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Thread: Opentherm BDR91 Cycles per Hour and Minimum On Time

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    Default Opentherm BDR91 Cycles per Hour and Minimum On Time

    Firstly does the Cycles Per Hour and Minimum On Time parameters have any relevant if the boiler is being controlled via an Opentherm Bridge?

    I currently use a BDR91 to control a motorised zone valve which in turn controls my UFH pump. I want to keep that pump running for as long as possible during an ON time schedule. I understand that the Cycles Per Hour and Min On Time parameters affect all BDR91 devices in the system. It was set to the default 6 per hour and 1 min.

    So given that I have OT and one BDR91 that is controlling my zone valve, what settings should I set to make sure the zone valve / pump stay on for as long as possible. I've changed it to 3 per hour and kept it at 1 min. Will that achieve what I want.

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    Don't know this for certain as I don't have OpenTherm, so take with a grain of salt but:

    1) Cycles per hour is irrelevant for the OpenTherm bridge as it is a TPI setting, however I suspect it will still control the cycles per hour for the heating zone valve if you have one.

    2) Minimum on time only applies to a boiler control relay. If you use OpenTherm it shouldn't do anything. Minimum on time does not apply to heating or hot water zone valves as far as I know, only a boiler control relay.
    Last edited by DBMandrake; 14th August 2017 at 08:55 AM.

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    Hoping one of the Honeywell guys or Richard is able to chime in.

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    I'm surprised those parameters are exposed in the UI when using OpenTherm.

    P.

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    Me too, hence my question. So it's either a bug or they are left to control all the other relay devices on the system.

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    Quote Originally Posted by bruce_miranda View Post
    Me too, hence my question. So it's either a bug or they are left to control all the other relay devices on the system.
    We know that the heating zone valve still TPI modulates even with OpenTherm (remember the thread where we discussed whether this was a bad idea or not with OpenTherm and whether it should just stay open when there is any heating demand...) so it stands to reason that the cycle rate setting will still apply, as it does on a system like mine with a boiler control relay and heating zone valve relay where the cycle rate affects boiler control relay and heating zone valve relay.

    The minimum on time setting being visible may just be an interface vestige though - it's probably just easier to hide/show the entire screen than hide individual settings. Likewise if you don't have a boiler control relay (or OpenTherm bridge) you can't access the cycle rate setting for the heating zone valve even though it is cycling!
    Last edited by DBMandrake; 14th August 2017 at 09:56 AM.

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    Automated Home Legend paulockenden's Avatar
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    But in an S-Plan system the heating BDR still cycles, but these parameters aren't exposed.

    The parameter is really for how the boiler relay is controlled (others things fall out from that), so I'm surprised that the parameters don't get hidden (as per S plan) if there isn't a boiler relay switching on and off.

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    Automated Home Legend top brake's Avatar
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    Quote Originally Posted by bruce_miranda View Post
    Firstly does the Cycles Per Hour and Minimum On Time parameters have any relevant if the boiler is being controlled via an Opentherm Bridge?

    I currently use a BDR91 to control a motorised zone valve which in turn controls my UFH pump. I want to keep that pump running for as long as possible during an ON time schedule. I understand that the Cycles Per Hour and Min On Time parameters affect all BDR91 devices in the system. It was set to the default 6 per hour and 1 min.

    So given that I have OT and one BDR91 that is controlling my zone valve, what settings should I set to make sure the zone valve / pump stay on for as long as possible. I've changed it to 3 per hour and kept it at 1 min. Will that achieve what I want.
    The cycle rate and min run time parameters only apply when you have BDR91 in the system.
    In the case of a OT Bridge for the heat demand and zone valve either at system level or zone level the cycle rate and run time parameters will TPI the zone valves according to demand and system charcteristics.

    Lowering the cycles and increasing the minimum run time will reduce control authority leading to less stable zone temperature control. the defaults are set optimal for condensing gas boilers
    I work for Resideo, posts are personal and my own views.

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    Thanks for coming in on this @top brake.

    So cycle rate and min run time will be ignored for my OT Bridge, hence my Boiler will not be using them. But will the cycle rate and min on time parameters affect my other BDR91s used for either Hot Water or Motorised Zone Valve?

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    Quote Originally Posted by bruce_miranda View Post
    Thanks for coming in on this @top brake.

    So cycle rate and min run time will be ignored for my OT Bridge, hence my Boiler will not be using them. But will the cycle rate and min on time parameters affect my other BDR91s used for either Hot Water or Motorised Zone Valve?
    Exactly that yes. Although not the hot water. That's just on or off.

    That's why an optimal modulation control system will not have a heating zone valve. Ref fig4 in appendix.
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