HR92 Observations on temperature and control processing

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  • g6ejd
    Automated Home Guru
    • Oct 2016
    • 153

    HR92 Observations on temperature and control processing

    1. Now I have the ability to do an IR Image assessment (thanks PaulO for the FLIR link) I have discovered in some instances of my 14 HR92's that the ambient temperature around them does not correspond with the set temperature or displayed temperature and typically some (but not all) of the HR92's read about 1°C higher than actual. So have have used the calibration facility to remove 1°C (setting = -1) where the HR92 is over reading.

    2. What's been happening is there are a few locations where as I call it, a micro-climate exists around the HR92, for example one is in the corner of a room with little or no air movement around it and I can now see this creates a hotter spot and the offset is now corrected for. So you may wish to consider the same scenarios in your installations. Of course you can compensate by increasing the zone temperature to above that expected, perhaps 23°C for an achieved 21°C ambient.

    3. Recently I set only one zone to come on early; our bedroom and heard the valve open at 04:00 (actually 04:02) as programmed, the heating began as I lay watching the room temperature projected on the ceiling () but to my surprised I heard the valve close a little after about 10-mins, the room turn-around time is about 20-mins from cold, then it turned (I presume) the heat off even more as it approached temperature. I found this interesting because it was the only zone on at that time, so why constrict water flow to achieve heat control, why not use the TPI to turn off demand earlier and leave the valve open? Interesting control logic I think.
  • Rameses
    Industry Expert
    • Nov 2014
    • 446

    #2
    3) to keep heat there and not lose the stored heat in the rads. If another zone had kicked in it would have flushed your lovely warm rad which was still perfectly delivering heat to the room. The system was utilising the stored heat and then calculating the demise curve, depending how long you asked the setpoint for it would have kicked in again. Btw Not sure what size room you or what temp you were going from and too but 20mins is not a long time.
    getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

    Comment

    • Rameses
      Industry Expert
      • Nov 2014
      • 446

      #3
      2) good observation and yes. This is why many customers also opt for dts91 to get a sensing point in the room nearer their comfort area. Or just set it to what you feel comfortable anyway. Measurement and what you feel are key factors. The system provides an idex of measurement but what you feel comfortable at is ultimately your choice and what we think is most important
      getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

      Comment

      • g6ejd
        Automated Home Guru
        • Oct 2016
        • 153

        #4
        Originally posted by Rameses View Post
        3) to keep heat there and not lose the stored heat in the rads. If another zone had kicked in it would have flushed your lovely warm rad which was still perfectly delivering heat to the room. The system was utilising the stored heat and then calculating the demise curve, depending how long you asked the setpoint for it would have kicked in again. Btw Not sure what size room you or what temp you were going from and too but 20mins is not a long time.
        Thanks for the insight, but if I I understand you correctly my system does not have a boiler relay and so can't utilise stored heat, if the pump is on, the boiler is on. I might get an additional BDR91 to utilise that capability in this Y-plan system. The house is fairly new and so needs little heat to get it to temperature, the bedroom is 6Mx3.5M and has two radiators one at either end.

        Comment

        • Rameses
          Industry Expert
          • Nov 2014
          • 446

          #5
          Originally posted by g6ejd View Post
          Thanks for the insight, but if I I understand you correctly my system does not have a boiler relay and so can't utilise stored heat, if the pump is on, the boiler is on. I might get an additional BDR91 to utilise that capability in this Y-plan system. The house is fairly new and so needs little heat to get it to temperature, the bedroom is 6Mx3.5M and has two radiators one at either end.
          Yes but if your pump /boiler turns of its not as if your radiators are suddenly ambient room temp are they? Two large bodies of water and metal filled with 60+ degrees water will continue to drive heat for some time.
          Last edited by Rameses; 29 November 2016, 10:29 AM.
          getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

          Comment

          • rcopus
            Automated Home Jr Member
            • Nov 2014
            • 49

            #6
            Originally posted by Rameses View Post
            3) to keep heat there and not lose the stored heat in the rads. If another zone had kicked in it would have flushed your lovely warm rad which was still perfectly delivering heat to the room.
            This doesn't make sense on a modern heating system, every radiator in my house has its own flow and return feed so there's no way to flush it with cold water by another HR92 onpening.

