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Thread: Evohome opentherm

  1. #1
    Automated Home Lurker
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    Default Evohome opentherm

    Just about to order my evohome parts.
    Question about the opentherm. I have a Y plan wiring.
    I will be fitting the HR92 to the radiators, I believe if you fit opentherm to the boiler, then you dont require BDR 91. I have Unvented hot water cylinder.
    How is the hotwater controlled without the BDR91 fitted, TIA

  2. #2
    Automated Home Jr Member
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    Jan 2019
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    My understanding (and please wait for those more qualified to comment) is that you will need to retain your zone valve. If you didnít have it then the HW would be heated every time the CH calls for heat.

    You would also need the domestic hot water package for evohome. With this set up and installed the EH controller would then call for max heat when there is a DHW demand and modulate for the CH. in addition you will need to isolate the switched live from the zone valve to the boiler. The correct way to do this is to disconnect the permanent live and switched live from the valve. The motor live is retained to power the actuator and will be via a BDR91

  3. #3
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    Default

    Thank you for your reply, I did think I would need theBDR91, but when I read about the opentherm controller, saying it did away with the BDR91, this did confuse me. So I take it the opentherm isnt used for the hotwater cylinder due to lower temp when boiler is modulating.
    Is it better to change the 3 port to a 2 port only on the hot water as the radiators will have the HR92 ?

  4. #4
    Automated Home Jr Member
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    OpenTherm controlled via evohome will fire the boiler at max if the hot water package is added, as it know the call for heat is for DHW rather than CH.

    You can do either 2port or 3 port valve it should work with both. Retaining the 3 port or changing from y to s plan gives you the benefit that you can row back from OpenTherm to a standard switched live call for heat should you desire it

  5. #5
    Automated Home Lurker
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    Apr 2021
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    Thanks for the information, really appreciate it.

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