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Thread: New home, mammoth Evohome install

  1. #11
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    Sorry to be the bearer of bad news Kevin, but the controller hasn't changed. There is actually a new BDR91 released for 2020 and that has the heat pump support in it. The same old controller from over 5 years ago has been updated with new firmware to support this new BDR91 (see pictures of the new BDR91). Unhelpfully the part number has stayed the same. But if you can read the radio, then you will see that a new function has been added to the BDR91.
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  2. #12
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    Where can Im buy the BDR91T new version 2024?

    Thanks

  3. #13
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    Unless you are planning on running a Heat Pump with the BDR91, there is little point in trying to find the new one.

  4. #14
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    I would like to get this version somewhere, (heatpump owner).

    The product codes are indeed confusing. However I currently have the BDR91A1000, previously controlled the central heating boiler and new the hybrid heatpump. That product code is obviously different from BDR91T1004.

    Is there a physical difference besides the symbol as well? I would expect this new version has 2 contacts; one to heat and one to cool. Is this a correct expectation?
    Last edited by Fmb; 24th November 2020 at 10:15 PM.

  5. #15
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    No difference in the connections as far as I could tell. But the device type in the radio signals does show a new type. Also in the Appliance Control screen there is Heatpump option in 19.33

  6. #16
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    Quote Originally Posted by bruce_miranda View Post
    No difference in the connections as far as I could tell. But the device type in the radio signals does show a new type. Also in the Appliance Control screen there is Heatpump option in 19.33
    Ok, thnx. If the physical connection is the same (so one on/off connection) I do not yet understand why a new relais is needed to support a heatpump.

  7. #17
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    I think it must have something to do with TDI. The BDR91 does TDI on its own depending on the Heat Demand sent by the Controller. It could be that the Heat Pump needs to be switched on/off differently than a traditional boiler or zone.

  8. #18
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    Quote Originally Posted by bruce_miranda View Post
    I think it must have something to do with TDI. The BDR91 does TDI on its own depending on the Heat Demand sent by the Controller. It could be that the Heat Pump needs to be switched on/off differently than a traditional boiler or zone.
    If indeed the BDR91(T) does something smarter than just switch on/off as "told" bij the Evohome, I would assume that the most important difference is that it tries to keep the heatpump "on" for a longer period of time. However as far as I understand the BDR91 "only" switches on/off by a (is this correct English?) potential-free contact. There is not much smart to do on top of that. For an opentherm communication-box this would make sense, but for an on/off communication-box?

  9. #19
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    The Cycle Rate and Minimum On Time settings affect all BDRs in the system (and I wish it didn't). But its not the controller that is actually switching the BDR91 on and off each time. The controller just passes the setting information to the BDR91 once. Then on an ongoing basis it passes the Heat Demand required and the BDR91 then decides how to turn on and off to best meet that Heat Demand using the Cycle Rate and On Time settings.
    So my guess is that when the BDR91T is set up as a Heat Pump relay, it must have a different algorithm that it follows for it's on off cycle, to fulfil the Heat Demand requested by the controller.
    In fact it's a shame we cannot use this Heat Pump setting for the Zone Valve etc because I use my BDR91 to control an UFH pump and I could do with keeping that on longer. Rather than the BDR91 just doing TPI on that, like it would do with the boiler.
    Last edited by bruce_miranda; 25th November 2020 at 09:02 AM.

  10. #20
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    I've attached two graphs which might explain how a normal BDR91 works.
    The first step graph is the Heat Demand the controller is sending the BDR91. Notice how the Heat Demand required, doesn't switch on and off, it goes from 100% down to 0% in stages.
    The second graph is actually tracking the time the BDR91 stays on and off. I have a Cycle Rate set to 6 per hour. Notice how the amount of time the BDR91 stays on within a particular hour reduces as the Heat Demand required reduces.
    My guess is that it's this part that must differently when it's paired as a Heat Pump Relay.
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