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Thread: Evohome, DHW priority and x-plan

  1. #1
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    Question Evohome, DHW priority and x-plan

    I'm looking at replacing my boiler, probably with Intergas. I already have EvoHome complete with DHW control.

    The question I have is around DHW priority. There are a number of articles that suggest that x-plan is the way to go, so you can have hotter water heating the cylinder than heats the rads.

    However, Evohome has a DHW priority function, and I was wondering how that works, especially when utilised in conjunction with OpenTherm? Does anyone have any knowledge or experience of this?

    PS Any recommendations for Intergas installers in the Bedford area appreciated.

    Thanks

  2. #2
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    In order to use HW priority, you will need BDR91 controlling the HW and CH loops, in addition to your OTB that fires the boiler.

  3. #3
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    Is that true with x-plan? I thought in that case the CH loop opened when the DHW loop was closed. ie just one BDR91 on the DHW loop.

  4. #4
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    The Evohome now allows you to implement DHWP regardless of the way the system is piped.
    You could always implement DHWP with an X plan with a single relay and using a Normally Open Valve.

  5. #5
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    Thanks. I guess that was what I was trying to determine. To make sure I have this correct. DHWP is a new feature in the latest firmware. Before that existed x-plan would have been the way to do it, but with the latest firmware DHWP can be achieved with y-plan or s-plan.

    Any advantage with either method?

  6. #6
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    Not that I can think of, the software now just gives existing installations the option. What we fought for though was the forced DHWP setting that couldn't be turned off. Thank goodness Resideo listening and made that switchable

  7. #7
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    Doesn't work well with Opentherm. I ended up changing my NC Motorised valve to a NO one as per the old system before the Firmware update.

    The issue is Opentherm fires the boiler for low flow temps, while the BDR91 keeps the CH motorised valve closed because of the TPI cycling issues. So defeats the object. Found on many occasions the boiler firing and CH valve closed, hence the change to a NO.

  8. #8
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    I absolutely hate that TPI is built into the firmware of the BDR91. It should have been overrideable. Worse still the relay settings available in the Controller then go and apply to all Relays in the system.
    This is one reason why I moved away from using a BDR91 for my UFH.

  9. #9
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    So the TPI issue is probably sufficient to send me down the x-plan route. Out of interest, does the evohome opentherm implementation allow different flow temperatures for DHW and CH? (I believe this is achieved by the wiring with x-plan and intergas, just interested from a theoretical perspective)

  10. #10
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    Quote Originally Posted by lloyd View Post
    So the TPI issue is probably sufficient to send me down the x-plan route. Out of interest, does the evohome opentherm implementation allow different flow temperatures for DHW and CH? (I believe this is achieved by the wiring with x-plan and intergas, just interested from a theoretical perspective)
    Nope,another missed opportunity, even though the OTB is able to distinguish between CH and HW workloads, it doesn't do much with it. Except when heating HW it goes from 10C to 90C demand only. Whereas with CH, it can do several stops between 10C and 90C. What you really need is a way for the boiler to raise the max flow clamp ceiling when doing HW. Intergas allows that. I have been able to do that on my Vaillant too. I previously did it programmatically using Domoticz and an eBus interface. Now I use the Vaillant wiring Centre and the microswitch inside the HW zone valve.

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