OpenTherm control behaviour

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts
  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    Didn’t someone come to the conclusion that the proportional band is even narrower when using OT?

    Comment

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

      Frustrating as you buy something based on all the spec saying it’s better and more efficient, and turns out it’s not that brilliant!

      Makes me wonder if I’d be better off ditching the OT all together?

      Comment

      • DBMandrake
        Automated Home Legend
        • Sep 2014
        • 2361

        Originally posted by paulockenden View Post
        Didn’t someone come to the conclusion that the proportional band is even narrower when using OT?
        Probably hearsay.

        Actually measuring the width of the proportional band with any degree of certainty or accuracy is more complicated than it appears, and certainly couldn't be done without direct access to the heat demand figures sent from HR92's and that sent to the OpenTherm bridge, and full precision un-rounded, unbiased temperature readings captured using something like a Domoticz.

        Has anyone done this or are they just eyeballing the rounded, biased temperature displayed by Evohome itself and comparing that to the boiler flow temperature ? If so, good luck with that. The half degree biasing of the temperature reading towards the set-point alone would make such a test erroneous.
        Last edited by DBMandrake; 29 November 2017, 11:50 PM.

        Comment

        • paulockenden
          Automated Home Legend
          • Apr 2015
          • 1719

          Originally posted by DBMandrake View Post
          Probably hearsay.
          I think it was Richard who said he'd managed to measure it using an OpenTherm monitor.

          Unless I've remembered incorrectly.

          P.

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            Originally posted by kimber.kimber View Post
            Frustrating as you buy something based on all the spec saying it’s better and more efficient, and turns out it’s not that brilliant!

            Makes me wonder if I’d be better off ditching the OT all together?
            It is better than the ALL Off or ALL On Boiler Relay. But is it better by a country mile, NO.

            Comment

            • paulockenden
              Automated Home Legend
              • Apr 2015
              • 1719

              I think it's the "It could have been so much better" aspects that people find most disappointing.

              But an OT Bridge which just takes the same parameter that cycles the BDR91 and converts it to a demand figures instead also could gave been so much better too. There's a HUGE difference between morning startup where lots of zones suddenly call for heat, and a schedule change during the day where just one room needs a two degree boost. To fire the boiler full-pelt for the latter is just asking for problems.

              Comment

              • HenGus
                Automated Home Legend
                • May 2014
                • 1001

                Originally posted by paulockenden View Post
                I think it's the "It could have been so much better" aspects that people find most disappointing.

                But an OT Bridge which just takes the same parameter that cycles the BDR91 and converts it to a demand figures instead also could gave been so much better too. There's a HUGE difference between morning startup where lots of zones suddenly call for heat, and a schedule change during the day where just one room needs a two degree boost. To fire the boiler full-pelt for the latter is just asking for problems.
                OT most certainly works well within the parameters set by the boiler and the control. That said, I am now pretty well convinced that once all the zones are up to temperature then there is little value in setting Economy as my gas usage shows that 2 hours of savings on Economy will more than be offset by the increased usage when Economy is turned off. The said, I am talking low gas usage here with 19 radiators/12 zones all at set temperatures of 15C and above with a constant state usage of 0.4C3Ms/hour. The boiler calculated flow temperature seems to vary from 20C (no demand for heat) to 30 to 57C (with demand for heat with zones in their set target temperature range) to 70C (initial start up and Economy up to Normal operation).

                I agree that the HR92 range does seem to be a bit too narrow, and I can see merit in having Economy as user-defined temperature reduction. In my situation, a 2C reduction might give better economy that a 3C reduction.

                Comment

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

                  So my current schedule turns the heating down to 5degs when we don’t need the heating system on. As the house is old, uninsulated (to the most part) and with lots of single glazing, the heat disappears pretty quickly and rooms can drop to 10degs or slightly less over night. This does mean that heating works pretty hard the get the rooms back up to temp. Do you think I’d be better off having the set back at a higher temp. My understanding at this though is that the flow temp is lower, so I don’t know if it would actually get the radiators warm.

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    Originally posted by paulockenden View Post
                    There's a HUGE difference between morning startup where lots of zones suddenly call for heat, and a schedule change during the day where just one room needs a two degree boost. To fire the boiler full-pelt for the latter is just asking for problems.
                    And if you have a large boiler that is sized to heat the entire house at once to a comfortable temperature, that single zone dropping below the proportionate band with the boiler being driven at full causes all sort of problems. Ask me!

                    Comment

                    • HenGus
                      Automated Home Legend
                      • May 2014
                      • 1001

                      Originally posted by bruce_miranda View Post
                      And if you have a large boiler that is sized to heat the entire house at once to a comfortable temperature, that single zone dropping below the proportionate band with the boiler being driven at full causes all sort of problems. Ask me!
                      There does seem to be an issue here. I agree that it makes sense to demand a max flow temperature at start up but doing so at other times really doesn't take into account such things as people turning up HR92s in their bedroom. A wider temperature band might solve this problem but I suspect that users might notice the higher zone temperature variation. I am fortunate that no one fiddles with the HR92s.

                      All set temperatures were reached in my home just after 8am this morning with a flow max temperature of 70C. The boiler is now 'ticking along' with a flow temperature of 47C and return temperature of 39C. Gas usage in the first hour was 1.4CM3s but only 0.4CM3s in the second. I am therefore just letting the system do its own thing.

                      Comment

                      • paulockenden
                        Automated Home Legend
                        • Apr 2015
                        • 1719

                        I think this could be solved in the controller.

                        Right now it appears to simply to do a max() of all of the zone demands to calculate the value it sends to the BDR91 or OT bridge. If, instead, it did an average I think this situation would be avoided.

                        When a new zone kicked in, wanting full heat the boiler would be turned up a bit, but not to maximum.

