OpenTherm control behaviour

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  • kimber.kimber
    Automated Home Sr Member
    • Jan 2017
    • 89

    OpenTherm control behaviour

    Morning all!

    So I finally installed the evohome system yesterday. TRVs on all rads, set up as per the Honeywell diagram with a two port switching valve for hot water only, and an ABV. I'm using a Ideal Vogue 32 system boiler, with opentherm control.

    After some initial issues with binding, I completed a factory reset and cleared the binding in all of the hr92s. System seemed to fire up properly, the rads got nice a hot, and the TRVs shut off as they should.

    However, looking at the boiler this morning, it's stating a 20% efficiency, which surprised me somewhat! I wanted to check a couple of details to see what people think.

    When there's no heat demand, the pump runs for quite a while, which I think I was expecting, but also the boiler intermittently fires for short bursts. I've taken a couple of photos of the boiler graphs to show what I mean. I've tried reducing the boiler temp to move it towards the condensing point, bit that has has no effect.

    I should point out that currently not alll of the rads are plumbed in, and the hot water isn't switched over from the Aga, so that technically the boiler is oversized for the system as it stands today, but I wouldn't have expected that lower efficiency.

    Thanks in advance.

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    Last edited by kimber.kimber; 14 April 2017, 12:43 PM.
  • kimber.kimber
    Automated Home Sr Member
    • Jan 2017
    • 89

    #2
    So for example, now non of the zones are calling for heat, yet the boiler pump is running and then intermittently cycling the flame as per the pictures abo e.

    On of the rooms is near its 16 degree set point, but we've just checked the rad and it's cold, so it can't be requesting heat.

    Pipe after the auto bypass is warm, so it must be running through that.

    Comment

    • paulockenden
      Automated Home Legend
      • Apr 2015
      • 1719

      #3
      Something is calling for heat. It's a real pity that Evohome doesn't let you know what's causing it.

      The only solution is Domoticz with an HGI80 (or equivalent) to see the lower level data.

      Or, I suppose, there could be a problem with the OpenTherm/Boiler interface.

      Comment

      • kimber.kimber
        Automated Home Sr Member
        • Jan 2017
        • 89

        #4
        Thanks Paul. Is it worth trying another factory reset and rebind? Just wondering if it's hanging on to something wrongly. But I'm pretty sure I managed it right last time round.

        Comment

        • StephenC
          Automated Home Guru
          • Feb 2017
          • 102

          #5
          Originally posted by kimber.kimber View Post
          So for example, now non of the zones are calling for heat, yet the boiler pump is running and then intermittently cycling the flame as per the pictures abo e.

          On of the rooms is near its 16 degree set point, but we've just checked the rad and it's cold, so it can't be requesting heat.

          Pipe after the auto bypass is warm, so it must be running through that.
          Interesting - I get that as well. With Opentherm connected, the pump in my Viessmann 111-W storage combi runs for a long time when it finally does ramp down the flow temp.

          I have a towel rail that stays warm just as you say (equivalent to a auto bypass pipe) with no other radiators with HR92 warm at all.

          It's almost as if one zone/HR92 is calling for a minimal 1%-10% heat, but not opening the valve enough to actually let any water through!

          Didn't ever see this with BR91 Relay that i know of. Could this be radiator balance issues, even slight?
          Last edited by StephenC; 14 April 2017, 01:22 PM.

          Comment

          • Alan C
            Automated Home Jr Member
            • Apr 2017
            • 10

            #6
            There was a post in another thread from someone who works for Honeywell that may help. Although we have no method of seeing which zone is calling for heat, I understand that Honeywell's technical section can see this through their connection to the Evohome (assuming you have connected it to Wi-Fi).

            Regards,

            Alan.

            Comment

            • kimber.kimber
              Automated Home Sr Member
              • Jan 2017
              • 89

              #7
              Originally posted by StephenC View Post
              Interesting - I get that as well. With Opentherm connected, the pump in my Viessmann 111-W storage combi runs for a long time when it finally does ramp down the flow temp.

              I have a towel rail that stays warm just as you say (equivalent to a auto bypass pipe) with no other radiators with HR92 warm at all.

              It's almost as if one zone/HR92 is calling for a minimal 1%-10% heat, but not opening the valve enough to actually let any water through!

              Didn't ever see this with BR91 Relay that i know of. Could this be radiator balance issues, even slight?


              Interesting thought on the balancing. The rads haven't been balanced yet, due to the few more rads that need adding. Unfortunately it's going to be about a week before they are added into the system.

              Comment

              • kimber.kimber
                Automated Home Sr Member
                • Jan 2017
                • 89

                #8
                Originally posted by Alan C View Post
                There was a post in another thread from someone who works for Honeywell that may help. Although we have no method of seeing which zone is calling for heat, I understand that Honeywell's technical section can see this through their connection to the Evohome (assuming you have connected it to Wi-Fi).

