Opentherm + EvoHome + Viessmann 111-W overshoots

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  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    Yeah, I'm probably over sensitive when it comes to system with two control inputs.

    It all goes back to when I worked for a large Dutch electrical manufacturer (you can probably guess who) in the '80s. There was a large server room there, with separate climate control systems at each end of the room. Both were set to the same temperature, but the thermostat on one was a few degrees out, so these two HUGE machines just spent their time fighting each other. One trying to cool the room by one degree but never getting there, and one trying to heat it by one degree...

    Since then I've always been nervous of any system that has two uncoupled controls driving it.

    Comment

    • blowlamp
      Automated Home Sr Member
      • Apr 2017
      • 98

      Starting with an unheated bedroom measuring at 16 degrees, I raised the TRV setpoint to 19 degrees and closed the door.
      About 90 minutes later, the room temperature was just on 18 degrees & starting to feel more cosy.

      The iSense controller has 6 strategies to call upon and tune to one's liking including an option that ignores
      weather compensation & utilises measured room temperature only.
      I selected said option and checked the requested flow setpoint which was higher than 'usual' @ about 46 degrees.


      Martin.

      Comment

      • HenGus
        Automated Home Legend
        • May 2014
        • 1001

        Originally posted by StephenC View Post
        I can't see one on my Viessmann Vitodens 111-W for info.
        I'm expecting if it was on the boiler to be on the outside or near the OT connections to be clear, so nothing obvious that I can see.
        There is no OT logo on the Atag boiler or in the accompanying documentation other than a reference in the wiring diagram to:

        Bus = Opentherm Controller/room thermostat (when used with Atag 3 port diverter valve and One Controller Kit) with a warning not to connect 230Volt.

        Comment

        • StephenC
          Automated Home Guru
          • Feb 2017
          • 102

          Originally posted by blowlamp View Post
          Starting with an unheated bedroom measuring at 16 degrees, I raised the TRV setpoint to 19 degrees and closed the door.
          About 90 minutes later, the room temperature was just on 18 degrees & starting to feel more cosy.

          The iSense controller has 6 strategies to call upon and tune to one's liking including an option that ignores
          weather compensation & utilises measured room temperature only.
          I selected said option and checked the requested flow setpoint which was higher than 'usual' @ about 46 degrees.


          Martin.
          Hi Martin,

          That's interesting. Thank you for the comparison system info.


          With your weather comp "off", it looks to be essentially the same method as EvoHome (room sensing).

          It does look to be a different algorithm used over Opentherm protocol (that's expected as it would be their own), and indeed very unlike Honeywell's design choice to prioritise fast room warmup above all else (max ch flow temp's requsted where actual room temps are measured 1.5 degrees c below target set point temps)


          You'll not likely see overshoots with your setup I'd expect as the set point is approached gently. Is that fair? Are you happy with those warmup times?

          What is your boilers max ch flow temp configured as?
          Last edited by StephenC; 3 May 2017, 09:08 PM.

          Comment

          • blowlamp
            Automated Home Sr Member
            • Apr 2017
            • 98

            I've never really had to consider warm up times as the system more or less takes care of itself.
            The 90 minute warm up was reasonable when I think of how the room felt before, so no complaint from me there.

            As for overshoots, well they happen occasionally - usually ~ 0.5 degree - and seem to rectify pretty quickly.

            Max flow temperature is set to 70 degrees but it's never been near that to my knowledge.

            The Honeywell overshoots under OpenTherm are a bit strange, I must say.
            I kind of wonder why the proportional band is only +/- 1.5 degrees before proper modulation kicks in after what seems like pulsing its way to that point. Maybe a definable (wider) proportional band would help with systems such as yours to reduce the pulses & increase modulation? All guesswork on my part though.



            Martin.

            Comment

            • blowlamp
              Automated Home Sr Member
              • Apr 2017
              • 98

              This document is quite interesting. In-situ monitoring of efficiencies of
              condensing boilers – TPI control project extension


              Martin.

              Comment

              • Stefo
                Automated Home Lurker
                • Oct 2018
                • 2

                SHAME on you, Viessmann!

                I bought this overpriced Viesmann 111-W with the same HoneyWell Evohome with 5 HR92 radiator valve controllers. The seller didn't know about the problem, although I bought everything from one seller. After the Viessmann combi boiler was installed, the guy who had to connect the Honeywell switch showed me this forum thread!!!
                I am angry now, because a year and a half after start of this forum thread there is still no resolution/workaround. The Viessmann guy that I talked with didn't know about this, I still don't have problems because the boiler is not fully connected, but of course he replied that the boiler probably will be working "as expected" and this isnt' a reason to return back the boiler.
                Now I can only return back the OpenTherm relay and I use the on/off relay, because nobody wants to have 80 degrees celsius hot radiators!
                Heating with natural gas is something new in my neighbourhood and no one from my neighbours/friends knew about the systems and smart solutions, so I had to be the first one that faced with the problems. Now I am sure that when my neighbours ask me what to recommend, the Viessmann definitely will be something to run away from ...