            I factory reset my Evohome and stated from scratch yesterday because the temperatures were either below set-point of shot way above them, hopefully it will 'learn' something better after the reset.

            Part of the problem (in my setup) I see with how the valves work is that they don't open anywhere near as much as they could in order to heat a room. It seems Evohome would happily run the boiler for longer/more often than just open the valve more. My house is brand new and the radiators all reach a temperature too hot to touch within minutes.
            Then there's the opposite problem with valves not being closed enough when another zone demands heat causing the zone that was at temperature to massively overshoot.
            I've also noticed that when a room needs to heat up from cold (say 18 to 20.5) it runs the boiler for immense amount of time to the point where 3 towel radiators can't dissipate the heat and the boiler starts cycling. Then I end up with an overshoot because the boiler could of heated up the radiator in that zone for 5 mins and then waiter another 5 mins before adding more heat etc.

            I thought the logic on Evohome was more intelligent than just having a constant demand at the boiler until the zone is nearer the set point. In my setup this just results in 20+ minutes of the boiler trying to heat water that's already returning very hot to where the boiler can't modulate any lower and starts cycling. Unfortunately due to the fact the boiler put in by the house builder is a BAXI it further compounds the problem by automatically relighting three minutes after turning the burner off regardless of what the flow return temp is, which of course is still too high causing the burner to cut out again seconds after relighting.

            Comment

            • Rameses
              Industry Expert
              • Nov 2014
              • 446

              #7
              Originally posted by rcopus View Post
              This doesn't make sense on a modern heating system, every radiator in my house has its own flow and return feed so there's no way to flush it with cold water by another HR92 onpening.

              I factory reset my Evohome and stated from scratch yesterday because the temperatures were either below set-point of shot way above them, hopefully it will 'learn' something better after the reset.

              Part of the problem (in my setup) I see with how the valves work is that they don't open anywhere near as much as they could in order to heat a room. It seems Evohome would happily run the boiler for longer/more often than just open the valve more. My house is brand new and the radiators all reach a temperature too hot to touch within minutes.
              Then there's the opposite problem with valves not being closed enough when another zone demands heat causing the zone that was at temperature to massively overshoot.
              I've also noticed that when a room needs to heat up from cold (say 18 to 20.5) it runs the boiler for immense amount of time to the point where 3 towel radiators can't dissipate the heat and the boiler starts cycling. Then I end up with an overshoot because the boiler could of heated up the radiator in that zone for 5 mins and then waiter another 5 mins before adding more heat etc.

              I thought the logic on Evohome was more intelligent than just having a constant demand at the boiler until the zone is nearer the set point. In my setup this just results in 20+ minutes of the boiler trying to heat water that's already returning very hot to where the boiler can't modulate any lower and starts cycling. Unfortunately due to the fact the boiler put in by the house builder is a BAXI it further compounds the problem by automatically relighting three minutes after turning the burner off regardless of what the flow return temp is, which of course is still too high causing the burner to cut out again seconds after relighting.
              Give it 7-10 days then DM me your Honeywell login and we will look at how your system is doing.
              getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

              Comment

              • DBMandrake
                Automated Home Legend
                • Sep 2014
                • 2361

                #8
                Originally posted by g6ejd View Post
                1. Now I have the ability to do an IR Image assessment (thanks PaulO for the FLIR link) I have discovered in some instances of my 14 HR92's that the ambient temperature around them does not correspond with the set temperature or displayed temperature and typically some (but not all) of the HR92's read about 1°C higher than actual. So have have used the calibration facility to remove 1°C (setting = -1) where the HR92 is over reading.
                It's also been my experience that HR92's over read by about 1 degree when the radiator is hot due to direct heat pickup so most of mine (not all) are set to -1 calibrate as well. Of course this over-reading only applies when the radiator is hot so when the radiator is cold the indicated temperature with calibrate -1 is now 1 degree below actual and this can cause the radiator to come back on again when the room is not in fact too cold. Also when there is no convection in the room due to the radiator being cold you get temperature stratification between the floor and ceiling - if I check with an IR gun there is more than 2 degrees difference between the floor and ceiling for example. Because the HR92 is close to the floor I find it typically reads an additional degree lower than a wall mounted DTS92 due to the area near the floor being genuinely colder. (So together with -1 calibrate the reading is typically 2 degrees lower than the "real" reading when the radiator is cold)

