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Thread: Evohome and OpenTherm - possible bug with HW implementation?

  1. #1
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    Default Evohome and OpenTherm - possible bug with HW implementation?

    I think I have uncovered a bug with the Evohome and OpenTherm implementation.

    When there is a HW heating scheduled the OpenTherm sets the boiler to 90C (or max boiler). However even after the Evohome HW reaches temperature and the BDR91 is no longer calling for heat, the boiler is still set to a target of 90C.

    If during this time the CH calls for heat the target for the boiler is still set to 90C, regardless of HW demand or not. The 90C target seems to be set based on the HW schedule rather than actual demand.

    Anyone else able to confirm what I am seeing?

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    Hi Bruce,

    I don't have the OpenTherm bridge, but I've been monitoring the heat demand messages on the Evohome system and here's my understanding of the DHW messages. When the DHW setpoint is reached the boiler demand value drops to 0 and the 'boiler relay' BDR91 will switch off, but the DHW valve BDR91 will remain open for the duration of the 'Hot Water Overrun' period. I'm guessing that the OpenTherm bridge uses the DHW valve messages to control the 90deg selection. What this means is that I would expect the 90deg setting to remain during the overrun period, but once the DHW valve BDR91 switches off I'd have expected the flow temp to drop back to the normal CH setting. Just guesswork on my part, but what do you think?

    Dan

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    I have the BDR91 controlling the DHW valve. There is no BDR91 boiler relay because of the Opentherm.

    What I am seeing is that when the HW reaches the set point the DHW BDR91 switches the valve off. There is no over run and that is driven by the boiler.

    However the Opentherm bridge is still sitting at a 90C target valve even though the Evohome knows the HW has reached setpoint and has turned the BDR91 for the DHW off.

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    Hi Bruce,

    Thanks for the additional info and explaining how the bridge replaces the boiler relay, sorry for my confusion. Does the boiler correctly shutdown shortly after the DHW BDR91 switches off and the setpoint is reached? I wondered if the bridge might be missing some of the DHW messages and this is causing the 90deg to be retained?
    If not, then it does sound like a bug to me.

    Dan

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    My boiler shuts down simply because the on-off is still controlled by the zone valve microswitches.

    The OpenTherm module couldn't control the firing of my boiler but does control the target flow temperature, which is all I really wanted.

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    OK great, now I get it and I've taken a quick look at your previous posts relating to the OpenTherm set-up. I agree that it sounds like a bug with the OpenTherm bridge as the only functionality it can provide in your set-up is to modulate the flow temp which its not doing. It's a real pain that there's no user indication/feedback on the OT messages (I'm still investigating this with help from others).

    Dan

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    There must be something related to that over-run feature that you saw with a normal boiler relay. Right now my HW is beyond the set point and I not in a DHW heating schedule. I then put the HW schedule on, but because the HW was over the set-point the BDR91 didn't switch the valve open, but more importantly the OpenTherm didn't touch the flow temperature too.

    So it could be that each time I looked at the flow temperature previously the OpenTherm was in this over-run mode, except that my boiler isn't listening to any over-run commands.

    How long is the boiler relay set to over-run beyond a demand being satisfied?

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    Here's how it works on my non-OpenTherm Evohome system with a 'boiler relay' BDR91 and DHW valve BDR91 setup. There's an overrun parameter under the DHW settings on the Evohome controller which can be adjusted. This causes the DHW BDR91 to remain on after the setpoint is hit for the specified period (10min in my case). The boiler relay BDR91 switches off immediately when the setpoint is reached and leaves my boiler to follow its own overrun procedure with the integrated pump continuing to run, but with the burner off allowing the remaining heat to be used.

    Dan

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    Quote Originally Posted by bruce_miranda View Post
    I think I have uncovered a bug with the Evohome and OpenTherm implementation.

    When there is a HW heating scheduled the OpenTherm sets the boiler to 90C (or max boiler). However even after the Evohome HW reaches temperature and the BDR91 is no longer calling for heat, the boiler is still set to a target of 90C.

    If during this time the CH calls for heat the target for the boiler is still set to 90C, regardless of HW demand or not. The 90C target seems to be set based on the HW schedule rather than actual demand.

    Anyone else able to confirm what I am seeing?
    That's not typical behaviour

    Must be something odd with your installation

    May I ask that you use the Honeywell support process here and email in details of your system and issue experiences to uksalesengineering@honeywell.com
    I work for Resideo, posts are personal and my own views.

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    Ok will do.

    I also checked and my HW is set to 0mins overrun.

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