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Thread: Opentherm + EvoHome + Viessmann 111-W overshoots

  1. #11
    Automated Home Legend paulockenden's Avatar
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    If it's below the 1.5 degree proportional band then surely it's correct to call for maximum heat? That's exactly what a BDR91 would do. It's only once you hit the proportional band that the boiler should throttle back.

  2. #12
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    Boiler firing up to max even when there is little demand, is something I too have seen. Infact it appears that Honeywell's OT implementation would rather go to max more often than absolutely needed - they seem to prefer more heat than just enough. However on a Vaillant the max can be adjusted. Not being able to adjust it is just a very silly. Also the max on the OT is 90C, clearly your boilers appear to limit the max to 81/82 which might be the full boiler max. What luck it wasn't able to get to 90!

    Honeywell's own manuals suggest that learning time is a week and 0.5C overshoots are to be considered normal.
    Last edited by bruce_miranda; 10th March 2017 at 10:10 AM.

  3. #13
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    Quote Originally Posted by paulockenden View Post
    If it's below the 1.5 degree proportional band then surely it's correct to call for maximum heat? That's exactly what a BDR91 would do. It's only once you hit the proportional band that the boiler should throttle back.
    Yes that's exactly right. Even a single zone that is at least 1.5 degrees below its set point will call for maximum heat from the boiler - a BDR91 will be on 100% of the time, and OpenTherm would call for maximum allowed flow temperature. This is normal. It's only within the proportional band that any "learning" can be done.

    The issue here is that with the BDR91 the maximum flow temperature of the boiler can be adjusted on the boiler, while when OpenTherm is in use the boiler is not allowing the maximum flow temperature to be set.

    This seems like an implementation/design flaw in the boiler as far as I can see, as other boilers allow the maximum flow temperature to be set when OpenTherm is in use. (And why wouldn't they ?) If it was me I wouldn't be very happy and I would be beating a path to Viessmann technical support, albeit probably not with much hope of a proper solution as OpenTherm seems to be treated as the ugly step child by most boiler manufacturers...
    Last edited by DBMandrake; 10th March 2017 at 11:06 AM.

  4. #14
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    Agreed. In this case I would certainly be putting the BDR91 back on. The boiler is wasting energy to get to a good 10C over what it should have.

  5. #15
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    Quote Originally Posted by DBMandrake View Post
    ...If it was me I wouldn't be very happy and I would be beating a path to Viessmann technical support, albeit probably not with much hope of a proper solution as OpenTherm seems to be treated as the ugly step child by most boiler manufacturers...
    Can confirm that ... here's the answer I got on twitter : The open therm controls are a learning controller so it will adapt once it understands the boiler it’s connected to (1/2) / beyond that you may need to speak with Honeywell directly as we don’t hold data on third party products. Many thanks ViessmannUK
    I'll mail their support with a better problem statement ... wait and see ...

  6. #16
    Automated Home Legend paulockenden's Avatar
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    It's a real shame that OpenTherm is such a "both parties blaming each other" thing.

    It could/should be SO much better than this.

  7. #17
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    Quote Originally Posted by paulockenden View Post
    It's a real shame that OpenTherm is such a "both parties blaming each other" thing.

    It could/should be SO much better than this.
    Especially when it would only take a simple OpenTherm protocol monitor and someone with a half decent understanding of the protocol spec (it's not a particularly complex protocol!) to work out what the issue is and figure out which side the blame lies on, or whether it's a misunderstanding of a poorly defined part of the spec.

    Asking for a flame to be turned on and setting a flow temperature is not rocket science.

  8. #18
    Automated Home Legend paulockenden's Avatar
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    Maybe it's just TOO open. Perhaps it needs some kind of certification, both for boilers and controls.

  9. #19
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    Quote Originally Posted by paulockenden View Post
    Maybe it's just TOO open. Perhaps it needs some kind of certification, both for boilers and controls.
    Well volunteered

  10. #20
    Automated Home Legend paulockenden's Avatar
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    Quote Originally Posted by dty View Post
    Well volunteered
    You haven't seen my lab rates!!!!

    P.

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