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Thread: Opentherm + EvoHome + Viessmann 111-W overshoots

  1. #21
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    If it's any help I can absolutely confirm that the boiler is constantly telling the OT bridge what the flow and return temperatures are besides the flame status and several other items. So if the OT bridge is indeed learning then it can because it's in a two way conversation with the boiler, not just telling it how high and when to fire. I'm just not too sure that the OT bridge has the learning function built in. In fact the OT bridge passes some value back to the evohome controller, but so far no idea what that value is, or what the controller does with it.

  2. #22
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    Thank all for the responses here.

    Opentherm (for me and a few other it seems) does seem to be a little less useful than the theory says, for example here:

    http://www.honeywelluk.com/Documents..._FINAL_WEB.pdf

    You would think that as the rooms approached the set point, OT would start lowering the CH boiler flow temp to avoid the overshoot. That was my understanding from the diagram in that PDF anyway.

    If as Bruce_miranda is saying it is true that the flow temps also are reported back to the OT system, there surely can't be hardly any way an overshoot can happen (excluding other heat sources for example solar gain, oven on etc.).

    I know it's only been 4 or so days, but I've not seen any improvement in the control of the system so far.

    I will of course wait the 10-14 days noted here to give it a chance to be fair, but imagine this was a new system for a new user with small children: scalding hot rads for 10-14 days (assuming learning works)?? That can't be right :-(
    Last edited by StephenC; 10th March 2017 at 11:38 PM.

  3. #23
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    Quote Originally Posted by Stephen Crown View Post
    Thank all for the responses here.

    Opentherm (for me and a few other it seems) does seem to be a little less useful than the theory says, for example here:

    http://www.honeywelluk.com/Documents..._FINAL_WEB.pdf

    You would think that as the rooms approached the set point, OT would start lowering the CH boiler flow temp to avoid the overshoot. That was my understanding from the diagram in that PDF anyway.
    That's exactly what it does. However it only varies the flow temperature when the zones are in the "proportional band" which is +/- 1.5 degrees around the set points. Any room that is more than 1.5 degrees below its current set point will trigger a maximum demand until it approaches (within 1.5 degrees) the target. Then it will start to modulate the flow temperature down to the minimum flow temperature required by the radiator with the greatest demand.
    If as Bruce_miranda is saying is true and that flow temps also are reported back to the OT system, there surely can't be hardly any way an overshoot can happen (excluding other heat sources for example solar gain, oven on etc.).
    The reason you're getting overshoot is because your maximum allowed flow temperature is just way too high for the conditions. Overshoot is inevitable in these conditions. I don't even use OpenTherm and if I turn my flow temperature up to maximum (about 75) in anything other than freezing weather I get overshoots in most of the rooms. Turn it down to a more suitable temperature and most rooms do not overshoot.

    It's caused by the thermal mass of the radiators and the amount of heat that they can put out in the time that it takes the radiator to cool after the valve has closed down. It's not a problem specific to OpenTherm, just a problem specific to having your maximum (100% demand) flow temperature way too high for your house/radiators, because your boiler won't let you control it.
    I know it's only been 4 or so days, but I've not seen any improvement in the control of the system so far.
    You won't see any improvement if you can't set a more reasonable maximum flow temperature. If the boiler won't let you do so and Viessman are disinclined to do anything to help you then you might have to give up on OpenTherm and go back to the BDR91!

    Did you ask them about whether Weather Compensation can still limit the maximum flow temperature when OpenTherm is in use ? As a possible workaround ? IF it did work correctly this would in theory work better than going back to a BDR91, as the weather compensation would adjust your peak flow temperature to match the heat loss of the building. (EG lower flow temperature in warmer weather)
    Last edited by DBMandrake; 10th March 2017 at 05:00 PM.

  4. #24
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    Quote Originally Posted by DBMandrake View Post
    Did you ask them about whether Weather Compensation can still limit the maximum flow temperature when OpenTherm is in use ? As a possible workaround ? IF it did work correctly this would in theory work better than going back to a BDR91, as the weather compensation would adjust your peak flow temperature to match the heat loss of the building. (EG lower flow temperature in warmer weather)
    I am going to get in touch with HoneyWell and Viessmann next week as I have been busy with work this week, and also wanted to give it a chance to learn to control better.

    I don't really want to go down the weather comp route if i can as that is more money, and I had made the decision to go OT (rather than just weather comp) based on the logic stated here that I agreed with. I also don't want to go back to the BDR91 if possible - but i understand now if weather comp is not financially viable, or does not technically work (well or at all) with OT, i will be left with little choice but go back. It then begs the question: what is the point of Viessmann stating the boiler can be integrated with OT?

    If weather comp and OT are stated to work well together (i.e. weather comp essentially just sets a dynamic "cap" on the max CH flow temp, and OT then dials the CH flow down when max flow temp not needed) i may well seek that as a solution - and also treat it as an experiment of sorts, and will report results back here.:-)

    Having looked through the boiler manual on page 58, it does seem that weather comp would offer some additional level of manual control on top of the dynamic flow temp configuration:

    The boiler CH flow temp dial that is currently disabled through OT, becomes a weather compensated mode "curve adjustment" dial, giving back that vital ultimate control over the max CH flow temp to the end user.wether comp curves.jpg

    see screen grab attached for what i mean.
    Last edited by StephenC; 10th March 2017 at 07:36 PM.

