OpenTherm control behaviour

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  • DBMandrake
    Automated Home Legend
    • Sep 2014
    • 2361

    #31
    Originally posted by StephenC View Post
    Ok, so have monitored very crudely via the onboard HR92 config/display the Valve Positions for a few radiators.

    What I am seeing is:

    For a couple of radiators, 0.5 degrees C setpoint increase higher than reported room temp is giving a valve position of 98% and 100% (2 radiators). Both old radiators and circa 10 year old valves. 1 has a danfoss adaptor for the HR92, the other HR92 is straight on.

    These radiators overshoot *consistently* as they always seem to get 81/82 degree ch flow temps through them via Opentherm which makes sense now based on valve position.

    I have set the stroke to 1 (was 0) and also re-calibrated he HR92 on one of these radiators to see if there is any difference.

    For another radiator, 0.5 degrees C setpoint increase higher than reported room temp is giving 64% and 1.0 degrees C giving 79%. This radiator is about 2 years old and the HR92 is directly fitted.

    This radiators overshoots *reguarly* as as it always seems to get high temp degree ch flow temps through it via Opentherm.


    After all, even 64% of 90 degrees C is 57.6 degrees, and 79% of 90 degreesC is 71.1 so still fairly high ch flow temps for such a small increase in temp request (note my boiler is not providing the Honeywell R8810 Opentherm bridge a max ch flow temp, therefore the issue I have is that the R8810 Opentherm Bridge makes an assumption of 90 degrees C from what we can tell)
    You misunderstand what the percentage is, it's a percentage of pin travel on the valve, not a percentage of water flow through the radiator, nor a percentage of flow temperature. There is a very non-linear relationship between the pin travel and the water flow, as well as the actual surface temperature of the radiator, which is part of the challenge for the HR92's heuristics to adapt to.

    The first X percent of the pin travel (typically 30-40%) is effectively "lost travel" because the valve remains non-flowing until some threshold of pin movement is reached. The water flow then starts to increase quite rapidly over only about 20% of the movement and then is typically flowing fully after another 20%.

    There is also a non linear relationship between the heat demand sent from controller to the BDR91/OpenTherm bridge, vs the heat demand sent from an HR92 to controller. For example a heat demand of 20% from an HR92 usually means the valve is still closed, whereas a 20% heat demand sent to a BDR91 would in fact fire the boiler for 20% of the time.

    My suggestion for checking the pin position was not in relation to overshoots, but in relation to what you were saying about the boiler cycling on when all radiators were cold. If you can hear the hiss from a radiator, try making 0.5 degree adjustments around the current temperature directly on the HR92 until you find the point where the valve JUST starts flowing slightly. Then check the valve position. If it's between about 30 and 40 that's fine, however if its say 60 or over then it would probably cause the boiler to come on when the valve is not quite open yet.
    So a newer/different valve and more appropriate valve open percentages does not seem to massively affect overshoots for me though.
    Overshoots are usually caused by oversized radiators or too high a flow temperature. In a time delayed negative feedback system like this if the gain is too high it will become unstable and overshoot/oscillate. Flow temperature and radiator size are both factors that affect the overall loop gain of the feedback loop from radiator to room to thermostat back to radiator valve.

    Reducing flow temperature to only what you need is the easiest solution but it only works if the radiator sizings in all rooms are correctly balanced with respect to other rooms, (otherwise if you need a high flow temp for a room with an under-performing radiator other rooms may overshoot) and if you tweak the flow temperature with changes in outdoor temperature. (Or let a weather compensation system do it for you) I tend to adjust my flow temperature based on the weather - about 60 in summer, 65 in spring/autumn or mild weather and 70-75 in winter, which is what works for my radiators.




    Now this is just a small sample and not very practical to keep monitoring other than spot checks.

    Of course if most radiators are within 0.5 degrees and valve slightly (or fully as some of mine seem to be) open, and then just one zone/HR92 calls for max heat, it's going to throw the other zones off by sending through significantly hotter water through than was requested, and expected. This will result in overshoots as I am seeing.
    Yes, but if your maximum flow temperature is too high you're always going to get overshoots, because there will always be times when some rooms are up to temperature and maintaining their temperature with a low average flow temperature, followed by a room scheduled to come on (like a bedroom warming up for bedtime) which will send the flow temperature to maximum and cause overshoots in other rooms.

    If your maximum flow temperature is appropriately set the overshoot will be minor (say 0.5 degrees) before the other HR92's figure out what happened and start throttling back, however if the flow temperature is way too high you may well see a 1-1.5 degree overshoot in other rooms caused by a room that was previously off being scheduled to come on to several degrees above it's current temperature.