                        Comment

                        • DBMandrake
                          Automated Home Legend
                          • Sep 2014
                          • 2361

                          Originally posted by paulockenden View Post
                          I think this could be solved in the controller.

                          Right now it appears to simply to do a max() of all of the zone demands to calculate the value it sends to the BDR91 or OT bridge. If, instead, it did an average I think this situation would be avoided.

                          When a new zone kicked in, wanting full heat the boiler would be turned up a bit, but not to maximum.
                          Nice idea but not workable unfortunately.

                          The controller has to consider that this "new" zone may in fact need full boiler flow temperature to achieve its set point - as both the set point and the thermal characteristics of the radiator and room could be very different from other rooms.

                          I think most houses have a room or two with undersized (or obscured or blocked) radiators that struggle to meet their target in winter. In our house our dining room, (partially blocked 2m long rad - colder at the bottom - needs replacing, have already tried to flush it out on the lawn..) living room (3 old style double panel radiators without convection fins) and hallway (not sure why that one struggles as it is quite large and is a convector, but it does, possibly the hallway and front door are draughty) and in the depth of winter they all need full flow temperature just to reach a comfy 20/21 degrees.

                          On the other hand some of our radiators are oversized - in particular both upstairs bedrooms, and overshoot is more of a problem there. In these circumstances if the controller were just to average the demands of the different rooms the ones that struggled would simply never reach their set points. So doing a max() of all the demands is actually necessary if you expect all rooms to achieve their set points if there is a significant variance in the abilities of the radiators from one room to another...

                          I talked about a possible solution in another thread as you may recall, something that could actually be implemented purely in controller software and which I think would help minimise overshoot -

                          1) When there is a sudden jump in the combined heat demand of all the rooms (after the max() calculation is made) for example one cold zone suddenly being scheduled to be on, don't just suddenly turn the boiler up to maximum, ramp up the heat demand sent to the boiler progressively over a 10-15 minute period so that other zones have time to sense a small amount overshoot and start closing their valves a bit. Because you are only ramping it up not limiting the maximum you still ensure that rooms that struggle will get there - they may just take about 10-15 minutes longer to get there, but I think that is an acceptable compromise.

                          In any kind of feedback system with a long time constant like this, responding well to a "step change" is the hardest thing to get right. So don't cause a step change in the flow temperature in the first place. Ramp it up smoothly over a reasonable amount of time.

                          2) Increase the wake-up frequency of HR92's for a while when a step change in demand and therefore flow temperature has occurred - it looks from the protocol that the sleep time of HR92's is controlled by a message from the controller, potentially dynamically, although I don't think they are varied at the moment. Having the HR92 respond more quickly to sensed temperature changes will lessen the degree of overshoot when another zone has called for a large increase in flow temperature. Once conditions have stabilised again and all zones are back in the proportional band you can go back to less frequent wake-ups to conserve battery life.

                          Combining 1 and 2 together should in theory greatly improve the temperature control of zones at equilibrium when a new zone comes online and calls for a lot of heat, but won't prevent a weak zone from reaching its target, at most delay it slightly.
                          Last edited by DBMandrake; 2 December 2017, 08:28 PM.

                          Comment

                          • paulockenden
                            Automated Home Legend
                            • Apr 2015
                            • 1719

                            Originally posted by DBMandrake View Post
                            Nice idea but not workable unfortunately.

                            The controller has to consider that this "new" zone may in fact need full boiler flow temperature to achieve its set point - as both the set point and the thermal characteristics of the radiator and room could be very different from other rooms.
                            The temperature of the water in the heating loop will be higher than the temperature in the zone in question, so the new zone WILL heat up. Just not as quickly as with the boiler going full tilt.

                            Also, if we're using an average (or perhaps a weighted average) to calculate the boiler demand the heating loop water will get a bit hotter for a while - not not the stupidly max-temp hotter that currently happens.

                            P.

                            Comment

                            • DBMandrake
                              Automated Home Legend
                              • Sep 2014
                              • 2361

                              Originally posted by paulockenden View Post
                              The temperature of the water in the heating loop will be higher than the temperature in the zone in question, so the new zone WILL heat up. Just not as quickly as with the boiler going full tilt.
                              You're not quite getting what I'm saying. For any given radiator/room/outside temperature combination there will be a minimum flow temperature with the radiator valve fully open and flowing as much as possible that will be able to reach the room set point.

                              If the flow temperature is lower than this the room wont get up to the set point. Period. Not more slowly, it just won't make it at all. If most of your rooms only need a flow temperature of 50 degrees under certain conditions to meet their set point but one room comes online that needs 65 degrees but we just take the average of the different zone demands and end up at 52 degrees, that room wont ever reach the set point target...it will sit there forever with the HR92 fully open and the room below the target.

                              The flow temperature needs to be at least as high as the most demanding room's requirements - the radiator valves in the other room can then close to reduce how much heat they put out at that boiler flow temperature.
                              Also, if we're using an average (or perhaps a weighted average) to calculate the boiler demand the heating loop water will get a bit hotter for a while - not not the stupidly max-temp hotter that currently happens.

                              P.
                              A time weighted average could actually be a good way to do it - but a time weighted average of the combined (max()) demand of all rooms. A time weighted average is just a low pass filter function after all.

                              Comment

                              • The EVOHOME Shop
                                Site Sponsor
                                • Dec 2014
                                • 483

                                Originally posted by paulockenden View Post
                                I think it was Richard who said he'd managed to measure it using an OpenTherm monitor.

                                Unless I've remembered incorrectly.

                                P.
                                It was and nothing hearsay about it...

                                The control bandwidth for OpenTherm is 1 degree either side of temperature setpoint of the zone.

                                Comment

                                Working...
                                X