                Comment

                • bruce_miranda
                  Automated Home Legend
                  • Jul 2014
                  • 2307

                  Yes, I agree with the sentiments about Viessmann, especially when this is such an easy fix. Allowing the boiler to ramp up all the way to it's physical max of +80C is just nuts.

                  I have a Vaillant, and there is plenty wrong with that boiler but at least I have control on the Max.

                  There is a way you can resolve this, if you are determined to use the Honeywell kit. But it involves sticking an OT gateway in between the boiler and the Honeywell OT Bridge. What that does is alters the Honeywell message to provide a Max flow OT message that the boiler is looking for, but one that Honeywell doesn't send, as it's optional in the OT spec.

                  Comment

                  • blowlamp
                    Automated Home Sr Member
                    • Apr 2017
                    • 98

                    Originally posted by bruce_miranda View Post
                    Yes, I agree with the sentiments about Viessmann, especially when this is such an easy fix. Allowing the boiler to ramp up all the way to it's physical max of +80C is just nuts.

                    I have a Vaillant, and there is plenty wrong with that boiler but at least I have control on the Max.

                    There is a way you can resolve this, if you are determined to use the Honeywell kit. But it involves sticking an OT gateway in between the boiler and the Honeywell OT Bridge. What that does is alters the Honeywell message to provide a Max flow OT message that the boiler is looking for, but one that Honeywell doesn't send, as it's optional in the OT spec.


                    It's stupid that it's optional as it turns out in practice to be vital for correct operation.

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      Optional values, by definition, don't need sending. The boiler should use a default, if not specified. All other boilers do.

                      Comment

                      • blowlamp
                        Automated Home Sr Member
                        • Apr 2017
                        • 98

                        Originally posted by bruce_miranda View Post
                        Optional values, by definition, don't need sending. The boiler should use a default, if not specified. All other boilers do.
                        Someone made the wrong judgment about what was optional in Evohome OpenTherm. Once under OpenTherm control the Viessmann expects to be controlled by Evohome and when it isn't it defaults to maximum output - as do many other boilers when they're loosely controlled in this way.

                        I'd have expected Honeywell's flagship product to have ample OpenTherm instructions rather than the absolute minimum.

                        Comment

                        • paulockenden
                          Automated Home Legend
                          • Apr 2015
                          • 1719

                          Let's be honest, the OpenTherm spec is a bit crap.

                          Sometimes a heating control will have a minimal implementation, assuming the boiler will make sensible choices for the rest.
                          Sometimes a boiler will have a minimal implementation, assuming the heating control will make sensible choices for the rest.

                          The problem comes when these two meet!

                          Comment

                          • blowlamp
                            Automated Home Sr Member
                            • Apr 2017
                            • 98

                            Originally posted by paulockenden View Post
                            Let's be honest, the OpenTherm spec is a bit crap.

                            Sometimes a heating control will have a minimal implementation, assuming the boiler will make sensible choices for the rest.
                            Sometimes a boiler will have a minimal implementation, assuming the heating control will make sensible choices for the rest.

                            The problem comes when these two meet!

                            The protocol itself seems pretty good, it's mainly what's specified as being optional where problems arise.
                            Put bluntly, shouldn't it be obvious that any OpenTherm controller must be capable of limiting a boiler's maximum output, given that at its core its purpose is to manage flow temperture?

                            Comment

                            • Stefo
                              Automated Home Lurker
                              • Oct 2018
                              • 2

                              Originally posted by bruce_miranda View Post
                              ...There is a way you can resolve this, if you are determined to use the Honeywell kit. But it involves sticking an OT gateway in between the boiler and the Honeywell OT Bridge. What that does is alters the Honeywell message to provide a Max flow OT message that the boiler is looking for, but one that Honeywell doesn't send, as it's optional in the OT spec.
                              I am sorry for the stupid question, but is there something out of the box that I can get from somewhere to perform this gateway function?
                              I think that device as Raspberry Pi could do the magic, but someone has to have enough understanding about programming it for receiving/sending correct protocol messages from both sides, I have no such low-level expertise to do it myself

                              Comment

                              • HenGus
                                Automated Home Legend
                                • May 2014
                                • 1001

                                Originally posted by blowlamp View Post
                                This is the latest piece of research from BRE:




                                Probably not relevant to the OT debate, but it does show current thinking on heating controls.

                                Comment

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