    Comment

    • StephenC
      Automated Home Guru
      • Feb 2017
      • 102

      #32
      Originally posted by HenGus View Post
      If higher than anticipated boiler flow temperatures are detected then Evohome will adapt. My advice, after 3 years of Evohome ownership, is 'let Evohome do its own thing'. It learns and it settles down. I have never changed any of the HR92 parameters and I get approximately a 2 year life out the batteries. In the early days, if I thought that a given zone was a little too hot or cold for comfort, I just adjusted the set temperature for that zone. There are pages and pages on the accuracy of HR92s. Accuracy depends in part on the location of the HR92 within the zone and local obstacles. If the boiler is pushing out a flow at 80C, it doesn't follow that one or more radiators will overshoot: in my experience, you will just hear more individual HR92 motor activity. Opentherm seems to reduce HR92 activity which may in part be due to the boiler change and reducing boiler temperature flows.
      Note: I've already stated I'm moving back to BRD91 Relay control from Opentherm due to interoperability issues with Viessmann (no way of limiting max ch flow temp), and lack adaptability from EvoHome/HR92s. I am providing info back only at this point.


      However, I'm afraid your statements are not accurate from my own actual experience:

      1. EvoHome has not adapted the flow temp after approx 5 weeks Opentherm use that I can see. Mostly I see max ch flow temp. Not that often do I see lower. That's not a Viessmann issue. Others have also mentioned EvoHome seems to request max ch flow temp more often than appears necessary. If I do see lower flow temps, it's normally much much lower, and the pump runs for ages with no flame for very long periods of time (due to low heat loss and low flow temp)

      2. If the boiler is pushing out 80 degrees C, and the rad size and heat loss don't match (as in my case) the HR92's have no chance at all I've found to control that. Massive and consistent overshoots. They also don't seem to learn to request less heat (in theory lower temp).

      Again, to be clear, I am reverting back to the much better (for my system) BDR91 Relay control.

      We need to be mindful that not everyone's setup is the same, and that what works for one person (yours clearly and I am glad it does work for you), it doesn't always work the same elsewhere due to other factors/variables.

      I have found out the hard way, Opentherm theory and reality are not the same. I can now see why installers don't want to go near more complicated controls and Opentherm in particular. Just too problematic in many situations. But of course works fine in others....
      Last edited by StephenC; 15 April 2017, 09:49 AM.

      Comment

      • DBMandrake
        Automated Home Legend
        • Sep 2014
        • 2361

        #33
        Originally posted by StephenC View Post
        Note: I've already stated I'm moving back to BRD91 Relay control from Opentherm due to interoperability issues with Viessmann (no way of limiting max ch flow temp), and lack adaptability from EvoHome/HR92s. I am providing info back only at this point.
        Yep I'm aware of that.
        1. EvoHome has not adapted the flow temp after approx 5 weeks Opentherm use that I can see. Mostly I see max ch flow temp. Not that often do I see lower. That's not a Viessmann issue. Others have also mentioned EvoHome seems to request max ch flow temp more often than appears necessary. If I do see lower flow temps, it's normally much much lower, and the pump runs for ages with no flame for very long periods of time (due to low heat loss and low flow temp)
        Which is exactly why you need a manual maximum flow temperature adjustment. Evohome will only drop the flow temperature from maximum if all the zones are within the proportional band.

        I've said on multiple occasions in your threads that the HR92's would not adapt to an excessively high flow temperature (despite claims by others) and overshoots would be inevitable. I hate to be right but...
        2. If the boiler is pushing out 80 degrees C, and the rad size and heat loss don't match (as in my case) the HR92's have no chance at all I've found to control that. Massive and consistent overshoots. They also don't seem to learn to request less heat (in theory lower temp).
        Umm, that's exactly the point I've been making all along - if the flow temperature is too high overshoots are inevitable. We're on the same side of the argument here...
        We need to be mindful that not everyone's setup is the same, and that what works for one person (yours clearly and I am glad it does work for you), it doesn't always work the same elsewhere due to other factors/variables.

        I have found out the hard way, Opentherm theory and reality are not the same. I can now see why installers don't want to go near more complicated controls and Opentherm in particular. Just too problematic in many situations. But of course works fine in others....
        I don't use OpenTherm - I use a BDR91, (ancient boiler) and it works OK for me.

        I do have to reduce my flow temperature in warmer weather to avoid overshoots and overly hot radiators though, and as I don't have weather compensation (ancient boiler again!) I have to do this manually. If I leave the flow temp set high year round the control of room temperatures and comfort in general is not very good, hence why I like the idea of weather compensation even though I can't use it myself.

