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Thread: Evohome - poor temperature regulation

  1. #101
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    I'm open to suggestions.

  2. #102
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    Well, I would ignore the two hallways for now and be happy with the fact that these will come on every time you have HR92 or hot water demand. You can even throttle these radiators back using the current manual hand-wheel valve on the radiator so the hallway does not get too warm.

    A quick way of doing things temporarily would be to bind in the two BDR91's controlling the heating motorised zone valves together as 'boiler relays'. I appreciate you (at present) will get the radiators come on in the hallways when you have HR92 or hot water demand, hence I suggest throttling them back or just make sure your hot water scheduling is the same as a larger cooler HR92 controlled zone for now.

    In a perfect world, I would be getting rid of your two heating motorised zone valves (don't physically have to take them out, just electrically disconnect them and put lever in the manually open position), HR92's on all radiators, 1x BDR91 'boiler relay' (to enable cycle rate and minimum on time feature to fine tune your system) and 'hot water only' for your hot water using 1x BDR91 and CS92. Also ensure you have an Automatic Bypass Valve when you go HR92's all rads.

    You can use your DTS92's as the sensors in any of your rooms that are HR92 controlled, should you feel they would benefit over being controlled locally at the HR92.

    At least with the above configuration the evohome system will genuinely only heat the room(s) you are in (with exceptions of the hallway for now) to the correct set point at the correct timings. Hot water doesn't take long to heat, so don't worry too much about this heating the hallways for now as the energy will not be wasted.

    Forgot to say, make sure you delete the two hallway zones.
    Last edited by The EVOHOME Shop; 19th March 2015 at 12:04 PM.

  3. #103
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    Quote Originally Posted by erik View Post
    G4RHL, sadly that doesn't work in my case. I've been testing while being out of the house whole day, perfect conditions, and saw very much sub-perfect results.

    But with the old wired Honeywell Round I'm seeing very good results. So I know that Honeywell CAN work very well in my home. I just wish it had zoning
    Erik, you will like this! Sitting in my front lounge this morning I could hear the boiler going on and off, sometimes on for less than a minute but the gaps between varied from a few minutes to 10 or more. Reason? Perhaps because it is a warm day here at present, downstairs (4 zones) is set to 18 and all were being kept to that or close to it - 17.5 to 18.5 although the radiator next to me has just gone up to 19. On the other hand is not this what heating is meant to do - maintain a temperature close to that set?

    As I typed the last sentence I heard the boiler come on for a short time, then go off, then back on again. In each case for less than a minute but no doubt the reason for that is 4 zones wanting 18 and all increasing or decreasing in temperature at different rates and therefore one is calling for heat when another does not need it. Again that seems normal to me.

    It makes me wonder whether I should combine 3 of the zones into one. I kept them separate initially to get used to it all but now three months in more often than not three of the zones are at similar temperatures most of the time and doors between them tend to be left open. Am learning all the time!!

  4. #104
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    I'm almost always testing with just 1 zone at a time enabled to make sure the different zones aren't affecting each other's heat demand etc. If there's heat demand, I know exactly wich zone is responsible for it.

  5. #105
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    Quote Originally Posted by erik View Post
    Observation/test result: yesterday, I reset/rebinded everything and put my living room to zone 2 instead of zone 1 and the result had not improved. Actually, I noticed the HR92's were being closed while still asking for heat. I changed the HR92's to full stroke and tested again today. The change resolved the HR92's being closed while asking for heat. However, I'm now seeing the same behavior again: putting in heat every 10 minutes for an hour or two. Creating a build up to at least 0.5 above setpoint (Evohome shows 0.5 above setpoint. My own thermometer actually goes up a whole degree). Afterwards, it cools down for about 2 hours, right untill setpoint is reached. Then it starts heating again, building up the 0.5-1.0 overshoot again. So the conclusion is: using zone 2 instead of zone 1 does not improve my setup.

    If anyone has other suggestions of things I might try, I'd like to hear it.
    I will jump back in on my own post now. Wife has basically got fed up of me constantly watching the Evohome controller like a hawk, and told me to give it a rest for a bit...
    Some other interesting discussions going on, please continue with them.

    So I rebound in a different order, and like Erik found, it makes no difference to the bathroom overshoot. There goes that theory.

    So since I did my full reset, I still have not been able to reproduce the 'repeated driven overshoot' that I used to get. I am even wondering if I only saw that with thermostat mode, and that the rest of the time I have been seeing effects of TPI.

    In the case of the bathroom, this is more like a passive overshoot...it is the smallest room so I would expect that room to be hardest to control (plus I don't really mind it drying the towels, and I could always use the lockshield valve if need be). Bedrooms and lounge are well controlled.

    Certainly, some of the behaviours I have observed are explained by an over-zealous proportional-integral controller (i.e. the integral component causes overshoot). One case that really threw me at first, having come from a proportional only thermostat, was the following. Set point and actual = 17. Then override set point to something obscene like 23. After an hour, when the actual temperature gets to 19, then set it back to 17 again. You will find that not only will the temperature keep increasing due to the heat in the system, the large integral-error term that has build up will mean for the next 30 mins or so, it will actually keep calling for heat.
    This is obviously an unrealistic case but demonstrates what can happen if you give it a big disturbance.

    Large disturbances to the system can really make it unstable, so I have now changed my program to avoid any steps in temperature of more than a few degrees. It is much better.

