Opentherm + EvoHome + Viessmann 111-W overshoots

[StephenC]

Stephen - As per agreement - please provide feedback to customer services ref your case. The senior tech will pick up with you.

You should have had an email, please check spam etc. But sorry you didnt get the exact timing notification.

Hi Rameses,

Just been on phone to Honeywell support. They are looking at this now. I wasn't sure however if this was related to the discussion I had yesterday.

Stephen

[StephenC]

Update:


The R8810 OT bridge issue and associated Viessmann boiler "A9 error/fault" was resolved by reversing the telemetry configuration that was made on the Evohome controller. A reboot of the boiler (and therefore R8810 OT Bridge) was all that was needed after this and all errors related to "A9" were resolved. So no boiler fault, nor R8810 OT Bridge hardware fault in reality! Phew.

Note: This config change was was originally made to enable Honeywell to gather more info to diagnose the issue with high flow temps and also for "appearing" to fire the boiler for longer than was surely needed, therefore this is not something likely to affect anyone else to be clear. I have posted this really just in case it ever comes up for someone else in the future as it seems OT comms issues are rare.

I have also been kindly walked through the high level summary of R8810 OT bridge to/from boiler comms relating to the high flow temp issues (so not complete conversation to be clear):

• R8810 OT Bridge requests Boiler Max CH Flow temp configured setting - whether implicitly set (i.e. boiler safety cut-off temps) or explicitly set (i.e. dial/digital controls by end user)

• R8810 OT Bridge records the value and sets this temp for Evohome controller requests of 100% heat demand

• R8810 OT Bridge retrieves/is sent actual *aggregated* demand from Evohome controller as a % (not a temp to be clear) - and any zone/HR92 under 1.5 degrees below set-point will in theory send a 100% heat demand itself to the Evohome controller

• R8810 OT Bridge translates this % value into a temperature value *x* (Honeywell algorithms here)

• R8810 OT Bridge communicates with the attached boiler and asks for a CH Flow temps of *x* - where *x* is the translated % heat demand in the previous step

• Boiler responds and confirms OT request and starts CH pump, then fires boiler to desired temperature

I have asked if Honeywell can provide some logs to provide some data/evidence to the conversation I've hopefully repeated correctly above, and will post anything non-confidential here if I get anything. I would like to take this data/evidence to Viessmann UK in the first instance and prove to them that their boiler is the fault here by sending back the reconfigurable max CH flow temp value.

In essence, Honeywell have informed me that Viessmann seem to have locked out 3rd party OT controllers/bridges from having end user configurable CH flow temp maximums (via direct on-board boiler configuration). This is either intentional (vendor lock in?) - or un-intentional poor interface design, where Viessmann have ONLY considered their own OT controller, and we believe must somehow have the ability to control a max CH flow temp.

I'm likely going to go-ahead now and get weather compensation installed as a work-around, but Honeywell have advised the HR92 devices will learn to deal with/cope with the high flow temperatures over time. I'd like that control back however to dial the max CH flow temp to a more skin friendly ~65-68 degrees or so, with the ability to adjust the heating curve on-demand if needed (via the on-board dial). Again, I'll report back the finding here for others and future reference.

Lastly, thanks go to Honeywell for providing great support on this issue, where Viessmann have dismissed it as "not their problem".

[bruce_miranda]

I can confirm that the way you have described the interaction between OT and boiler is exactly what I have seen by studying the logs between my Vaillant boiler and the OT bridge. What is strange however is the fact that your boiler is letting the OT override it's own max setting when infact, in my case atleast, the OT bridge never asks the boiler to amend it's max setting. It simply receives the settings from the boiler. Now it is true that the OT bridge kicks the boiler into max for high heat demand but because that max is configured by the boiler it all just works fine. If you are interested, I can post the actual interaction between OT and the boiler. In your case the boiler appears to give up its own control to the OT bridge for some reason and so OT max is the boiler's physical max as opposed to what it's been set as.

[StephenC]

Maybe I didn't explain it well enough in my post - but what you said is happening here: the R8810 OT Bridge is simply receiving the Max flow CH temp from the boiler (and set at the boiler whether implicit (boiler safety cut-off temps) or explicit (dial/digital controls by end user).

As the Viessmann detects OT - for some (stupid) reason, it expects the OT device to be calling the shots, when in fact Honeywell have stated this is not how it works, it simply receives the max CH flow temp config from the boiler and aligns this in its config to 100% heat demand.

[StephenC]

I believe this may be the controller Viessmann were referring to - a Viessmann CM737.

Looks just like a Honeywell which is ironic :-), and has boiler parameters that are available to be set, including CH max flow temp....:


Viessmann-CM737.jpg

Retrieved from:

http://viessmanndirect.co.uk/files/c0875299-d8b8-459e-8287-a5af009f441d/CM737%20install%20instructions%20%20final.pdf

[dty]

That is so obviously a white-labelled Honeywell product! Even the model number starting with CM gives it away. It looks virtually identical to my decade-old CM907 in my old house!