        (As I use an external digital flow temperature controller with an old boiler I've thought of replacing it with one that supports weather compensation, but nobody seems to make one! So I'd have to design one myself, probably based around an arduino, but that's a lot of work which I have little time for)

        Yes it's unfortunate that you can't get OpenTherm working well for you, but it's because your boiler won't allow you to set the maximum flow temperature and allows the Evohome to call for maximum possible flow temperature. This is never going to work well.

        For anyone who can adjust the maximum flow temperature with OpenTherm it should work well, and there are many reports from such people of it working very well for them and calling for rather low flow temperatures.

        In your case you have no real choice but to go back to a BDR91 until (or if) Viessmann get their act together. Unfortunate, but it's best to be pragmatic. It's what I'd do in the same circumstances.
        Last edited by DBMandrake; 15 April 2017, 11:40 AM.

        Comment

        • HenGus
          Automated Home Legend
          • May 2014
          • 1001

          #34
          Originally posted by DBMandrake View Post

          For anyone who can adjust the maximum flow temperature with OpenTherm it should work well, and there are many reports from such people of it working very well for them and calling for rather low flow temperatures.

          In your case you have no real choice but to go back to a BDR91 until (or if) Viessmann get their act together. Unfortunate, but it's best to be pragmatic. It's what I'd do in the same circumstances.
          And the above is exactly what I will do if I have any concerns about Opentherm/Evohome. I appreciate that with Spring now with us, it is not a good time to assess heating controls/boilers. However, as posted many times in various threads on this forum, Opentherm/Evohome does not play nicely with all Opentherm-compatible boilers. Posters with Ideal and Intergas boilers also have reported Opentherm issues; that said, I thought that the Viessmann 100 problem could be solved with a HW junction box? I admit that I do not have the necessary expertise to judge where the problem sits; however, simple logic would suggest that as Honeywell designed the Openthterm protocol then the problem most likely sits with the boiler manufacturers. It is interesting to note that in The Netherlands where Opentherm seems to be a requirement, there are not forums of unhappy home owners that I could find in my research.

          Clearly, this issue is more complicated than my non-engineering mind thought:

          Last edited by HenGus; 15 April 2017, 12:41 PM.

          Comment

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

            #35
            So I've managed to log in and check my gas usage for the first time after the system installation. Compared to the few days before we turned the old boiler off... its bloody gone up!!

            I don't get it! We've gone from a 70's gas guzzling boiler, undersized radiators, no TRVs and no thermostat, to a new efficient boiler, new properly sized radiators, zone heating (so some rooms not on) and TRVs! It doesn't really make sense!!!

            I hope this changes within a week or so of the learning phase happening, or there is going to be some particularly uncomfortable calls happening with Ideal and Honeywell...
            Last edited by kimber.kimber; 17 April 2017, 08:02 PM.

            Comment

            • HenGus
              Automated Home Legend
              • May 2014
              • 1001

              #36
              Originally posted by kimber.kimber View Post
              So I've managed to log in and check my gas usage for the first time after the system installation. Compared to the few days before we turned the old boiler off... its bloody gone up!!

              I don't get it! We've gone from a 70's gas guzzling boiler, undersized radiators, no TRVs and no thermostat, to a new efficient boiler, new properly sized radiators, zone heating (so some rooms not on) and TRVs! It doesn't really make sense!!!

              I hope this changes within a week or so of the learning phase happening, or there is going to be some particularly uncomfortable calls happening with Ideal and Honeywell...
              Three to 4 days isn't enough time for Evohome to get into its stride. I can assure you - three years on - that the problem doesn't sit with Evohome. With my old Ideal boiler, I was averaging gas usage savings of c.15%. Learning can take a couple of weeks or more. During the learning period, the HR92s tend to hunt around the set temperatures leading to extra heat demands etc.

              Comment

              • DBMandrake
                Automated Home Legend
                • Sep 2014
                • 2361

                #37
                Originally posted by HenGus View Post
                Three to 4 days isn't enough time for Evohome to get into its stride. I can assure you - three years on - that the problem doesn't sit with Evohome. With my old Ideal boiler, I was averaging gas usage savings of c.15%. Learning can take a couple of weeks or more. During the learning period, the HR92s tend to hunt around the set temperatures leading to extra heat demands etc.
                Also, what temperatures do you have scheduled at different times of the day for each room and just as importantly what night time set back do you have scheduled ?