                A remote sensor like a DTS92 solves both these issues as it is both away from the radiator and up away from the floor. (Recommended height, 1.2 metres) No calibration offset is required and the reading is always correct regardless of whether the radiator is hot or not. (As long as the DTS92 is well positioned and at least 1.5 metres from the radiator)
                2. What's been happening is there are a few locations where as I call it, a micro-climate exists around the HR92, for example one is in the corner of a room with little or no air movement around it and I can now see this creates a hotter spot and the offset is now corrected for. So you may wish to consider the same scenarios in your installations. Of course you can compensate by increasing the zone temperature to above that expected, perhaps 23°C for an achieved 21°C ambient.
                Yes this can certainly be an issue, especially if the HR92 is right in a corner, boxed in or otherwise obscured - if you don't get a good strong convection current around the room the HR92 will read too high, possibly several degrees, leading to a room that is colder than claimed. This can also be a problem with old fashioned non-convector panels as they (not surprisingly) don't convect very well and rely more on direct IR radiation. If you're seeing more than one degree of offset you're probably better off with a remote wall mounted sensor near the occupants, as applying a large correction like 2 or 3 degrees will only be right under specific circumstances and only make the problem of the radiator coming on when it shouldn't (as described above) even worse. The radiator will tend to cycle on and off a lot if the HR92 is suffering from localised heating and poor convection instead of maintaining a steady temperature.

                Our kitchen has this problem as the radiator is boxed in by two kitchen cabinets and a worktop, the HR92 is literally about 2 inches from the side of the cabinet. It doesn't get a good reading of the room temperature at all and tends to cycle a lot and shows a large amount of overshoot on my graphs, although the overshoot is really only the temperature near the HR92 overshooting not the room as a whole. Again a remote DTS92 would solve this completely but I'm not bothered enough about the exact temperature of the kitchen to spend the money on that - as long as its warm when I'm up for breakfast I'm happy.
                3. Recently I set only one zone to come on early; our bedroom and heard the valve open at 04:00 (actually 04:02) as programmed, the heating began as I lay watching the room temperature projected on the ceiling () but to my surprised I heard the valve close a little after about 10-mins, the room turn-around time is about 20-mins from cold, then it turned (I presume) the heat off even more as it approached temperature. I found this interesting because it was the only zone on at that time, so why constrict water flow to achieve heat control, why not use the TPI to turn off demand earlier and leave the valve open? Interesting control logic I think.
                It's normal for it to start closing the valve progressively well in advance of reaching the target - its a PID controller so has a differential component to the control - if the temperature in the room rises quickly (as it does in our bedroom too, because there is a fairly large type 22 double panel convector radiator) then it will learn this and learn to prevent overshoot by progressively closing the valve as the target is reached, ideally stopping on a dime at the correct temperature. Radiators have a lot of thermal inertial due to the heat storage of the steel and water which would cause an overshoot of 2-3 degrees or more if it waited until the target was reached before turning off the radiator.

                Keep in mind that just because you can hear the motor turning to close a bit does not mean the flow is always actually reduced - there is quite a bit of "dead band" on either side of the active part of the valve operation. For example if the black wheel can turn 10 turns between fully closed and fully open, you'll probably find that from the point where the valve starts flowing until it is fully flowing is only about 2-3 turns. (Typically 40-80% indicated valve opening in the menu is the full useful range of the TRV valve body)

                So when a large temperature jump is scheduled it will fully open the valve, as the target approaches it will keep making small adjustments to progressively close it but initially these adjustments won't have any real effect on the flow of water until you approach the action point of the valve.

                Regarding TPI - it actually does both valve control and TPI (via sending heat demand messages) in parallel so the final result is a combination of the two. The way it works is quite clever.

                The heat demand sent from a single HR92 is quite "low" in proportion to how far its valve is open. What this means is that when only one radiator is calling for heat (for example the bedroom at night) and only a small amount of heat is required for example to maintain 16 degrees, it reaches a balance where the valve is largely open but the TPI duty cycle is low. I have seen 80% indicated valve opening and an average flow temperature of 50 degrees for example. However as soon as other zones start calling for heat the total call for heat goes up, TPI increases the duty cycle so the average flow temperature goes up, now the HR92 will detect that the room is starting to get too hot so it will close the valve a lot more. Now you might have 4 zones running, the original bedroom zone now has the valve open 40% but with a 70 degree flow temperature to maintain the same average amount of heat going into the room.