[bruce_miranda]

It looks like a Honeywell because it is a Honeywell! See CM707 and CM727. CM937 or CM957
I can see what's happening here. The boiler is expecting a parameter to be set via the OT controller and when none is set it just goes to physical max. The Evohome OT bridge has no configurable parameters, known.

[StephenC]

The boiler is expecting a parameter to be set via the OT controller and when none is set it just goes to physical max.

Indeed, which the OT specification doesn't seem to cover off from speaking to Honeywell. i.e. both devices are "within spec" of the standard, and Viessmann have coded their side of the interface for their own controllers only in mind.

All I can find online is the older v2.2. OT spec (v3.0 seems to be the current spec, but not convinced that would be in use here anyway as it is fairly new):

https://www.domoticaforum.eu/uploaded/Ard%20M/Opentherm%20Protocol%20v2-2.pdf

The Evohome OT bridge has no configurable parameters, known.

One thing I did establish and not mention in the above comms steps between R8810 OT Bridge and boiler is that the R8810 firmware is *not* realistically up-gradable. Thats not the same as not possible to be clear.


As we know, the R8810 OT bridge deals with temps, and the Evohome controller deals with heat demand percentages % (except stored hot water of course), so I believe it's very unlikely therefore that the issue we (Viessmann owners at least) have with regards to a lack of end user control over max CH flow temps on the Evohome controller side, will be resolved in the near future.

[bruce_miranda]

In section 5.2 it clearly states that 25 Boiler temperature is Not mandatory. So why is your boiler expecting the OT bridge to set one?

[StephenC]

Vendor lock in do you think? After all, I'm guessing they want heating engineers to deploy Viessmann controls with the boilers. This is one way of making it difficult for them to go with another manufacturer for those controls. I don't read it that the boiler is doing anything wrong against the OT spec, its just withholding standard boiler ability to control the value once it detects OT integration which I think (correct me if you disagree) is outside of the OT spec as it's a boiler feature.


On a related note, I'm interested generally now if other 3rd party controllers with OT support (e.g. nest 3rd gen) have inbuilt max ch flow temp config ability. If they don't, they're all going to start to run into this problem surely?



I've ordered (reluctantly) the overpriced Viessmann weather comp kit (basically a thermistor in a Viessmann branded box). I was told this would set dynamic max ch flow temp values for OT to then read in, so we shall see. I will keep the R8810 OT bridge in place.

Knowing what I know now, I would say Honeywell OT (R8810) is not recommended for Viessmann boilers unless they sort this issue out. Which of course is only going to be on new boilers. I'd also suggest sticking with the basic (but reliable) BDR91 Relay with TPI, and go straight to adding Weather comp and do that alone if further efficiency savings are the objective.

This is a boiler manufacturer specific piece of advice of course however as the reality and experience here has shown to be different to the logic and common sense in than we and others on here expected.

[bruce_miranda]

I would say Viessmann boiler is at fault here because if the OT spec say that setting a boiler's flow temperature is optional, then the boiler should NOT assume that it's flow temperature will be set by the OT controller. If it does, it should oblige, otherwise it should stick with it's own setting.

I'll have a good read of the OT spec sheet. Because if the Boiler flow command is a one time setting, then maybe there is a way around it. i.e. you build an OT interface that simply sets the flow temperature. I don't know if you can add and subtract OT devices from the bus at will.

[bruce_miranda]

Another stone at Viessmann. The manual for the CM737 states. "90°C or obtained from the boiler" and then the footnote states "1) Only if the setting is allowed by the heating appliance. Standard settings and limits can be set by the heating appliance." Clearly their boiler is not operating to their own manual (or Honeywell's )

The CM737 is almost identical to the CM937, except that the 937 isn't available in the UK. https://products.ecc.emea.honeywell....4nl01r0809.pdf

[bruce_miranda]

Stephen - you wouldn't happen to have a T87RF round room stat in the house? You could pair that with your OT Bridge and see if that solves the problem, albeit single zone. If it does then the Evohome to OT Bridge is an issue.

[StephenC]

Yes, I recently got one to work around HR92 placement issues on the bathroom radiator that was also right next to a heated towel rail (without an HR92). I got it in a pack with the R8810 actually.

Were you referring to the overshoot issue and max ch flow issue?

Good idea.

[bruce_miranda]

Yes. Delete any previous bindings on both the T87RF and OT Bridge. Just try to pair the T87RF with the OT Bridge like a single zone OT thermostat. And then observe what happens on the boiler when it calls for heat. Again the T87RF has no configurable boiler flow temperature parameter like the CM737 or the CM937, but would be interesting to see how the boiler works in this mode.