    When settled - and that means after 2 hours of each zone being at the set point - the system can maintain the desired zone temperatures perfectly (within 0.5), even when zones are at temperatures of 3 degrees different from each other. It works very well if you hold temperatures constant in all zones, and do not have too many switch points.

    This accumulated integral error effect (which having googled it, is called 'integral wind up') does not however explain the 1.5 degree driven bouncing that I definitely see in thermostat mode (which I don't use anyway). I have not been able to reproduce this effect since my reset (which is both good and bad).

    So the warning is: this is a complicated, intertwined system driven by a P-I algorithm. In my house, it takes 2-3 hours to settle to an equilibrium so that each zone is at the temperature required, and receiving the heat required to maintain that temperature for the loop temperature. if you 'fiddle' with the system to see if it can hold a temperature or see how the other zones behave, it will disturb everything, and will take a few hours to find that new equilibrium (in mild weather...only 30-60 minutes in cold weather so you have a larger sink for heat).
    Of course the controller is also trying to take that demand, and put it into 'lumps' for the boiler. From what I have seen, it is also trying to decide when the temperature error is small and not to trigger the boiler and wear it out. For example, if all zones are at set point except one zone which is 0.5 lower than set, it won't do anything for 30 mins or until another zone also needs heat before firing. This is called a 'deadband' according to my googling :-)

    See, if someone had put that in the manual, that that we have is a series of closely coupled non-linear systems driven by a P-I algorithm, I would have treated it much more gently! I guess people don't write user manuals for physicists.

  6. #106
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    Set point and actual = 17. Then override set point to something obscene like 23. After an hour, when the actual temperature gets to 19, then set it back to 17 again. You will find that not only will the temperature keep increasing due to the heat in the system, the large integral-error term that has build up will mean for the next 30 mins or so, it will actually keep calling for heat.
    Haven't noticed that yet (haven't tried it) and would be pretty bad if it's true. A setpoint change like that should instantly (give or take 5 minutes) override any integral etc. Just shut the valves and stop calling for heat immediatly (if there's no other zones calling for heat).

  7. #107
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    With regard to the HR92 valves.

    As I said, I have found that if zones have only just reached a stable temperature (say after 30 min), and you introduce temperature switch points in one zone, it affects all the other zones as well.
    You get a increase in the other zones, sometimes by 1 - 1.5 degrees, and sometimes for several hours. The settling time of this system in my house seems to be about 1-2 hours under light load. (FYI in colder weather, it is much more stable.)

    Now when all zones have been stable for several hours at requested temperature, if I turn one zone up, the others will not heat much. Or at all, they will close their valves.
    But as I said if the zones have only just reached temperature recently (or are even slightly above), and I turn up one zone, all of the zones will tend to then overshoot as the radiator valves are still open.

    Typically, the valves are 80% open when < T-0.5. 30 % open at T-0.5. Sometimes they are still 10-20% open at T+0.5 as well, if it has just overshot.
    I believe this is because of the accumulated integral error again, the system still has low demand in there, which takes a little time to get rid of.
    After an hour or so at temperature, the valves can even be 0%-closed, at T+0.5. I have even seem them closed at exactly the setpoint.

    I tested this by looking at the bathroom valve when I gave it a big temperature step. Then at T 0.5 degrees higher than set point it was still 73% open! It got to 1.5 degrees over before thinking it would close.

    It takes a little while for for the accumulated integral error to be 'used up', hence you get demand overshoot and radiator valves open.

    One thing I will say, is that the thermal comfort I get from this system is far better than the old system - provided I leave it alone!
    Last edited by greyhound1234; 20th March 2015 at 08:40 AM.

  8. #108
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    A comment for Honeywell.
    PI controllers are designed to drive 'symmetric' systems - those which can be driven up or down. Heating systems are asymmetric, they can only be driven up, but come down by heat loss only. There seems to be no capability in the system to deal with integral overshoot at low load conditions.
    My recommendation would be to implement one or more of the following:
    - We know the system is not stable to temperature steps exceeding a few degrees. Implement temperature change requests in a linear or gradual fashion e.g. 1 degree per hour.
    - Do not start accumulating integral effects, until a zone is at Tset minus 1 degree. This will reduce overshoot when large deviations are requested.
    - Purge the accumulated integral term if the temperature is 0.5 degrees above setpoint and rising.

    If you do this, you will get less complaints from technically-minded people who like to fiddle with their heating system and set complex schedules...
    Last edited by greyhound1234; 20th March 2015 at 08:45 AM.

  9. #109
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    A question for Honeywell:
    The minimum run time setting (default 1 minute).
    Is this the minimum the controller will fire for, or is the telling the controller what the boiler overrun is (for the learning algorithm?).

    Just that I have noticed whilst set to 1 minute, sometimes the controller will call for heat for several minutes, but other times I have seen it click on and off in 10 seconds!
    If it is to tell the system, if you pulse for 10 seconds, you will run the heating for a minute and 10 seconds? It may need to know this to improve the learning.

    In which case, I need to set it to 2 minutes since that is what the pump overrun is on my boiler.

  10. #110
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    thanks for all your detailed analysis/findings, greyhound.

    I think this one:
    There seems to be no capability in the system to deal with integral overshoot at low load conditions.
    is probably the most important.

    afaik, the minimum run time setting is suposed to be the minimum time that the relay sends an ON signal to the boiler. So 10 seconds should not happen. I've seen this happening as well sometimes though, but not very often. I think only after setpoint changes.

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