                It automatically finds the optimum equilibrium that satisfies the individual rooms with the lowest TPI duty cycle and flow temperature possible. The only drawback is there can be noticeable overshoots in one zone like the bedroom when other zones switch on as the first zone was relying on the low TPI duty cycle to maintain the right temperature so it has to detect an overshoot and close the valve reactively - it has no knowledge that other zones have just caused the TPI duty cycle to increase dramatically. All it knows is that its own room is suddenly getting warmer and that it needs to close the valve down and signal less heat demand.
                Last edited by DBMandrake; 29 November 2016, 11:17 AM.

                Comment

                • DBMandrake
                  Automated Home Legend
                  • Sep 2014
                  • 2361

                  #9
                  Originally posted by rcopus View Post
                  This doesn't make sense on a modern heating system, every radiator in my house has its own flow and return feed so there's no way to flush it with cold water by another HR92 onpening.
                  On the contrary, it can and does happen. If you have one radiator flowing and all the rest are off your boiler will be modulating its burner (or cycling on and off) to try to maintain your flow temperature based on the warm water coming back from that one radiator.

                  Now what happens if 5 cold radiators open simultaneously ? All that cold water will come rushing back to the return pipe of the boiler and cool the heat exchanger right down - even if the boiler is burning at maximum burn the output flow temperature can't help but drop dramatically, maybe 20 degrees initially, and it could take a good 5-10 minutes for the flow temperature to recover as the 5 additional radiators warm up. In the meantime if the first radiator was still flowing it would be cooled down by this drop in flow temperature.

                  I factory reset my Evohome and stated from scratch yesterday because the temperatures were either below set-point of shot way above them, hopefully it will 'learn' something better after the reset.

                  Part of the problem (in my setup) I see with how the valves work is that they don't open anywhere near as much as they could in order to heat a room. It seems Evohome would happily run the boiler for longer/more often than just open the valve more. My house is brand new and the radiators all reach a temperature too hot to touch within minutes.
                  Not sure what you mean there - how are you determining how far open the valves are ? And at the time you check what is the set point and what is the measured room temperature ? The HR92's will always open 100% if the set point is at least 1.5 degrees above the current measured temperature. If your radiators are not flowing properly under these conditions your valves might not be fully compatible with the HR92. (Pin dimensions/travel etc) You could try enabling full stroke mode if this is the case.
                  Then there's the opposite problem with valves not being closed enough when another zone demands heat causing the zone that was at temperature to massively overshoot.
                  What do you call massive ? It is true if you had one radiator running at a low temperature then another zone is scheduled to come on you might see an overshoot of up to about 1 degree initially.
                  I've also noticed that when a room needs to heat up from cold (say 18 to 20.5) it runs the boiler for immense amount of time to the point where 3 towel radiators can't dissipate the heat and the boiler starts cycling. Then I end up with an overshoot because the boiler could of heated up the radiator in that zone for 5 mins and then waiter another 5 mins before adding more heat etc.
                  I can't actually follow what you're trying to say there...if a room is below temperature it needs to call for heat from the boiler does it not ? Is it a modulating boiler or just an on/off thermostat type. (like mine)
                  I thought the logic on Evohome was more intelligent than just having a constant demand at the boiler until the zone is nearer the set point. In my setup this just results in 20+ minutes of the boiler trying to heat water that's already returning very hot to where the boiler can't modulate any lower and starts cycling.
                  Again, I don't really understand what your perceived issue is - I think you're misunderstanding how things are supposed to work.

                  If zones are 1.5 degrees or more below their set points the system will call for the boiler to be on 100% - why wouldn't it ? The rooms need to heat up...

                  If the return flow is too hot that suggests your pump speed may be set too high. If the boiler can't modulate low enough that suggests it might be oversized for the house... but there is nothing wrong with the boiler cycling of and on when the demand is low, and if its oversized for the house and doesn't have a sufficient modulation range then that's what it will have to do.

