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  1. #1
    Automated Home Jr Member
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    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 Guru
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    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 Jr Member
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    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
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    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

  5. #5
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    Happy New Year all.

    I'm afraid that Evohome's habit of sending CS=10 to the boiler for a short time is still bugging me.

    evo2.jpg

    Regards @kevinsmart's suggestion above, could anyone confirm that 'low load control' is implemented in Evohome? As far as I could read it's related to the T6 series thermostats. https://ensupport.getconnected.honey...language=en_US

    If so, can I switch 'low load control' off in Evohome?

    The implication is that flame on & CH return temp Opentherm messages from the boiler are monitored by Evohome. Could anyone confirm this?

    Thanks.
    Last edited by PJB; 2nd January 2021 at 03:22 PM. Reason: Additional question.

  6. #6
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    Quote Originally Posted by PJB View Post
    Happy New Year all.

    I'm afraid that Evohome's habit of sending CS=10 to the boiler for a short time is still bugging me.

    evo2.jpg

    Regards @kevinsmart's suggestion above, could anyone confirm that 'low load control' is implemented in Evohome? As far as I could read it's related to the T6 series thermostats. https://ensupport.getconnected.honey...language=en_US

    If so, can I switch 'low load control' off in Evohome?

    The implication is that flame on & CH return temp Opentherm messages from the boiler are monitored by Evohome. Could anyone confirm this?

    Thanks.
    Happy New Year! It would be helpful if you can share a graph which includes the boiler flow temperature to see whether it is overshooting and anti-cycling.

    Richard from the Evohome shop refers to low load control here:

    https://theevohomeshopforum.co.uk/vi...23908cee61e26a

    Unfortunately you can’t disable this, but with some custom firmware in your OTGW, you could probably alter it.
    Last edited by kevinsmart; 2nd January 2021 at 05:12 PM.

  7. #7
    Automated Home Jr Member
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    Quote Originally Posted by kevinsmart View Post
    Happy New Year! It would be helpful if you can share a graph which includes the boiler flow temperature to see whether it is overshooting and anti-cycling.

    Richard from the Evohome shop refers to low load control here:

    https://theevohomeshopforum.co.uk/vi...23908cee61e26a

    Unfortunately you can’t disable this, but with some custom firmware in your OTGW, you could probably alter it.
    OK, thanks. I see what's happening now. I must say that I'm not convinced that the 'low load control' is handing a reducing demand any better than the boiler does when left to it's own devices. Low load control doesn't kick in with every reducing demand and it sometimes kicks in when the demand is constant and relatively high (CS=50). Very odd.

    I'm intrigued by the thought of suppressing the short duration CS=10 command from Evohome controller with the OTGW. Is that what had you in mind?

    Thanks

  8. #8
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    Quote Originally Posted by PJB View Post
    I'm intrigued by the thought of suppressing the short duration CS=10 command from Evohome controller with the OTGW. Is that what had you in mind?

    Thanks
    Yes, you could suppress the CS=10 and start a timer. Stop the timer if CS>10 sent, otherwise issue CS=10 after timeout.

    I recall max modulation settings also change, so you might need to workaround those too.

    Still waiting for the installation in my new house to be completed. Can probably provide further insight when I’m in the guts of the OTGW firmware again.

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