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Thread: Opentherm and S Plan with Evohome HW priority

  1. #21
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    I still think that a CH valve on an OT system is unnecessary except for when the DHW switches on. That stops the sudden in-rush of HW into the CH circuit. Otherwise I think the TPI on the CH valve defeats the benefits of the OT modulation. This is another use case where I disagree with Honeywell's design decision to force TPI into every BDR. There should have been a simple On Off relay as a device. UFH, CH+OT would benefit from that.

  2. #22
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    Quote Originally Posted by MajorDanby View Post
    So for the last hour my system has been sitting at 11% demand. In that time the boiler has been alternating between being off and modulating down to a flow temperature of 37 degrees. Regarding the CH valve this has been both open and closed during that period, although it has certainly been closed and cycling the water through the bypass longer than it has been open
    Thanks, as I suspected. Glad it isn't just mine, just defeating the object of OT and HW priority features.

    Quote Originally Posted by bruce_miranda View Post
    I still think that a CH valve on an OT system is unnecessary except for when the DHW switches on. That stops the sudden in-rush of HW into the CH circuit. Otherwise I think the TPI on the CH valve defeats the benefits of the OT modulation. This is another use case where I disagree with Honeywell's design decision to force TPI into every BDR. There should have been a simple On Off relay as a device. UFH, CH+OT would benefit from that.
    Yes it does indeed. My boiler is firing with a low flow temp and with nowhere for it to go. I don't understand though, surely software based could just tell the CH BDR91 to stay open like the HW relay does. All they needed to do was to tell it a 100% demand whenever there is a OT demand like the HW BDR91 works.

  3. #23
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    That was the hack that I had programmed into Domoticz for when I was using a BDR91 for my UFH. For any Heat Demand greater than 0%, I was sending a 100% Heat Demsnd to the UFH BDR. That stopped it from doing TPI. Now I have moved to a HM80 and while that fixed some issues it highlighted others!

    This is where pairing a BDR91 to multiple devices really comes handy. So I basically paired my UFH BDR twice I.e. Once with the controller as a Zone Valve for the zone and then with my HGI80 for my hack

  4. #24
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    Ended up fitting a NO Motorised valve to sort the issue. Shame it couldn't be sorted software based, as in my eyes it's a relatively straight forward fix.

  5. #25
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    I am trying to see if I can force the newer BDR91T to bind as a Heat Pump relay. Because that does a TPI cycle only once per hour.

  6. #26
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    Quote Originally Posted by bruce_miranda View Post
    I am trying to see if I can force the newer BDR91T to bind as a Heat Pump relay. Because that does a TPI cycle only once per hour.
    Let me know how you go on.

    In terms of savings, I think i've saved 30% in gas over the last week. Doing readings over a longer period to see how it goes.

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