                  None of this is really anything to do with the Evohome.
                  Unfortunately due to the fact the boiler put in by the house builder is a BAXI it further compounds the problem by automatically relighting three minutes after turning the burner off regardless of what the flow return temp is, which of course is still too high causing the burner to cut out again seconds after relighting.
                  Sorry but that sounds like a design flaw or maladjustment of the boiler - and completely out of scope of the Evohome system.

                  Edit: one other thing occurs to me, do you have HR92's on all radiators, and if so do you have an automatic bypass valve in the system ?
                  Last edited by DBMandrake; 29 November 2016, 11:41 AM.

                  Comment

                  • rcopus
                    Automated Home Jr Member
                    • Nov 2014
                    • 49

                    #10
                    Originally posted by Rameses View Post
                    Give it 7-10 days then DM me your Honeywell login and we will look at how your system is doing.
                    Thanks Rameses thats kind of you.

                    Comment

                    • g6ejd
                      Automated Home Guru
                      • Oct 2016
                      • 153

                      #11
                      @DBMandrake
                      Yes, having added a -1°C calibration offset, today I have realised that on radiators that are off the difference is 1°C unsurprisingly, I have also used a high precision SHT35 thermometer and the HR92 results are with a few percentage points, interestingly I believe the HR92 reports temperature to 0.5°C resolution and the only device I've seen on the market that does that is the BOSCH 18B20 sensor, so my guess is it's one of those, either that or software is setting the resolution and a cheap bead thermocouple is being used.

                      Are you using hot water overrun in your system, I'm thinking of buying a 3rd BR91 to use that capability as a boiler relay in my Y-Plan system, but I recall you saying you've turned your time to zero? I can see some advantage in allowing the BDR91 to remove boiler demand whilst the pump continues to cycle residual heat through the water circuit.

                      Comment

                      • rcopus
                        Automated Home Jr Member
                        • Nov 2014
                        • 49

                        #12
                        On the contrary, it can and does happen. If you have one radiator flowing and all the rest are off your boiler will be modulating its burner (or cycling on and off) to try to maintain your flow temperature based on the warm water coming back from that one radiator.

                        Now what happens if 5 cold radiators open simultaneously ? All that cold water will come rushing back to the return pipe of the boiler and cool the heat exchanger right down - even if the boiler is burning at maximum burn the output flow temperature can't help but drop dramatically, maybe 20 degrees initially, and it could take a good 5-10 minutes for the flow temperature to recover as the 5 additional radiators warm up. In the meantime if the first radiator was still flowing it would be cooled down by this drop in flow temperature.
                        I don't disagree with what you're saying here in general and maybe my case is not typical. But I can see the flow and return temperatures of my boiler and I know that regardless of how many radiators I turn on, the boiler is never going to be that slow to react to the return flow difference and have any trouble at all in increasing the flow temperate to compensate. We're talking seconds, not minutes and certainly not 5-10 minutes, where a small amount of water may leave the boiler at a lower than previous temperature, and only a proportion of that is going to make it to the already 'on' radiator, in fact 1/6th of it given your 5 new radiators opening example above.

                        What do you call massive ? It is true if you had one radiator running at a low temperature then another zone is scheduled to come on you might see an overshoot of up to about 1 degree initially.
                        I have my sons bedroom set to 21c and when other zones were coming on-board it would go to 23.5c. This was happening in other zones too which meant in the current cold weather you start to feel cold once the over-shot temperature started to stabilise.

                        I can't actually follow what you're trying to say there...if a room is below temperature it needs to call for heat from the boiler does it not ? Is it a modulating boiler or just an on/off thermostat type. (like mine)
                        It's a modulating boiler. What I was trying to say is if I'm only trying to heat one room in my home from 18c to 20.5c this can be done without running the boiler continuously to the point where the temperature is reached and ultimately overshot. I think I'm expecting too much from the technology here.

                        If zones are 1.5 degrees or more below their set points the system will call for the boiler to be on 100% - why wouldn't it ? The rooms need to heat up...
                        My problem is that the continuous running (100% demand) of the boiler is only stopped once at the set-point (or very near it) but then the set point is always overshot.

