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Thread: Evohome : opentherm compatibility problem with Navien combi

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  1. #1
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    Default Evohome : opentherm compatibility problem with Navien combi

    Hi all,

    I've had an evohome setup, with a Honeywell Opentherm gateway, for just over a year now, working fine with my old Ferroli boiler.

    However, recently I had the boiler replaced with a shiny new Navien unit.

    As best I can tell, in Opentherm mode, the Navien disables front panel control of both the heating and the hot water. However, the Evohome does not seem to send the necessary command that the boiler expects, so hot water remains disabled.

    I signed up for the Evohome beta and it is now running on my Evotouch but this has not had any effect on the problem.

    To solve the problem the boiler installer has loaned me a BDR91 and put the boiler in regular switched thermostat mode, which works perfectly but obviously I lose the benefits of Opentherm.

    Is there anything I can do here other than hope that either the evohome or the boiler have their firmware updated to account for this scenario ?

  2. #2
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    Quote Originally Posted by bheading View Post
    Hi all,

    I've had an evohome setup, with a Honeywell Opentherm gateway, for just over a year now, working fine with my old Ferroli boiler.

    However, recently I had the boiler replaced with a shiny new Navien unit.

    As best I can tell, in Opentherm mode, the Navien disables front panel control of both the heating and the hot water. However, the Evohome does not seem to send the necessary command that the boiler expects, so hot water remains disabled.

    I signed up for the Evohome beta and it is now running on my Evotouch but this has not had any effect on the problem.

    To solve the problem the boiler installer has loaned me a BDR91 and put the boiler in regular switched thermostat mode, which works perfectly but obviously I lose the benefits of Opentherm.

    Is there anything I can do here other than hope that either the evohome or the boiler have their firmware updated to account for this scenario ?
    What kind of Navien boiler is it, a combi? So heating mode is working, but the hot water mode temperature is not set correctly?

    The issue could probably be debugged and fixed with an OpenTherm Gateway. It could be that the DHW setpoint temperature needs set as the boiler is not reporting it correctly. With this gateway intercepting communication between the Evohome OpenTherm bridge and the boiler, it can be programmed by its Opentherm Monitor application to set the DHW setpoint.

    https://www.nodo-shop.nl/en/openther...m-gateway.html

    You can buy one ready built.

    I've read that issues like this with other boilers (e.g. Vokera) can be resolved by plugging in the manufacturer's OpenTherm controller to set up the parameters. Then when the Evohome OpenTherm bridge is reconnected everything works. I'm not sure whether this is persistent in the case of a power loss though.

  3. #3
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    Kevin, yes, it is indeed a combi - I should have been clearer.

    I was also thinking that it looks like the best idea, short of a proper fix, is to purchase a gateway, which would be handy thing to keep around in any case. Power loss is rare around here but it can happen. The Navien does seem to keep its temp settings in some kind of non volatile RAM so it's possible we would be quite safe here.

    It's been suggested to be elsewhere that the bait and switch technique you mention could work too.

    I will investigate and report back.

  4. #4
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    I had another thought about this : I wonder if it might be possible to set a hot water setpoint to the Opentherm gateway using wireless, ie Rameses II.

    I know the wireless protocol is not the same as native Opentherm, but it seems likely that the gateway might support a command to set the DHW setpoint ..

    (edit):

    Interestingly, RAMSES II seems to have an Opentherm message type (https://github.com/Evsdd/The-Evohome...nTherm-Message) which seems to be used to send Opentherm messages directly. This makes the Honeywell OT gateway a fairly simple device which is presumably decoding RAMSES-II messages towards the boiler, and encoding them on the way back.

    If I can get an RF device for an Arduino/RPI that can inject Opentherm message #56, in theory this should work ..
    Last edited by bheading; 2nd September 2020 at 03:55 PM.

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