Why is the setpoint way over the set temperature?

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  • Woody
    Automated Home Jr Member
    • Feb 2015
    • 32

    Why is the setpoint way over the set temperature?

    As part of me catching up with the latest Evohome I have a question.

    Ever since installing the new Evohome controller I notice that one of my zones gets a higher setpoint than necessary. This zone is a hallway, with a small radiator, that is set for 18C during the day and evening, and 15C during the night.

    Every morning during warming-up the setpoint of this zone is set to 23C. Temperature then runs up quickly and the setpoint is adjusted to 18C. The temperature itself overshoots just a little, but I don't understand why Evohome selects a 23C setpoint when it only needs 18C. None of my other zones show this behavior, even when they have to warm up 2C they set the setpoint at the desired temperature.

    I already reinstalled the zone (deleted the zone, cleared the HR92 and re-learned it) but that did not change anything.

    Is this normal?

    setpoint overshoot.jpg
  • DBMandrake
    Automated Home Legend
    • Sep 2014
    • 2361

    #2
    A few questions -

    1) What firmware version is the controller running ? (Press settings then hold down Device Info for 20 seconds)

    2) How are you graphing the temperatures and set points ?

    3) Are you using Optimal Start ? (I'm assuming so)

    4) Have you changed the maximum zone temperature for that zone in the zone configuration to 23C ? (default is 35C)

    Comment

    • Woody
      Automated Home Jr Member
      • Feb 2015
      • 32

      #3
      Originally posted by DBMandrake View Post
      A few questions -
      1) AS: 02.00.19.31, WiFi: 02.00.17.00

      2) I'm graphing the temp data with a home automation system, Domoticz. This basically gets these temperatures every couple of minutes from Honeywell via my account and the API. The values are the same as the values displayed on the controller: temperature measured and temperature set per zone. Domoticz (running on an always-on RPi) charts this data per day, month and year which gives me a nice insight in the performance of Evohome.

      3) Yes, both optimal start and optimal stop are switched on

      4) Yes, Tmax is dialed down on all my zones to 23C. Which made me suspicious of seeing precisely that value being used by my hallway zone....


      Paul

      Comment

      • Woody
        Automated Home Jr Member
        • Feb 2015
        • 32

        #4
        Ah, seems I found an answer for this problem elsewhere in this forum. For some reason my brand-******** new Evohome controller has a beta FW version that exhibits this problem. Should be repaired in 19.33 so now I have to see how to lay my hands on that version. I read something about mailing someone to get that done. I'll look that up again and see if I can get this fixed.

        Comment

        • DBMandrake
          Automated Home Legend
          • Sep 2014
          • 2361

          #5
          Yep, known problem with the 19.31 beta, fixed in 19.33.

          A lot of people who didn't sign up for the beta test seem to have ended up with the first (quite buggy) beta on their system - some sort of error on Honeywell's part.

          Good news is that the second beta, 19.33 works very well, I've been very happy with it since my system updated to it, and apart from one minor niggle it feels very stable and polished.

          Comment

          • Woody
            Automated Home Jr Member
            • Feb 2015
            • 32

            #6
            I requested the beta with Honeywell. Let's see if that updates it to 19.33.

            Oh, and by the way, this forum seems to be more prudish than my grandmother; the intensifier I used to emphasize how new my Evohome is was replaced by a lot of asterisks :-))

            Comment

            • Woody
              Automated Home Jr Member
              • Feb 2015
              • 32

              #7
              19.33 was installed yesterday and the hallway zone is now behaving as expected; the setpoint only rises to the set temperature. Nice!

              One question about the 'advanced load scaling' option: does that also work with an on-off interface (bdr91) or only with an OT interface to the boiler?

              Comment

              • bruce_miranda
                Automated Home Legend
                • Jul 2014
                • 2307

                #8
                Actually only works with the BDR91, doesn't work with the OT as that is deemed to be an adaptive control anyway

                Comment

                • Woody
                  Automated Home Jr Member
                  • Feb 2015
                  • 32

                  #9
                  So scaling does not do away with the 'ask for 100% as soon as a setpoint differs more than 1.5C' using an R8810? That is a pity as that might solve the problem I raised elsewhere on this site. One would expect this also to work with OT which does not seem very adaptive, at least not when when used with a Remeha boiler.

                  Comment

                  • DBMandrake
                    Automated Home Legend
                    • Sep 2014
                    • 2361

                    #10
                    Yes it only works with a BDR91 - I find it works great, however I have mine set to "Partial" which we're lead to believe means that it doesn't take the outside temperature reading into account whereas Normal will scale the load from a zone down even more when the weather is warmer than when it is colder, which in theory is a good idea.

                    I found Normal too "aggressive" at reducing the heat demand to the point where some of my zones were struggling to get up to their set points. Partial works well for my system.

                    When you change the Advanced Scaling Mode setting you need to leave it for at least 2-3 days for the HR92's to adapt to the new behaviour, so you might find your zones initially overshoot (or more likely undershoot) a bit for a day or two until the system adapts to the change as it fundamentally alters the heat demand sent to the boiler.

                    The point of load scaling is to allow the system to adjust more gracefully to schedule changes and minimise overshoots. For example without load scaling if you have 5 zones running and up to equilibrium temperature then a 6th zone in a cold room - say a bedroom - is scheduled to come on and the zone was previously <1.5C below the set point, the boiler heat demand would go instantly to 100% and get the boiler up to maximum flow temperature very very quickly. The old algorithm just sent the highest heat demand from any zone to the boiler, so any one zone calling for 100% would cause the boiler to go to 100%.