                        If the return flow is too hot that suggests your pump speed may be set too high. If the boiler can't modulate low enough that suggests it might be oversized for the house... but there is nothing wrong with the boiler cycling of and on when the demand is low, and if its oversized for the house and doesn't have a sufficient modulation range then that's what it will have to do.
                        The combi boiler was sized based on two showers running at the same time, whether this means its oversized for the heating demand of the house I do not know?
                        My problem with the boiler is that when cycling it does not check whether the return temp has reduced or not. The boiler purely relights after 3 minutes regardless of any temperature. I know this is nothing to do with Evohome. Perhaps its me expecting too much from technology again, but quite why you'd design a boiler with anti-cycling that just relights after 3 minutes regardless, without checking the return temp just seems bizarre to me!


                        I have no doubt you are correct on the design of the system, the builder had the plumbers put in a conventional heating system and then the electricians did the Evohome installation after. The boiler supposedly has an integrated bypass, however I have no separate ABV. This means I have 3 towel radiators without HR92's that are 100% open on the system.
                        This is part of my frustration with the system that has led me to do a full-reset, I have 3 towel radiators 100% open at all times and the zone(s) in question that require heating, all radiators get too hot to touch almost instantly meaning the flow return to the boiler is very warm causing the cycling. In those instances I think its pointless having a 100% heat demand when clearly we can't get rid of enough heat to at least reduce the cycling.
                        Does this mean my radiators are too small as they don't get rid of enough heat? I doubt that as they get the rooms up to temperature from cold in 10-15 mins?
                        Or does it mean the boiler is too powerful? But I can't avoid that as a combi boiler is going to be sized based on the DHW requirements?

                        I would like to get an ABV fitted, however no flow and return pipework is visible in the house other than directly underneath the boiler. Given that I already have 3 towel radiators already 100% open though, the ABV is going to do nothing with regard to lowering the the return temperature. It may only increase the flow through the boiler which if anything will cause the cycling to happen sooner.

                        You mentioned pump speeds, is this something I can easily look at and change myself?
                        Last edited by rcopus; 29 November 2016, 01:12 PM.

                        Comment

                        • paulockenden
                          Automated Home Legend
                          • Apr 2015
                          • 1719

                          #13
                          Originally posted by g6ejd View Post
                          interestingly I believe the HR92 reports temperature to 0.5°C resolution and...
                          Nope. As you'll see if you ever get an HGI80 the HR92 reports with 0.1 degree resolution.

                          It's the controller which applies the coarser granularity, and also biases towards the setpoint.

                          Comment

                          • DBMandrake
                            Automated Home Legend
                            • Sep 2014
                            • 2361

                            #14
                            Originally posted by paulockenden View Post
                            Nope. As you'll see if you ever get an HGI80 the HR92 reports with 0.1 degree resolution.

                            It's the controller which applies the coarser granularity, and also biases towards the setpoint.
                            If you want to get really pedantic the measurement resolution is actually 0.01 degrees!

                            In the protocol the temperature reading is a 16 bit signed integer where the decimal temperature is multiplied by 100. So 21.56 degrees = 2156 as a signed 16 bit integer. Negative valves can be represented as well although the handling of negative readings by the Evotouch is somewhat buggy! (Try an HR92 or DTS92 in a freezer and you'll find out...)

                            If I look on my domoticz dashboard at the moment I see 41.35 for hot water and 14.14, 16.35, 16.63, 13.9, 15.75 and 8.85 degrees for various rooms. (No I'm not at home shivering, I can access it remotely )

                            So it does seem to be finer resolution than 0.1 degrees. That's not to say that the sensor will send an update for very small temperature changes - it almost certainly wont send an update if the reading went from 20.12 to 20.18 for example, and I'm not sure what the minimum temperature change to prompt an update is. But at the time updates are sent the resolution does appear to be two decimal places, and the DTS92 does seem to be very accurate - I see it agreeing with a standalone thermometer within less than 0.1 degrees if they are side by side and left to settle for a while.
                            Last edited by DBMandrake; 29 November 2016, 01:27 PM.

                            Comment

                            • DBMandrake
                              Automated Home Legend
                              • Sep 2014
                              • 2361

                              #15
                              Originally posted by rcopus View Post
                              I have my sons bedroom set to 21c and when other zones were coming on-board it would go to 23.5c. This was happening in other zones too which meant in the current cold weather you start to feel cold once the over-shot temperature started to stabilise.
                              It is a weakness of the system I'll agree, although I only see overshoots on the order of 1 degree in these circumstances - enough to notice but not to be uncomfortable. The problem is that a zone that has its valve open wide while the boiler duty cycle is low (like a night time bedroom radiator) is unaware of other zones coming on in the house increasing boiler demand to maximum so its not until the room starts to overshoot due to the flow temperature increase that it will react to the rise in room temperature and close its own valve down.

