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Thread: Evohome & Opentherm

  1. #1
    Automated Home Lurker
    Join Date
    Feb 2020
    Posts
    9

    Default Evohome & Opentherm

    Hello

    My setup:

    ATAG iC Economiser Plus27 combi. (installed Sept 2018)
    Evohome & OpenTherm Bridge
    HR92 rad valves in 5 zones. Plenty of bypass rads.
    Nodo-Shop OpenTherrm Gateway
    Home-Assistant monitoring all of the above, running on RaspiPi

    All works pretty well, bar some fun with the DHW ECO mode - solved now & probably best saved for another thread.

    I have a question regarding the Evohome implementation of OpenTherm. The graph below shows OpenTherm Setpoint against time, as an output from the OpenTherm Gateway, placed between the Opentherm Bridge & boiler. Apologies, the picture looks rather small in the preview. I think you can click for a larger view. There is no DHW demand on the boiler at this time.

    evo.jpg

    I've noticed that at times of reducing/reduced CH demand Evohome steps the setpoint down to 10deg and then after ~1 mins steps it back up to the reduced setpoint. Purple arrows in the graph above. This looks more like TPI control than OpenTherm. Has anyone else noticed this & could you comment on whether this is as expected, as designed or otherwise? This is before the 02.00.19.31 Beta firmware update btw.

    Many thanks.

  2. #2
    Automated Home Jr Member
    Join Date
    Sep 2018
    Posts
    44

    Default

    Honeywell call this “low load control”.

    When an appliance is no longer able to modulate lower the control switches from modulating control to low/off control.

    Off in OpenTherm is 10C.

  3. #3
    Automated Home Lurker
    Join Date
    Feb 2020
    Posts
    9

    Default

    OK thanks. That sounds plausible around 17:30 - 17:45 in the graph. But the appliance is able to modulate down .. down to 24deg as you can see. I don't understand why Evohome thinks the boiler can't modulate from 57deg to 52deg (first purple arrow), then 52deg to 47deg without going all the way down to 10deg for a minute.

  4. #4
    Automated Home Jr Member
    Join Date
    Sep 2018
    Posts
    44

    Default

    PJB, enable the flow temperature graph as well. I expect you’ll see your boiler overshooting the temperature demand and then anti-cycling.

    See my post here:

    https://www.automatedhome.co.uk/vbul...g-with-Evohome

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