                    This would usually have the effect of other zones that were at a nice equilibrium overshooting because those HR92's were counting on the low average flow temperature they were calling for until the 6th zone upset that nice balance.

                    Load scaling reduces the heat demand from each zone before sending it to the boiler, so a single zone calling for 100% does not in fact call for 100% from the boiler - it may get scaled down to 40% for example. Also the heat demand from zones is semi-cumulative. So an individual zone calling for 100% can't cause the boiler to go to 100%, but multiple zones calling for 100% can. The scaling ratio also adapts over time so for example the ratio between requested heat from the zone and the demand set to the boiler is different for each zone, learnt over time.

                    The end result is sudden changes to the set point caused by the schedule are "smoothed out" and the temperature changes a bit more gradually with much less impact on other zones. If you graph your zone temperatures you'll probably set the set point targets being hit and maintained much closer with much less overshoot than before. That's what I see anyway.

                    There is one exclusion with load scaling though - if you increase the temperature in a zone manually as a timed override, and the override is sufficiently large load scaling for that zone is cancelled. So you might still see an overshoot in other rooms when making a manual override which you wouldn't if it was just a normal schedule change.

                    This was a compromise introduced in 19.33 as without this (19.31) manual overrides to for example turn up a bathroom radiator could take too long until the radiator actually started to heat up. I think it's a good compromise though because the system stays stable with minimal overshoot when following the schedule but still allows you to get quick heat in any zone with a manual override.
                    Last edited by DBMandrake; 10 November 2020, 01:55 PM.

                    Comment

                    • Woody
                      Automated Home Jr Member
                      • Feb 2015
                      • 32

                      #11
                      Good explanation. Much clearer than the official one I read in the Residio mail after asking for the beta. Thanks!

                      I'll set load scaling to 'Normal', give it a few days and see what happens. BTW, in my system there are 6 HR92's and 7 HR80's. Does that influence load scaling at all?

                      Comment

                      • DBMandrake
                        Automated Home Legend
                        • Sep 2014
                        • 2361

                        #12
                        Originally posted by Woody View Post
                        Good explanation. Much clearer than the official one I read in the Residio mail after asking for the beta. Thanks!

                        I'll set load scaling to 'Normal', give it a few days and see what happens. BTW, in my system there are 6 HR92's and 7 HR80's. Does that influence load scaling at all?
                        I don't think the number of radiator valves make a difference to the learning time.

                        The individual radiator controllers all have some intelligence built into them (a self tuning PID controller) and it learns over time the response characteristics of the room - how much over the air heat demand and water flow causes the room temperature to rise at a certain rate, how early before the target temperature it has to start backing off to avoid overshoot etc...

                        Load scaling throws it a curve ball where the heat demand request from the individual radiator controllers is intelligently "scaled down" from what they actually asked for. So the adaption process is largely about the radiator controllers learning that they need to send a higher heat demand than they used to to get the same result. (Also the load scaling itself performed in the controller has its own separate adaption process)

                        So typically the initial reaction to enabling load scaling for the first time would be for some of your rooms to undershoot their targets by maybe a degree or so and then gradually adjust to get it right. Similar to what you might see if you got a sudden cold snap where the targets might be undershot for a day or so.

                        Comment

                        • Woody
                          Automated Home Jr Member
                          • Feb 2015
                          • 32

                          #13
                          After only two days load scaling behaves exemplary. No more 100% boiler requests just to heat up a bathroom. Out of my 11 zones only one still undershoots. This is always a difficult room (office) that has a lot of loose stuff in it (projects, measuring equipment, PC's, tools, materials, and what not) that all have to heat up in the morning. So I'll forgive Evohome that one. I am nicely surprised :-)

                          Comment

                          • DBMandrake
                            Automated Home Legend
                            • Sep 2014
                            • 2361

                            #14
                            Originally posted by Woody View Post
                            After only two days load scaling behaves exemplary. No more 100% boiler requests just to heat up a bathroom. Out of my 11 zones only one still undershoots. This is always a difficult room (office) that has a lot of loose stuff in it (projects, measuring equipment, PC's, tools, materials, and what not) that all have to heat up in the morning. So I'll forgive Evohome that one. I am nicely surprised :-)
                            Glad to hear its working well for you as it does for me.

                            On the room that undershoots do you mean it just doesn't hit its target temperature in time but does reach it later ? (In which case increasing the maximum optimal start lead time might help - I have mine set to 4 hours to allow for a couple of rooms which take a long time to heat up)

                            Or do you mean it reaches a temperature that is a bit below the target for the room on time, and maintains that lower temperature steadily for a long time? (This is what I would call an undershoot) In that case, it may still eventually adapt, although it sounds like the heat output in that room might be a bit marginal relative to the work to be done to heat up the room.

                            What is the heat demand reported for this undershooting room while it has stabilised at a below set point temperature ?
                            Last edited by DBMandrake; 13 November 2020, 11:05 AM.

                            Comment

                            • Woody
                              Automated Home Jr Member
                              • Feb 2015
                              • 32

                              #15
                              It reaches the target temp later and then more or less keeps it:

                              Undershoot.jpg

                              The optimal start time is set to 3 hours. By the graph it looks like the office heats up nicely for 2 hours after which it takes another hour for the last .5C to end up at the target temp. It is set to reach the target temp at 08:00. I think it might need another couple of days to get it right.

                              I haven't looked at the heat demand yet; will take a look next Monday.

                              Comment

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