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Thread: Evohome - poor temperature regulation

  1. #111
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    Quote Originally Posted by erik View Post
    thanks for all your detailed analysis/findings, greyhound.

    I think this one: 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.
    as I've mentioned previously I also see this all the time too

  2. #112
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    I'll post my summary that is under 'positive experiences' thread here as well to round this off.
    ---
    Initially, I was shocked at the lack of control, and overshooting behaviour I encountered. I was very concerned and assumed it was faulty.
    So naturally I did loads of fiddling with it to work out how well it held a certain temperature, what the behaviour of other zones was etc. This didn't help.

    One thing I found particularly surprising coming from a simpler thermostat, that it would be possible to get heat demand when all zones are above the set point. But this is possible with a proportional integral controller (i.e. TPI) if subject to a couple of large set point perturbations. As my feel for how the system behaves has improved, I have altered how I use the system and things are better.

    Some general observations
    - the more you change the temperature, the worse the control gets.
    - If you change the set point up by more than a few degrees, you will get an overshoot before it settles down.
    - For a given set of zone temperatures, it takes a few hours for the system to settle down, each zone to find the correct valve setting, and the boiler demand to stabilise.
    - Other zones will then overshoot again if one zone is increased again in this 2 hour 'settling period'. This will be most evident in rooms that are small and therefore heat up faster.
    - After the settling period of a zone, it won't then increase when other zones are set higher.
    - Once this happens, the control is very good.
    - It is better to schedule in a series of stepped rises that one big step. It is also better for thermal comfort to use slightly lower set point but held for longer, compared to a big increase for a short time.

    So basically, the more you fiddle the worse it gets. If you are perturbing the equilibrium the algorithm will take a few hours to settle down. During this period, you may get overshooting. You have to be 'gentle' with it.

    I have made a few suggestions for how Honeywell could avoid this effect (caused by a build up of the 'integral' part of the P-I algorithm), but it is easily avoided if you don't make huge whacking changes in set-point.

    For example, now after a few hours all zones are in equilibrium and I can achieve lounge - 18.0 as set, bedroom 16.0 as set. Perfect.
    Also comfort is way better than the old thermostat, which keeps the Wife happy!

    Energy use I will report back when I have a year of data. I average 4000 kWh/yr gas usually. But getting control of the upstairs was the main selling point for me, which has been achieved.

  3. #113
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    Are you saying it's going to take a few hours of settling down EVERY DAY (or actually even every time you have a 'big' change in setpoint?) before it achieves temperature as set?

  4. #114
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    Basically, yes. But it depends on your definition of 'achieve'. Can get to the temperatures easily enough, but to be stable at those temperatures a few hours.

    E.g. set 18.0 from 16.
    For me, time to pass 18.0 (to 18.5 or 19.0 sometimes) = 30 minutes.
    Time to settle back to 18.0 = 30 minutes. (give or take, depends on room and overshoot)
    Time to become stable at that set point = 30-60 minutes.

    With this system, it is not just achieving the set point but achieving stability at the set point. From my observations, that takes a little while. But once achieved, it is held quite nicely, even if you ramp up other zones again. The longer the zone is at temperature, the more stable it seems to be as well.
    So long unchanging schedules are best.

    I therefore have altered my scheduling so that rather than allowing living room to drop to 15 whilst at work, and bringing it up to 18 at 5pm, I only let it drop to 16.5 whilst at work and bring it up to 18 at like 2pm, so system is stable when I get home.
    Slightly more energy use one would think, but 2 hours I don't think is a big issue, since it does take a couple of hours for the thermal mass of a room to get to the air temperature anyway, which affects thermal comfort quite a bit.

  5. #115
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    Thanks for explaining.

    Doesn't seem like very smart behavior though. And in my case, it's not hapenning for some reason. Because even after running an entire day on 1 setpoint, temperature is still going up and down a lot. If we lived closed together, we could do some comparing. Too bad

  6. #116
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    Quote Originally Posted by erik View Post
    Thanks for explaining.

    Doesn't seem like very smart behavior though. And in my case, it's not hapenning for some reason. Because even after running an entire day on 1 setpoint, temperature is still going up and down a lot. If we lived closed together, we could do some comparing. Too bad
    If you set it the same temperature in zoned mode, and it is still unstable as you have said, then it is not behaving itself.
    Today is a 'low load' day, fairly warm here in UK, I have a well insulated modern house. My readings have been constant for half the day at: lounge 18.0 (set 18.0); bedroom 1 and 2 16.0 (set 16.0); bathroom 17.0 (set 16.0). The boiler comes on once an hour or so. There is no unstable behaviour.
    Given that I have noticed an improvement since resetting everything, that is all I can suggest. Failing that Honeywell should sort it out, fix/replace/refund whatever.

    You have already reset-rebound before I see. I can tell you exactly what I did...
    - Factory reset on the controller, after confirming yes to reset, it says all settings have been reset / bindings cleared (tick). I didn't press the tick else it would return to the main screen in thermostat mode (it is a pre-bound unit). I took the batteries out at that point instead. Left them out for 5 minutes.
    - I then went round each HR92, I unbound it (hold 15 sec in binding mode). I did a 'reset' on it from the menu. I then took the batteries out, and left them out for 5 minutes.
    - I then put batteries in and reassembled all the HR92s.
    - I put the batteries back in the Evohome controller.
    - It comes up that I do not have any bindings, and do I want installer or guided configuration. Chose guided configuration.
    - I added 1 zone in guided configuration (living room).
    - When binding, I put the HR92 in binding mode first. Then when Evohome says do I want to use the internal sensor, press no. Then press bind on HR92. Then I waited 30 seconds before pressing the green tick. Do I want to add more rads? No.
    - I repeated for other zones using the setup menu (add zone).
    - Then program a constant temperature in each zone during the day, a lower temperature at night, leave for a few days so it can learn the rooms before programming anything more complex.

    This is what I did.

  7. #117
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    just to be sure, you're also steering the boiler with a BDR91? And you are seeing it going ON only once per hour? That's truely amazing

    Thanks for your explanation. Did you reset the BDR91 as well? (not listed)

  8. #118
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    It is a bdr91. I didn't reset bdr91, but it did get rebound as part of guided configuration. Good luck.

  9. #119
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    it is a bdr91 and it's only going on once per hour? Wow, I'd pay to see that

    and HR92 totally set to default? No full stroke/torque? (sorry for asking so many questions )
    Last edited by erik; 20th March 2015 at 06:30 PM.

  10. #120
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    Yes, it is nice to have it behaving. No other changes. Normal stroke. 1 minute min cycle, 6 per hour. Happy to help.

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