New home, mammoth Evohome install

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  • kevinsmart
    Automated Home Ninja
    • Sep 2018
    • 257

    #31
    Some other updates.

    I spoke with Vokera about disabling the DHW preheat that was happening every hour over night even though I am using as a system boiler. Moving Jumper tag 5 to 6 disabled this.

    I’m experimenting with overriding the Evohome low load control algorithm on the basis that the HCC80s employ TPI anyway.
    Short term 10C requests are now substituted with a minimum control setpoint. That’s 40C on my boiler.

    Updated:

    But just overriding the 10C request is not enough. The maximum relative modulation is set to 0 in some cases stopping the burner. I’m overriding this too.

    Also Evohome disables central heating along with the 10C request, that needs to be overridden for the short term low load 10C setpoints.
    Last edited by kevinsmart; 30 January 2021, 09:02 PM.

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    • kevinsmart
      Automated Home Ninja
      • Sep 2018
      • 257

      #32
      My low load control workaround appears to be an improvement, less burner on/off cycles.

      Yes, eventually the boiler will anti-cycle but less than before, so I think it’s more efficient.

      Comment

      • PJB
        Automated Home Jr Member
        • Feb 2020
        • 16

        #33
        Originally posted by kevinsmart View Post
        My low load control workaround appears to be an improvement, less burner on/off cycles.

        Yes, eventually the boiler will anti-cycle but less than before, so I think it’s more efficient.
        I implemented something similar, inspired by your comments in the 'Evohome & Opentherm' thread.

        IMG_6077.jpg

        Blue line is the Evohome setpoint, complete with low load control CS=10 requests. Red line is the modified request that the boiler receives via the OTGW. I'm letting the boiler work out what it needs to do.

        As you say, Evohome seems to send some max modulation commands as well as the setpoint requests. I pretty sure by boiler (ATAG iC27) ignores them, which is just as well since they don't seem to coincide much with the heating demands. Hey ho. I overrode them anyway.

        Comment

        • kevinsmart
          Automated Home Ninja
          • Sep 2018
          • 257

          #34
          That’s great! How long has your tweak been running, are you happy with the results? Any overshoots?

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            #35
            Well done. You guys will soon be writing your own stand alone OTB code. In fact this was something I had planned to do. But I wanted to implement an OT to eBUS interface, which didn't need the VR33 and was a better implementation of the OTB. The sudden drops to 10C followed by the sudden rise to 90C make no sense to me at all. And they don't even appear to match a sudden heat demand surge too.

            Comment

            • PJB
              Automated Home Jr Member
              • Feb 2020
              • 16

              #36
              Originally posted by kevinsmart View Post
              That’s great! How long has your tweak been running, are you happy with the results? Any overshoots?
              Thanks. I’ve only had it working for a couple of days but so far so good. It took me a while to work out what the otgw was doing so I had a few false starts. I'm using a modified py-otgw-mqtt rather than the Home Assistant otgw integration. More moving parts but it’s allowed me to work out what’s going on.

              Regard overshoot .. no, no appreciable difference that I can see.

              Comment

              • PJB
                Automated Home Jr Member
                • Feb 2020
                • 16

                #37
                Originally posted by bruce_miranda View Post
                Well done. You guys will soon be writing your own stand alone OTB code. In fact this was something I had planned to do. But I wanted to implement an OT to eBUS interface, which didn't need the VR33 and was a better implementation of the OTB. The sudden drops to 10C followed by the sudden rise to 90C make no sense to me at all. And they don't even appear to match a sudden heat demand surge too.
                Thanks. Agreed, those CS=10 dips were crying out to be squashed. My only concern now is that I need otgw / py-otgw-mqtt & Home Assistant (all on raspi) to run flawlessly 24/7. Not sure how I feel about that, although the setup's been working very reliably for at least a year in a monitor only mode.