                Regards,

                Alan.
                I might put a quick call into Honeywell on Tuesday to see what they say. Interestingly, the efficiency has now gone up to a whopping 33%!

                Comment

                • kimber.kimber
                  Automated Home Sr Member
                  • Jan 2017
                  • 89

                  #9
                  Just one thought. Does the main controller need to be configured on one of the zones? At the moment, it's not, and HR92s are doing the control in all zones.

                  Comment

                  • DBMandrake
                    Automated Home Legend
                    • Sep 2014
                    • 2361

                    #10
                    Originally posted by kimber.kimber View Post
                    However, looking at the boiler this morning, it's stating a 20% efficiency, which surprised me somewhat! I wanted to check a couple of details to see what people think.
                    Well, how exactly is the boiler determining an efficiency of "20%" ? 20% of what ? Without knowing precisely what this is trying to measure I think you'll find its a red herring.
                    When there's no heat demand, the pump runs for quite a while, which I think I was expecting, but also the boiler intermittently fires for short bursts. I've taken a couple of photos of the boiler graphs to show what I mean. I've tried reducing the boiler temp to move it towards the condensing point, bit that has has no effect.
                    As Paul says, if the boiler fires for short bursts there is still some demand from one or more zones. How are you determining that there is really no demand ?

                    Are you familiar with how TPI works ? If not have a quick read about it as it will help to understand what is probably happening. On a TPI system just because zone temperatures may be up to temperature (or even slightly above) does not mean there is no demand. There is a +/- 1.5 degree proportional band around each zone's set point where there is potentially some demand, even if it is only 10%. (Which would result in the boiler firing for 1 minute out of 10 minutes)

                    There is also some biased rounding of the measured temperature for display, so even though you might have a room set to 20 degrees and the system might report a measured temperature of 20 degrees, in reality it may only be 19.4 degrees for example, because the measurement (for display purposes only) is biased towards the set point by up to 0.5 degrees and then rounded to the nearest 0.5 degrees. (Thus saying 20 in the above example) In that situation there is definitely demand as the temperature is 0.6 degrees below the set point, even though you don't realise that's the case.

                    Currently the only way to see the true measured temperatures without biasing towards the set point and rounding is to monitor the system using Domoticz, which as Paul says will also allow you to see the normally invisible "heat demand" request from each HR92, which is what actually calls for the boiler to fire.

                    If you adjust all the set points so they are at least 2 degrees below the current measured temperatures and wait a few minutes you should find the boiler goes off and stays off. If that is the case most likely the system is working normally. Unless the weather is really warm its normal for the boiler to run intermittently to maintain the set temperatures as a small amount of heat must be put into the house to equal the heat lost just to maintain the status quo.
                    Last edited by DBMandrake; 14 April 2017, 02:44 PM.

                    Comment

                    • DBMandrake
                      Automated Home Legend
                      • Sep 2014
                      • 2361

                      #11
                      Originally posted by StephenC View Post
                      Interesting - I get that as well. With Opentherm connected, the pump in my Viessmann 111-W storage combi runs for a long time when it finally does ramp down the flow temp.

                      I have a towel rail that stays warm just as you say (equivalent to a auto bypass pipe) with no other radiators with HR92 warm at all.

                      It's almost as if one zone/HR92 is calling for a minimal 1%-10% heat, but not opening the valve enough to actually let any water through!

                      Didn't ever see this with BR91 Relay that i know of. Could this be radiator balance issues, even slight?
                      It's probably related to the pin calibration of your TRV valve bodies, and the "minimum on time" setting which you get with a BDR91 but not with the OpenTherm bridge. I have seen similar symptoms (all radiators cold but the occasional cycling on of the boiler) on rare occasions even on a BDR91.

                      What I believe happens is this:

                      The HR92 has a certain range of pin travel when it opens and closes the valve. When you fit it to the valve it does a calibration where it pushes the pin down hard until a certain amount of force is reached and this is calibrated as the fully closed position. (0% on the reported valve position in option 10 in the menu) It then opens the valve a certain number of turns of the gear inside the HR92 (a greater number of turns in full stroke 1) or until it hits the limit stop. This gets calibrated as 100%.

                      This percentage is a proportional indication of how far the pin is pushed down or released with 0% being pushed right down. The HR92 sends this valve position percentage to the controller as a "heat demand" percentage which the controller then uses to determine whether there is sufficient demand to warrant firing the boiler, and if so by how much. (TPI duty cycle using a BDR91 or flow temperature using OpenTherm)

                      Here's the underlying issue - the HR92 doesn't really know for sure at what point of pin travel the valve actually starts letting water flow. In the fully closed position the rubber washer in the valve will be compressed. As the HR92 unwinds and lets the pin move out quite a lot of travel will happen where the washer is decompressing but still sealing the valve, therefore no water will flow.