                Better heating controls won't save you gas if you have your rooms set too hot or scheduled to be hot at times of the day when they don't need to be. Every extra degree higher uses a lot more gas, so your chosen schedule for each room will have a major impact on whether you save gas or use more gas than what you had before.

                My first winter of Evohome I used more gas than the previous winter!

                The main culprit turned out to be scheduling the downstairs rooms to 14 degrees at night which was causing the boiler to come on to keep those rooms warm unnecessarily. Lesson learnt and I now schedule everything except the bedrooms to 5 degrees at night (basically frost protection) and let optimal start figure out when to turn the rooms back on again to get them up to temperature in time for getting up.

                Another mistake was setting the bedroom too warm at night in a poorly insulated room. Very comfy but very gas wasting compared to putting on a warmer blanket as the boiler was running all night!

                Over time I've adjusted my set points so that rooms are no warmer than needed (just comfortable and no more) when occupied, and have greatly pared back the times that rooms are hot to only when really needed.

                One example - my Saturday schedule for the bathroom is 20 degrees at 7:30 which is warm for use in the morning, dropping to 18 at 10am and then dropping again to 16 at 8pm before dropping to 5 at 11pm. Yeah I could leave the bathroom at 20 degrees all day but it uses a lot more gas! Likewise spare rooms are only run at 16 degrees and for just a few hours a day to help prevent damp, and drop back to 5 at night like the rest of the house.

                During the work day when the house is empty all rooms are set back to 5 degrees so the boiler doesn't run at all.

                Think carefully about your pattern of room usage throughout the day and try to minimise on time for rooms when they are not likely to be occupied or in use by either setting them back a few degrees (if they might be used unexpectedly) or off (5) completely if they're not likely to be used. Make zoning work for you. For example after 8pm the only rooms in our house that are heating significantly are the living room, and in the late evening the bedroom is warming up. Other rooms like hall, bathroom and kitchen are set way back to 16 at 8pm before going off at 11pm.

                If you think you're using more gas than before I'd start by comparing your original schedule with your new room set points. The temptation is to have things warm and comfy throughout the house all the time but you certainly won't save any gas doing that.

                If I manually override most of my rooms to be 20 degrees for most of the day instead of following their more intricate and conservative schedules it nearly doubles the gas use according to my loop energy monitor! So zoning really does work to reduce consumption, as long as you make use of it. For zoning to work well you do need to try to keep doors closed as much as possible though.
                Last edited by DBMandrake; 17 April 2017, 09:24 PM.

                Comment

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

                  #38
                  Thanks for advance guys. I've worked pretty hard on the scheduling, although I know it's going to take a bit of adjustment over the next couple of weeks. TBH, this is one of the reasons I bought the system. I work an 8-5 job and the wife works in hospitality, so we have completely different patterns. TBH, 6 set point changes is a little limiting!

                  The below images show a couple of the schedules, for the lounge and the nursery. Ignore the name, it's where we're sleeping at the moment (little Kimber due end of June!). Spare rooms are all on a 15 degs permenantly, again to give them a little damp protection. I guess I'm just surprised that the gas is more considering how inefficient the old system was!

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                  Comment

                  • DBMandrake
                    Automated Home Legend
                    • Sep 2014
                    • 2361

                    #39
                    Originally posted by kimber.kimber View Post
                    Thanks for advance guys. I've worked pretty hard on the scheduling, although I know it's going to take a bit of adjustment over the next couple of weeks. TBH, this is one of the reasons I bought the system. I work an 8-5 job and the wife works in hospitality, so we have completely different patterns. TBH, 6 set point changes is a little limiting!
                    Yes I've hit the 6 set point limit myself and wished I had more.
                    The below images show a couple of the schedules, for the lounge and the nursery. Ignore the name, it's where we're sleeping at the moment (little Kimber due end of June!). Spare rooms are all on a 15 degs permenantly, again to give them a little damp protection. I guess I'm just surprised that the gas is more considering how inefficient the old system was!
                    Were your spare rooms on 24/7 with your old system ? I presume not, if it was just a standard ON/OFF timer for the whole system.

                    I'd start by changing your spare room schedules to only be 15 degrees when other rooms in the house are also scheduled to be on and the house is occupied, and 5 degrees in the night and during the work day, to make your spare room schedule more similar to your old system.

                    For example my spare rooms are set to 16 only from 6pm to 8pm weekdays, and from 7:30am to 8pm on weekends. At all other times 5 degrees. The room doesn't need to be warm 24/7 to avoid moisture build up and damp, as long as it warms up every day. If it did, everyone with old fashioned timers that turned the heating off completely at night and during the work day would have moisture problems...
                    Last edited by DBMandrake; 17 April 2017, 09:48 PM.