                Comment

                • kevinsmart
                  Automated Home Ninja
                  • Sep 2018
                  • 257

                  #38
                  Originally posted by PJB View Post
                  Thanks. Agreed, those CS=10 dips were crying out to be squashed. My only concern now is that I need otgw / py-otgw-mqtt & Home Assistant (all on raspi) to run flawlessly 24/7. Not sure how I feel about that, although the setup's been working very reliably for at least a year in a monitor only mode.
                  Yes, that’s my concern now too, I am reliant on the OTGW for OpenTherm to work properly. Without the OTGW, Evohome would be attempting to heat my UFH to 80C, as that is what my boiler reports as the Max CH Setpoint.

                  Should be a simple fix, and it has been requested before, for Evohome to specify the max DHW and CH temperatures.

                  I might order another OTGW, just in case it fails.

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    #39
                    I have never liked the idea of my DIY OTGW module sitting in the critical path. The eBUS monitor on the other hand sits to the side and even if the power to it fails, nothing stops.

                    Comment

                    • kevinsmart
                      Automated Home Ninja
                      • Sep 2018
                      • 257

                      #40
                      I can now see how sub-optimum Evohome’s OpenTherm low load control is.

                      Basically it appears to be invoked if the burner is observed to go out.

                      But this can naturally happen when, through falling demand, the control set point temperature is reduced.

                      Example with my boiler, that has +/-5C set point hysteresis:

                      Current 48C setpoint (70% demand) reduces to 40C (50% demand).

                      Current 48C flow temperature is above 40C demand + 5C upper limit so burner off.

                      Burner off observed, Evohome thinks the boiler can’t modulate lower so switches to periodic 10C off/40C on behaviour.

                      The reality that I can see with my workaround, the boiler is able to easily maintain the 40C request after one 3-minute anti-cycle delay.

                      Comment

                      • kevinsmart
                        Automated Home Ninja
                        • Sep 2018
                        • 257

                        #41
                        I learned that my boiler’s floor heating jumper tag setting is actually dynamically read.

                        If the jumper is inserted, OpenTherm will switch from a 40-80C range to 20-45C.

                        So, I’m going to connect an Arduino relay to the OTGW GPIO. When there is a demand for less than 40C, I’ll program the OTGW to close the relay.

                        Will be interesting to see if this reduces some cycling when zones are near their set points.
                        Last edited by kevinsmart; 6 February 2021, 04:37 PM.

                        Comment

                        • bruce_miranda
                          Automated Home Legend
                          • Jul 2014
                          • 2307

                          #42
                          What boiler are you running Kevin?

                          Comment

                          • kevinsmart
                            Automated Home Ninja
                            • Sep 2018
                            • 257

                            #43
                            Originally posted by bruce_miranda View Post
                            What boiler are you running Kevin?
                            Vokera Unica, and LPG, unfortunately Bruce. Will probably make do with it until the time is right to fit a GSHP.

                            Comment

                            • kevinsmart
                              Automated Home Ninja
                              • Sep 2018
                              • 257

                              #44
                              I’ve added my OTGW controlled Arduino relay to switch my boiler between standard heating (40-60C) and floor heating (20-39C) modes. Appears to work well.

                              Conveniently the OTGW has 4 pins above the PIC: GND, GPIO2, GPIO1, 5V perfect for this application. I added a 4-pin header cable to control the relay and a 2 pin male from the relay to bridge the boiler’s jumper.

                              So, I think that’s the install all done now, will monitor how it runs.

                              Comment

                              • kevinsmart
                                Automated Home Ninja
                                • Sep 2018
                                • 257

                                #45
                                So it turns out, after all that, that floor heating mode doesn’t work very well for my setup.

                                In this mode the boiler appears to reduce its hysteresis from +5/-5 to +2/-2C.

                                What this means, with low temperature requests, is that the boiler ignites and immediately exceeds its upper temperature limit, shutting down, anti-cycling for 3 minutes, and repeating.

                                So, I gave up on that idea.

                                Now I’m taking advantage of my Vokera boiler supporting reading of its return water temperature, unlike Intergas.

                                If the return water temperature exceeds the control setpoint I set CH mode off until it reduces.

                                So now rather than my boiler running for hours at 40C with OpenTherm requests < 30C, it will periodically heat and then turn off earlier.

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