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Thread: X-Plan

  1. #1
    Automated Home Sr Member
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    Default X-Plan

    I've seen a few references in various posts regarding a so called x-plan, but any links associated with the conversations lead to 404 errors. Can anyone point me to an explanation or diagram?

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  3. #3
    Automated Home Guru
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    Basically the CH 2-port is Normally Open and the DHW valve Normally Closed.

    When there is a Priority DHW demand the CH valve is closed and DHW opens, so all flow is to the cylinder.

    With Intergas boilers, typically a short circuited DHW NTC sensor is then connected, switching the boiler to a DHW lower power and higher flow temperature setting.

  4. #4
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    Thanks for those. Makes for interesting reading.

    One thing I'm still struggling to get my head around is why you need to have the CH 2-port normally open. Is it not more accurate to say that you want it closed whenever the DHW is called for? What am I missing?

  5. #5
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    Because default operation is OpenTherm heating with only HR91/2s controlling flow to radiators.

    When there is a demand to heat the DHW cylinder you don’t want 70C flow in your CH circuit leading to overshoots.

  6. #6
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    ok. I had assumed that if you were using OpenTherm then evoHome would simply open the CH / DHW valves as appropriate, and if you had enabled DHW priority that it would then always close the CH valve when heating the DHW. Obviously this is not the case.

  7. #7
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    Evohome support of DHW priority has only just been added to its firmware. Previously if you had CH and DHW valves you would wire them to the DHW BDR91 such CH closes when DHW opens.

    I’m not sure how the new support works with OpenTherm regarding any CH valve.

  8. #8
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    Presumably the temperature sensor connected to the boiler is not needed if you have evohome controlling the DHW.

  9. #9
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    Sure, if you are happy with the same max flow temperature and boiler settings for both DHW and CH.

    In my weather compensation setup, I disconnect the outside temperature sensor on DHW heating, detect it via OpenTherm Gateway firmware and boost the flow temperature to 70C, whereas for UFH CH, max flow temperature is 50C - Outside Temperature, reduced further by Evohome.

  10. #10
    Automated Home Sr Member
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    Ok so this is where my complete lack of knowledge of OpenTherm (and perhaps naivety about evohome) shows. I had assumed, probably wrongly, that if you were using OT with evohome, then evohome would boost the max flow temperature if DHW was being call for.

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