                    Comment

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

                      #40
                      Originally posted by DBMandrake View Post
                      Yes I've hit the 6 set point limit myself and wished I had more.

                      Were your spare rooms on 24/7 with your old system ? I presume not, if it was just a standard ON/OFF timer for the whole system.

                      I'd start by changing your spare room schedules to only be 15 degrees when other rooms in the house are also scheduled to be on and the house is occupied, and 5 degrees in the night and during the work day, to make your spare room schedule more similar to your old system.

                      For example my spare rooms are set to 16 only from 6pm to 8pm weekdays, and from 7:30am to 8pm on weekends. At all other times 5 degrees. The room doesn't need to be warm 24/7 to avoid moisture build up and damp, as long as it warms up every day. If it did, everyone with old fashioned timers that turned the heating off completely at night and during the work day would have moisture problems...
                      No the spare rooms were just set to come in and off with the rest of the house. I've actually just taken a look, and they are set at 10degs not 15. They rooms naturally hover at around 16/16.5 degrees, so I'll keep an eye and see how it goes.

                      TBH the house is pretty inefficient at the moment. Solid brick walls, single glazing etc. etc. We'll be doing a lot of work over the next few years to try and make it a little more efficient, without trying to screw anything else up!

                      I have a feeling the radiators may not help the learning process. In all the major rooms, we've installed cast radiators. Obviously they hold the heat for a long time, so I'm not sure if this will affect the way the TRVs are working.

                      I've not got the evohome base station set as a temp reading for any of the zones. Should I have?
                      Last edited by kimber.kimber; 17 April 2017, 10:12 PM.

                      Comment

                      • HenGus
                        Automated Home Legend
                        • May 2014
                        • 1001

                        #41
                        Originally posted by kimber.kimber View Post

                        I've not got the evohome base station set as a temp reading for any of the zones. Should I have?
                        It's your choice. I use the controller for my hall landing and stairs rather than one of the three HR92s in that zone.

                        Comment

                        • HenGus
                          Automated Home Legend
                          • May 2014
                          • 1001

                          #42
                          I continue to watch and listen to my new boiler. I would agree that when a zone is outwith the TPI range, the boiler flow temperature stays high - however, the pump speed drops off markedly as zones reach their set temperatures. The pump on my boiler seems to run continuously when any zone is within the TPI range. When all the zones are up to their set temperatures, the boiler temperature drops and the boiler gets firmly into the condensing mode. I had a boiler return temperature of 47C for most of yesterday afternoon when heating demands were low. The highest zone overshoot that I have seen is 1C.

                          By no means scientific, but my average daily cost has fallen by 25% compared to this time last year - and I increased the zone temperatures in my kitchen, hall and study by 2C.

                          Comment

                          • DBMandrake
                            Automated Home Legend
                            • Sep 2014
                            • 2361

                            #43
                            Originally posted by HenGus View Post
                            It's your choice. I use the controller for my hall landing and stairs rather than one of the three HR92s in that zone.
                            Using the controller as a remote sensor is more accurate than using an HR92, like any remote sensor, (although I found mine had to be calibrated to -1.0 degrees to match other thermostats) but it's only really usable if it stays on its mount and is not moved around or frequently handled.

                            Mine is mounted on the wall mount in the hallway and is left on the mount 99.9% of the time so I use it as a sensor, because why not. However if you take it out of the mount and handle it frequently or you use the desk mount I probably would not use it as a sensor as even a couple of minutes holding it will cause the temperature reading to skyrocket, leading to the radiator being turned off. Likewise if you took it off the desk mount and left it lying on the sofa for a while the reading would shoot up and falsely shut off the radiator. Or even moving its location from one part of the room to another would affect the comfort level for a given temperature reading.

                            Generally remote thermostats should be left in one location in the room. So if you think yours might end up being fairly mobile I wouldn't use it as a sensor, especially if it might end up being taken into a different room entirely!

                            Comment

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

                              #44
                              Cheers for the comments. Mines on the hall stand, as the wiring for the wall mount won't be run until we're ready to chase it and the box into the walls.

                              I use the app more than the pad, so it just sits happily in the stand, telling us the time!

                              I'm assuming if I want to set it as the temperature sensors for the hall, I just need to change the 'Temperature Sensor' from 'remote sensor' to 'evotouch sensor'.

                              Comment

                              • bruce_miranda
                                Automated Home Legend
                                • Jul 2014
                                • 2307

                                #45
                                Correct...

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