  5. #25
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    Quote Originally Posted by DBMandrake View Post
    Any room that is more than 1.5 degrees below its current set point will trigger a maximum demand until it approaches (within 1.5 degrees) the target. Then it will start to modulate the flow temperature down to the minimum flow temperature required by the radiator with the greatest demand.
    I'm not seeing this at all, it appears to stay at full boiler temp well after the set point has been reached. That was why I said I was surprised that it didn't dial the temp down when just under the set point.

    I'd love to capture that data from the boiler to evidence this (or indeed show me something that I'm not seeing on the digital readout on front of the boiler).

    I'm capturing data from EvoHome (set points and actual sensor readings) via Domoticz, does anyone have any suggestions for capturing the data at say 1 minute intervals from:

    Boiler: actual flow temp, actual return temp, requested flow temp, gas usage

    EvoHome: requested flow temp


    This would be needed to truly build a picture of what's happening, and also which part of the system is at fault.

    I do think Honeywell EvoHome via OT should be controlling the flow temp better, and not allowing it (ideally via configurable setting) to go high enough into unsafe temps regardless of the boiler type.
    Last edited by StephenC; 10th March 2017 at 11:41 PM.

  6. #26
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    Evohome monitoring is doable but beyond room temp and set points means you are going to have to invest in yet another item like an HGI80.

    Boiler monitoring is another can of worms. I can and do monitor the boiler but I am monitoring the eBUS side of the Vaillant and not the OT side of the Honeywell. So I am assuming that the OT bridge and the Vaillant eBUS adaptor are talking nicely and can understanding each other. I can however see what the Vaillant eBUS adaptor and the boiler are saying to each other. That gives me flow, return, target etc.

    I've not seen anyone on here monitor the OT bridge but I have seen it done. In my case I didn't bother with OT monitoring because it doesn't matter what the OT bridge says, if the boiler cannot understand and respond to what it's told to do. I am keen to understand what the controller and the OT bridge are saying to each other, and that is done via the HGI80 - albeit I still don't know what the OT bridge is saying to the controller.

    Paul's thread on boilers that work well with OT suddenly becomes even more important.

    Also remember this http://www.automatedhome.co.uk/vbull...ll=1#post30419 Seems like that make sure doesn't like OT a lot.
    Last edited by bruce_miranda; 12th March 2017 at 01:47 AM.

  7. #27
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    Thanks Bruce_miranda.

    I will look into further data logging to see what is possible - I would like to get more from the boiler to be honest, but can't find much info on the Viessmann 0x0 or 1xx series (entry level i guess) boilers. The 2xx and 3xx series seems to have more capability for datalogging.

    re: Paul's "OT compatible boiler" thread: I'll wait to post anything there until i have a working (or not) solution so i can keep the post/info to the point as it is more of an info collection thread.


    I haven't seen any learning (improvement in control) as of today - 7 days after the install for info.

    Have spoken directly to both Honeywell and Viessmann:

    1) Viessmann technical support:

    Opentherm works well with their own 1st party kit (there's a surprise). The flow temp dial on the front of the boiler apparently still works with their own OT kit allowing a user set max flow temp.
    They stated the problem had to be with Honeywell OT kit as it (R8810) was the kit requesting max temp, and the R8810 seemed to disable the front mounted CH flow temp dial. (can't say i disagree with them to be fair on this from my knowledge thus far)

    Was told to take up the issue with Honeywell.

    Don't know how true, but it was stated that Viessmann OT kit is made by........Honeywell!

    They also stated that weather compensation and OT do work well together according to Viessmann - and that is what their test setup is configured with (1st party Viessmann kit of course). They agreed that as a workaround, adding Viessmann weather comp to the Honeywell OT setup should allow the dial to work to set a heating curve, and therefore bring down to a user set figure for the CH flow temp.


    2) Honeywell technical support:

    I have raised a formal case (14575585) and was told (later via an email response) that the BDR91 is was using (!!) was not OT compatible and i would need an R8810......that wasn't a great start if i'm honest considering how I was careful with my wording when i reported the problem - and that i had just *upgraded* to an R8810 from a BDR91.

    I phoned back to clarify the case with the actual info and that i actually had the R8810, and now the issue is being "escalated" to product support. So i will wait to see what i get back.

    It would be useful if the Honeywell representatives on this form (e.g. Rameses) could provide some guidance as well if possible.
    Last edited by StephenC; 13th March 2017 at 04:33 PM.

  8. #28

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    How big is the radiator on the room you have overshoots? . evohome/HR92 tends to have a conservative dip. livingroom.jpg
    Your graphs (apart from hallway which looks ok) - seem like a big heat gain regardless what the valve can control

    (with your permission) support can see what your opentherm is trying to do (commands, etc). This might give you some insight.
    Last edited by Rameses; 13th March 2017 at 05:51 PM.
    getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

  10. #30
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    Hi Rameses,

    It's pretty much all rooms, Even the tiny WC rad which is single 600x400mm with solid walls and no external insulation.

    Running the radiators that were sized and installed for between 45/50(autumn/spring) to 60/65(winter)CH flow temps (i'd guess as that is what works for me with BDR91) at 81/82 degrees C is going to be a problem.

    Maintaining a set temp does not seem to cause an issue on the whole from what i can see, it is bringing rooms up to temp. I regularly see the boiler running - well mostly just the pump - at 26 or 31 or 35 or 41 degrees C whilst maintaining temps through the day/night, so it does appear to be dialing down when little heat demand is made.

    IMG_3127.jpgIMG_3126.jpg

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