                      From my observations it seems to be typical (at least on MY valves) that the pin will reach about a 30-40% reported opening (based on total calibrated pin travel) before any water starts to flow at all, and typically will be fully flowing by around 80%. Because of this the controller seems to be programmed to ignore heat demands of less than about 40% and won't fire the boiler until at least one zone exceeds this.

                      However what happens if due to valve pin dimensions, washer thickness, valve stickiness etc your valve doesn't let water start flowing until "60% open" ? It could very well be that the heat demand is sufficient to fire the boiler intermittently while the valve is not actually letting any water flow. And the system has no real way of knowing this. As far as it is concerned the HR92 is open enough that there must be some flow through that radiator, but there isn't.

                      If you use a BDR91 there is an additional factor - the minimum on time setting. By default its 1 minute (and can't be set any lower) and for the default 10 minute cycles it means that even if the controller thinks a valve is open enough to warrant firing the boiler, it won't do so until the required output from the boiler is at least 10%, and would thus exceed the minimum on time. So this would tend to avoid any brief firings where it might think a valve was slightly flowing when it actually wasn't.

                      With OpenTherm I don't think there is any minimum on time or minimum demand, so it would tend to try to fire the boiler with a very low flow temperature. (30 ??) This would bring the boiler on for the short time needed to get the flow up to 30, and probably leave the pump running a lot.

                      What can you do about this if it is firing a lot when radiators are not flowing ?

                      The first thing I would do is remove each HR92, unwind the black wheel and refit the HR92 to allow it to re-calibrate itself. Then give it a few days for the valves to fine tune their calibration.

                      If that doesn't help, next time you see the boiler intermittently firing when all radiators are cold I would nip around each HR92 and go into the settings, look at option 10 (valve position) and record what percentage it believes the valve is open. If you have a figure significantly higher than 40% and the boiler is heating up and the radiator is stone cold I would be suspicious of that valve body.

                      You could try enabling full stroke 1 mode in the HR92 settings, which increases the amount of torque to close the valve and also increases the number of turns of the motor between closed and open. The disadvantage of this mode is the batteries don't last as long and there is more tendency to get temperature overshoots, however on a couple of my valves I've been forced to use full stroke 1 mode.

                      The valve could also be sticky (causing hysteresis in the opening point depending on whether the valve is closing or opening) or it may not be 100% compatible with the HR92's due to for example a pin length that is a bit longer than average. Or if you are using an adaptor the adaptor might not be sitting right.
                      Last edited by DBMandrake; 14 April 2017, 03:14 PM.

                      Comment

                      • kimber.kimber
                        Automated Home Sr Member
                        • Jan 2017
                        • 89

                        #12
                        Cheers for this. I've literally just gone through my schedule and changed the low point from 16 to 5 degrees. I'm going to see if this stops the shuttling.

                        I was guessing that the efficiency was going off times etc etc. But your right, it could just be something for the end user to cooo over! Like I've just been doing.

                        I see how it goes with the reduced temps and report back. Maybe the low point just wasn't low enough.

                        Comment

                        • kimber.kimber
                          Automated Home Sr Member
                          • Jan 2017
                          • 89

                          #13
                          That's loads of information on the valves too! Will definitely have a look at that. Is there a way to force them to do their calibration again? I've literally had a whole new heating system, boiler, tank, rads and pipe work, so it shouldn't be sticking valves. I fitted all new Honeywell Valencia valves at the point of instal.

                          Comment

                          • DBMandrake
                            Automated Home Legend
                            • Sep 2014
                            • 2361

                            #14
                            Just to be clear, my second post was replying to StephenC's problem of the boiler firing while the radiators stay cold - that's not what's happening to you is it ?

                            I got the impression that your boiler is firing when you think it shouldn't be but that the radiators are slightly warm ?

                            Comment

                            • HenGus
                              Automated Home Legend
                              • May 2014
                              • 1001

                              #15
                              A week in, I have noticed that the HR92s cycle a lot less frequently under OT control than was the case with the BDR. My boiler seems to 'slow' down as zones reach set temperatures whilst still maintaining a high flow temperature. Only when all the zones are at their set temperatures do I see the temperature on the boiler fall back to a steady condensing flow temperature. (This was described this to me as Evohome OT treating heating as a single zone - presumably, doing all the sums before communicating with the boiler ). Last night, the boiler was ticking over at 54/44 for a while. I have not heard the boiler cycling when the house is warmer than the set temperatures, as was the case with my old boiler under BDR control.

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