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Thread: Opentherm + EvoHome + Viessmann 111-W overshoots

  1. #141
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  2. #142
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    SHAME on you, Viessmann!

    I bought this overpriced Viesmann 111-W with the same HoneyWell Evohome with 5 HR92 radiator valve controllers. The seller didn't know about the problem, although I bought everything from one seller. After the Viessmann combi boiler was installed, the guy who had to connect the Honeywell switch showed me this forum thread!!!
    I am angry now, because a year and a half after start of this forum thread there is still no resolution/workaround. The Viessmann guy that I talked with didn't know about this, I still don't have problems because the boiler is not fully connected, but of course he replied that the boiler probably will be working "as expected" and this isnt' a reason to return back the boiler.
    Now I can only return back the OpenTherm relay and I use the on/off relay, because nobody wants to have 80 degrees celsius hot radiators!
    Heating with natural gas is something new in my neighbourhood and no one from my neighbours/friends knew about the systems and smart solutions, so I had to be the first one that faced with the problems. Now I am sure that when my neighbours ask me what to recommend, the Viessmann definitely will be something to run away from ...

  3. #143
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    Yes, I agree with the sentiments about Viessmann, especially when this is such an easy fix. Allowing the boiler to ramp up all the way to it's physical max of +80C is just nuts.

    I have a Vaillant, and there is plenty wrong with that boiler but at least I have control on the Max.

    There is a way you can resolve this, if you are determined to use the Honeywell kit. But it involves sticking an OT gateway in between the boiler and the Honeywell OT Bridge. What that does is alters the Honeywell message to provide a Max flow OT message that the boiler is looking for, but one that Honeywell doesn't send, as it's optional in the OT spec.

  4. #144
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    Quote Originally Posted by bruce_miranda View Post
    Yes, I agree with the sentiments about Viessmann, especially when this is such an easy fix. Allowing the boiler to ramp up all the way to it's physical max of +80C is just nuts.

    I have a Vaillant, and there is plenty wrong with that boiler but at least I have control on the Max.

    There is a way you can resolve this, if you are determined to use the Honeywell kit. But it involves sticking an OT gateway in between the boiler and the Honeywell OT Bridge. What that does is alters the Honeywell message to provide a Max flow OT message that the boiler is looking for, but one that Honeywell doesn't send, as it's optional in the OT spec.


    It's stupid that it's optional as it turns out in practice to be vital for correct operation.

  5. #145
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    Optional values, by definition, don't need sending. The boiler should use a default, if not specified. All other boilers do.

  6. #146
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    Quote Originally Posted by bruce_miranda View Post
    Optional values, by definition, don't need sending. The boiler should use a default, if not specified. All other boilers do.
    Someone made the wrong judgment about what was optional in Evohome OpenTherm. Once under OpenTherm control the Viessmann expects to be controlled by Evohome and when it isn't it defaults to maximum output - as do many other boilers when they're loosely controlled in this way.

    I'd have expected Honeywell's flagship product to have ample OpenTherm instructions rather than the absolute minimum.

  7. #147
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    Let's be honest, the OpenTherm spec is a bit crap.

    Sometimes a heating control will have a minimal implementation, assuming the boiler will make sensible choices for the rest.
    Sometimes a boiler will have a minimal implementation, assuming the heating control will make sensible choices for the rest.

    The problem comes when these two meet!

  8. #148
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    Quote Originally Posted by paulockenden View Post
    Let's be honest, the OpenTherm spec is a bit crap.

    Sometimes a heating control will have a minimal implementation, assuming the boiler will make sensible choices for the rest.
    Sometimes a boiler will have a minimal implementation, assuming the heating control will make sensible choices for the rest.

    The problem comes when these two meet!

    The protocol itself seems pretty good, it's mainly what's specified as being optional where problems arise.
    Put bluntly, shouldn't it be obvious that any OpenTherm controller must be capable of limiting a boiler's maximum output, given that at its core its purpose is to manage flow temperture?

  9. #149
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    Quote Originally Posted by bruce_miranda View Post
    ...There is a way you can resolve this, if you are determined to use the Honeywell kit. But it involves sticking an OT gateway in between the boiler and the Honeywell OT Bridge. What that does is alters the Honeywell message to provide a Max flow OT message that the boiler is looking for, but one that Honeywell doesn't send, as it's optional in the OT spec.
    I am sorry for the stupid question, but is there something out of the box that I can get from somewhere to perform this gateway function?
    I think that device as Raspberry Pi could do the magic, but someone has to have enough understanding about programming it for receiving/sending correct protocol messages from both sides, I have no such low-level expertise to do it myself

  10. #150
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    Quote Originally Posted by blowlamp View Post
    This is the latest piece of research from BRE:


    https://assets.publishing.service.go...on-tpi-bre.pdf

    Probably not relevant to the OT debate, but it does show current thinking on heating controls.

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