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Thread: OpenTherm control behaviour

  1. #171
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    Quote Originally Posted by paulockenden View Post
    I suspect that a BDR91 boiler relay and an OT Bridge are effectively working off the same signal from the controller.

    The only difference is how they deal with anything less than 100% heat demand. With the BDR91 it's by pulsing the boiler on and off, whilst with the OT bridge it's by passing the relative heat demand through to the boiler. The controller doesn't micro-manage either device.
    I suspect you're broadly right. I don't think it's the exact same message otherwise you wouldn't need to tell the controller whether you were using a BDR or OT.

    I really must find the time to figure out why my radio firmware can't receive the messages from the OT bridge (I suspect a timing issue), fix it and then get some OT messages over to DanD for analysis.

  2. #172
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    Could it be as simple as the BDR telling the boiler to turn on or off where as the OT tells it turn the temp up or down? If itís not specifying an exact temp, would explain a few things.

  3. #173
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    DanD has a lot of OT messages from me already which he has looked at. But there is definitely an issue with the implementation or interpretation of the OT Bridge and Domoticz. Currently my OT Bridge Heat Demand messages seem to track those of my Kitchen zone. Which is the only zone in the house which used a BDR91/TR87 combination for an UFH zone. The thing is I have a Boiler Device and an CH Valve Device too, but I have neither of those two as BDR91s. I suspect this is all wrapped up in the issue with the various heat demand messages from the BDR91 and how Domoticz is currently reading them.

  4. #174
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    Quote Originally Posted by fergie View Post
    Could it be as simple as the BDR telling the boiler to turn on or off where as the OT tells it turn the temp up or down? If it’s not specifying an exact temp, would explain a few things.
    As well as specifying an absolute temperature, OT also allows you to set a boiler modulation from 0-100% (see 5.3.8.2). That would map directly into the heat demand percentage from the controller (which a BDR91 converts into a duty cycle). So maybe the bridge is doing that, rather than specifying an absolute temperature. Makes sense if so, because would be by far the easiest way to implement it.

    P.

  5. #175
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    I really, *REALLY* must get round to building my OpenTherm gateway!

    I'm 99% sure (from the eBus messages I see on my Vaillant boiler) that it just alters the flow temperature and leaves the boiler to figure out its own modulation.

  6. #176
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    I did offer to build it for you , do some testing and then send it back to you pre-built.

    OK I can confirm that the Boiler OT device in Domoticz is not correct. I'll take this away and discuss with Dan.

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