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Thread: Which OpenTherm boilers work well with Evohome?

  1. #1
    Automated Home Legend paulockenden's Avatar
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    Default Which OpenTherm boilers work well with Evohome?

    There's lots of info spread across many threads here, but I thought it might be worth having a reference thread, with all of the info needed in one place.

    So, for those using Evohome with Opentherm I wonder whether you'd be kind enough to comment with:

    - Boiler make and model
    - Boiler type (combi / system / open vent)
    - Whether any additional boiler hardware was needed to enable OpenTherm
    - How well it works, any niggles, etc.
    - If you upgraded to this from an initial BDR91 install, have you noticed much difference to comfort or costs?

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    Automated Home Legend top brake's Avatar
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    New ATAG, Ideal Vogue, Ravenheat OT all very good with evohome
    Some reliability issues with Viessmann mean I can't personally recommend them
    I have a Vaillant with VR33 OT adapter


    Need to consider minimum modulation to suit the min load
    Stored hot water best handled using boiler hot water priority kit where possible or if not available the evohome hot water kit enables a very easy implementation
    I work for Resideo, posts are personal and my own views.

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    Automated Home Legend top brake's Avatar
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    Since upgrade to OT my boiler is quieter and lower flow temperatures
    I work for Resideo, posts are personal and my own views.

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    Valliant 438 Open Vented boiler with VR33 and undocumented wiring modifications. Stored hot water handled by Evohome HW kit. However I find that OT is running the boiler at full more often than I would have thought was necessary to satisfy the CH only demand.

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    Quote Originally Posted by top brake View Post
    New ATAG, Ideal Vogue, Ravenheat OT all very good with evohome
    Some reliability issues with Viessmann mean I can't personally recommend them
    TB - what is the issue with Viessmann? I see that you have not mentioned Intergas: I assume that the multi-zone modulating issue hasn't been solved.

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    Viessman 100w Openvent boiler- Running Opentherm. No extra demand box required to get max flow temp for HW. Flow temp for CH modulates up and down well, however everytime there is a change in flow temp requimrent the boiler shuts down and switches the pump off for 40 seconds.

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    To add to my comment above- I have also discovered that the Boiler cycle settings and HW Pump overrun time in the Evocontroller doesn’t work with Opentherm and Viessman.

    Not major issues just more annoying niggles that really should work

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    Quote Originally Posted by Norfolkboy View Post
    To add to my comment above- I have also discovered that the Boiler cycle settings and HW Pump overrun time in the Evocontroller doesn’t work with Opentherm and Viessman.

    Not major issues just more annoying niggles that really should work
    The boiler cycles per hour and minimum on time settings aren't applicable to OpenTherm - they are specifically to control TPI of a boiler relay, and OpenTherm doesn't use TPI.

    I would have thought that hot water overrun would work though. Are you sure it isn't ? If you enable hot water overrun your boiler should stop firing when hot water temperature is satisfied (and also assuming there is no heating demand at all!) but the hot water zone valve will remain open for the specified number of minutes.

    I THINK (but am not certain off the top of my head) that the hot water overrun is disabled if the heating zone valve is currently open - EG there is currently some heating demand, but I'd have to double check.

    Do you have the switch contacts (orange and white wires) on your hot water zone valve connected to a hot water demand or hot water boost input on your boiler by any chance ? If you do, this will defeat hot water overrun as it may cause the boiler to fire even if OpenTherm is not demanding the boiler to run for hot water. Thus the boiler will continue to fire during the period that Evohome is trying to do hot water overrun. Only if the system is wired in such a way that the boiler can be turned off while the zone valve is kept open is hot water overrun possible.
    Last edited by DBMandrake; 19th January 2017 at 11:01 AM.

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    you also need to make sure that the cylinder thermostat is kept higher than the evohome HW desired temperature otherwise the thermostat will close the zone valve, making any over run setting useless.

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    I posted on another thread about a strange reply that I received from Ideal about its Vogue System Boiler and hot water demand. We have since exchanged a couple of further e-mails about Opentherm. Today, I received this reply:

    With OpenTherm what normally happens is that the controller (Evohome) would modulate the flow temperature leaving the boiler in accordance with demand and also temperatures it is sensing. I am looking at the Evohome technical information at the moment, but cannot find how, or indeed if it adversely affects the flow temperature when there is a hot water demand as well as a central heating demand. Traditionally a hot water demand should always fire the boiler up at the maximum rate, therefore heating up the cylinder as quickly as possible.

    It may be that you would need further clarification from Honeywell as to how the Evohome does regulate flow temperature when there is a demand for hot water.

    Any thoughts, as I assume the question applies to all condensing boilers using Evohome with Opentherm?

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