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Thread: Evohome, S-Plan and OpenTherm

  1. #1
    Automated Home Jr Member
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    Default Evohome, S-Plan and OpenTherm

    Hi guys,

    Any advice on how to install OpenTherm on an existing EH/S-plan system?

    I appreciate that it is not necessary with EH to even have any zone valves, but Iíd like to retain them for if I ever moved home.

    Iím just not quite sure how it should be wired with the opentherm bridge / zone valves calling for boiler heat.

    Do I simply disconnect the boiler feedback from the valves or can I leave them in situ? All rooms in my house are controlled by HR92s

    Thanks

  2. #2
    Automated Home Jr Member
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    Once you have your boiler Opentherm enabled, do you not just replace the BDR91 Relay Box with a R8810A1018 OpenTherm Bridge (R8810)?

  3. #3
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    My BDRs switch the zone valves rather than the boiler for DHW and CH. The switched live (orange) from the valves then provide the boiler with the call for heat.

    I assume that the simple answer here is that I remove the switched live from terminal 10 of the wiring centre to the boiler which will still allow the bdrs to actuate the valves but will then allow the R8810 to control the heat demand.

    I guess that Iím just trying to look for confirmation that this is the correct method, and also if there is then a risk that the zone valves could fail in the closed position yet open therm calls for heat with effectively a closed circuit

    EDIT: After speaking with Honeywell support, I think Iíve answered my own question.

    From both zone valves the recommend advice appears to be to remove the grey and orange wires (permanent live and switches live respectively) from the zone valves. This would allow the valves to actuate but not send the on/off call to heat to the boiler. This is fine, but my worry still remains about the valves failing in the closed position. The boiler could be brought on but pumping into a closed circuit. What are the implications of this?

    Thanks
    Last edited by MajorDanby; 11th October 2021 at 02:23 PM.

  4. #4
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    I'm in exactly the same position as you and I'm trying to understand what is going to trigger the zone valve for the CH - as far as I'm aware EH is rebound to the R8810 and the BDR91 is replaced, so there's nothing to trigger the valve in the first place...

    Did you find a solution/get an answer?

    In the short term I'm thinking I'll need to lock the CH valve open but potentially longer term replace it with a normally open valve which is triggered closed by the HW valve opening (which in turn is triggered by the BDR91), but I'm not sure how sensible that is with a heat-only/system boiler.

  5. #5
    Automated Home Jr Member
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    Yeah, itís actually fairly simple.

    Leave the existing system configuration as is - I.e the room stat or HR92 calls for heat from the evohome controller which in turn activates the bdr91 and opens the zone valve via the motor live (brown)

    You need to install the open therm via the low voltage OpenTherm terminal in the boiler.

    At the same time the boiler s/l needs to be disconnected.

    On the EH controller access the installer menu and select boiler control as OpenTherm.

    This should then work.

    To tidy it up you should disconnect the boiler s/l from the wiring centre.

    This would obviously leave a terminal powered unnecessarily in the wiring centre via the zone valve switched live (orange) so you can also fully remove the permanent live (grey) and switched live (orange) from the zone valve if you wish.

    I didnít do this as if I ever wanted to go back to switched live boiler control itís just a case of reinstalling two connections

  6. #6
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    Interesting, thanks for that.

    So you actually have the Evohome bound to both a BDR91 and the OpenTherm bridge for heating?

  7. #7
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    Yes, if you go into the installer menu and then system devices you will see an option for boiler control, which will be set to Ďnoneí. From this menu you can select boiler control as opentherm which will then take you through the binding process for the opentherm bridge

  8. #8
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    Quote Originally Posted by MajorDanby View Post
    Yes, if you go into the installer menu and then system devices you will see an option for boiler control, which will be set to ‘none’. From this menu you can select boiler control as opentherm which will then take you through the binding process for the opentherm bridge
    Thanks, got it.

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