                              It would be good if it was forewarned that the flow temperature was about to go up so it could take proactive measures but it doesn't look like the wireless protocol allows this kind of information to be passed back to the HR92's.
                              It's a modulating boiler. What I was trying to say is if I'm only trying to heat one room in my home from 18c to 20.5c this can be done without running the boiler continuously to the point where the temperature is reached and ultimately overshot. I think I'm expecting too much from the technology here.
                              Well it shouldn't actually run the boiler continuously in these conditions and it shouldn't overshoot either. It's possible that it hasn't had enough "learning" time yet. How long has it been installed ? It actually takes several weeks of regular use with a consistent schedule (not constantly changing the schedule around) for it to adapt to the characteristics of your system and radiators and stop overshooting.

                              What should happen is that if you start at 18 degrees and change to 20.5, the boiler demand will be 100% until 19 degrees then will rapidly drop back to a lower demand and at the same time the valve on the radiator will progressively close. It should keep progressively closing and reducing demand as the target is approached and in theory not overshoot.

                              Some rooms are very difficult to heat without some overshoot though. It depends on your flow temperature, size of your radiators, heat loss in the room, location of the temperature sensor and so on. I have some rooms that do overshoot a bit and other rooms that rise quickly and effortlessly to the target without overshooting at all. It just depends on the room.
                              My problem is that the continuous running (100% demand) of the boiler is only stopped once at the set-point (or very near it) but then the set point is always overshot.
                              As I said above, it doesn't sound like it has had time to adapt. It's not normal for it to run at 100% demand until very close to the set point unless the radiator is struggling to reach the set point and it is basically requiring full output to reach the target. My hallway is a bit like that in winter due to an undersized radiator - if I set it to 20 the radiator valve is fully open and boiler duty cycle is high to just make the target. If I drop it to 19 I don't have an issue. A bigger radiator would help too.
                              My problem with the boiler is that when cycling it does not check whether the return temp has reduced or not. The boiler purely relights after 3 minutes regardless of any temperature. I know this is nothing to do with Evohome. Perhaps its me expecting too much from technology again, but quite why you'd design a boiler with anti-cycling that just relights after 3 minutes regardless, without checking the return temp just seems bizarre to me!
                              Seems bizarre to me too, and bad design. Not knowing the design of that boiler I can't really comment on whether its normal behaviour from it or not but it does seem stupid.
                              I have no doubt you are correct on the design of the system, the builder had the plumbers put in a conventional heating system and then the electricians did the Evohome installation after. The boiler supposedly has an integrated bypass, however I have no separate ABV. This means I have 3 towel radiators without HR92's that are 100% open on the system.

                              This is part of my frustration with the system that has led me to do a full-reset, I have 3 towel radiators 100% open at all times and the zone(s) in question that require heating, all radiators get too hot to touch almost instantly meaning the flow return to the boiler is very warm causing the cycling. In those instances I think its pointless having a 100% heat demand when clearly we can't get rid of enough heat to at least reduce the cycling.
                              Have the lockshield valves on your towel radiators been balanced properly or have they just been left wide open ? If they're just wide open then when there is little demand from other radiators the bulk of the flow is just going to go through your towel radiators with little temperature drop and contribute to a high return flow temperature. Closing them down a little bit might help here.

                              I don't have any always on bypass radiators on my system both the bathroom and hallway radiators have HR92's and there is an external automatic bypass valve installed in the system.
                              You mentioned pump speeds, is this something I can easily look at and change myself?
                              Probably. Most pumps have three fixed speeds and typically get set on high speed even though medium is more often appropriate. Too high a speed will lead to high return flow temperatures even when radiators are open.

                              If its fairly new it might be a modulating pump though that varies its speed based on demand. If it is a modulating pump then having some towel rails fully open would tend to cause the pump to run fast all the time as modulating pumps only slow down when they detect flow resistance from closing radiators.
                              Last edited by DBMandrake; 29 November 2016, 01:47 PM.

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