the OT bridge it's by passing the relative heat demand through to the boiler. The controller doesn't micro-manage either device.

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            • dty
              Automated Home Ninja
              • Aug 2016
              • 489

              Originally posted by paulockenden View Post
              I suspect that a BDR91 boiler relay and an OT Bridge are effectively working off the same signal from the controller.

              The only difference is how they deal with anything less than 100% heat demand. With the BDR91 it's by pulsing the boiler on and off, whilst with the OT bridge it's by passing the relative heat demand through to the boiler. The controller doesn't micro-manage either device.
              I suspect you're broadly right. I don't think it's the exact same message otherwise you wouldn't need to tell the controller whether you were using a BDR or OT.

              I really must find the time to figure out why my radio firmware can't receive the messages from the OT bridge (I suspect a timing issue), fix it and then get some OT messages over to DanD for analysis.

              Comment

              • fergie
                Automated Home Sr Member
                • Mar 2017
                • 92

                Could it be as simple as the BDR telling the boiler to turn on or off where as the OT tells it turn the temp up or down? If it’s not specifying an exact temp, would explain a few things.

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                • bruce_miranda
                  Automated Home Legend
                  • Jul 2014
                  • 2307

                  DanD has a lot of OT messages from me already which he has looked at. But there is definitely an issue with the implementation or interpretation of the OT Bridge and Domoticz. Currently my OT Bridge Heat Demand messages seem to track those of my Kitchen zone. Which is the only zone in the house which used a BDR91/TR87 combination for an UFH zone. The thing is I have a Boiler Device and an CH Valve Device too, but I have neither of those two as BDR91s. I suspect this is all wrapped up in the issue with the various heat demand messages from the BDR91 and how Domoticz is currently reading them.

                  Comment

                  • paulockenden
                    Automated Home Legend
                    • Apr 2015
                    • 1719

                    Originally posted by fergie View Post
                    Could it be as simple as the BDR telling the boiler to turn on or off where as the OT tells it turn the temp up or down? If it’s not specifying an exact temp, would explain a few things.
                    As well as specifying an absolute temperature, OT also allows you to set a boiler modulation from 0-100% (see 5.3.8.2). That would map directly into the heat demand percentage from the controller (which a BDR91 converts into a duty cycle). So maybe the bridge is doing that, rather than specifying an absolute temperature. Makes sense if so, because would be by far the easiest way to implement it.

                    P.

                    Comment

                    • dty
                      Automated Home Ninja
                      • Aug 2016
                      • 489

                      I really, *REALLY* must get round to building my OpenTherm gateway!

                      I'm 99% sure (from the eBus messages I see on my Vaillant boiler) that it just alters the flow temperature and leaves the boiler to figure out its own modulation.

                      Comment

                      • bruce_miranda
                        Automated Home Legend
                        • Jul 2014
                        • 2307

                        I did offer to build it for you , do some testing and then send it back to you pre-built.

                        OK I can confirm that the Boiler OT device in Domoticz is not correct. I'll take this away and discuss with Dan.

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