If you are happy to keep the HW control separate from the CH then that's fine but it requires some thought and schedule disciplines. The issue is that they both won't be controllable from a single app. And you will need to set the boiler max as required when within HW heating cycle. Just an on-off won't do when combined with Opentherm. Imagine this - No CH demand, So OT will set the target flow to 10C, then your HW demand kicks in but the boiler now will need to override that target with a higher than normal target. When you get back to CH, you then again don't need that high target. You can work around this but.....
Which OpenTherm boilers work well with Evohome?
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Originally posted by mtmcgavock View Posthowever using the Opentherm will disable the boiler flow temperature controller on the front. Whether this is an issue or not seems to be debatable
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The boiler doesn't run at it's physical max all the time as the Opentherm controls what flow temperature it requires? Also the max flow temperature it reaches is only the max temperature that could be set using the physical controller if the Opentherm wasn't connected, as the boiler was designed to do this no damage will be done running it at this temperature and many other boilers by other manufactures I have no doubt people will have set on their maximum. Whilst it might be debatable this maybe not the most efficient way of running a system it will certainly do no harm to the boiler.
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I'm in full agreement with Bruce, and I have the (bad) experience to back it up - not just theory.
Honeywell Technical support also stated it was dumb for a boiler to not have an installer or end user max ch flow temp setting even during Opentherm control.
To quote Viessmann Technical support:
"Dear Stephen,
If the Honeywell control worked fine whilst using the switch live demand through the BDR91 then there is no logical reason why, when connected to the open therm through a R8810 Bridge, this should be any different, the boiler control logic should not be overridden by the Evohome, If the boiler is set to 65 degrees for example then the Evohome should not force the boiler to 81/82 degrees.
We suggest you ask Honeywell if any settings can be made to the Evohome to adjust this issue, potentially, if the boiler is being forced to 82 degrees this is very close to the boiler limit stat and could potentially cause the boiler to lock out and no heating would be available until the boiler is reset.
All of our boilers are fully modulating and have had open therm connection for the past 10 years without any issue.
Kind Regards
Lee Pedley
Viessmann Technical Support "
Then this from the same person 3 weeks later once I (who doesn't work in the heating industry...) showed them evidence they were wrong:
"Dear Sir,
Thank you for your recent enquiry.
Following your initial enquiry, we have looked into the issue and can advise that when an opentherm device is connected into our boiler opentherm connections then it takes over full control of the boiler, the controls on the front of the boiler are overidden as you correctly identified in the US manual. Weather Compensation is also disabled so would not give functional control back if fitted.
If a switch live option is available on the Honeywell controller and is connected to our switch live connections on the boiler, then the controls on the front of the boiler can be used, however, we do not know what affect this will have on the functionality of the Honeywell controller. The communication protocol used by the Honeywell controls are unknown to us, therefore, we cannot comment or advise on how it will actually control / modulate our boiler.
We trust this clarifies your enquiry.
Kind Regards
Lee Pedley
Technical Supervisor
Viessmann Ltd
Hortonwood 30
Telford TF1 7YP"
Funny thing is, guess what: I've been having boiler faults in the last week or so, and have had to have Viessmann out for warranty repairs that they have stated was likely caused by excessively high heating temperatures causing radiators to expand/contract and thereby creating debris into the system. The system was new 2 years ago, and was powerflushed for nearly a day so I know it was clean then.
I was starting to see 89 degrees C in the last week for central heating. Ludicrous.
I've gone back to BDR91 in last few days (phew), with weather compensation. This is working well, albeit a little dumb (but much better than fixed temp). Ironically I miss the fast room warm up times that Opentherm brought! I don't however miss the 3-5 degree overshoots I had been getting in the warmer weather though. No amount of HR92 learning was able to deal with excessively high ch temps with radiator sizing designed for 50-60 degree temps. What is needed even with Opentherm control, is a relevant temp limit ability based on the house heat loss and rad sizing characteristics.
Viessmann have no idea themselves about Opentherm, that is quite clear, and at best are designed with vendor lock in, so anyone considering Opentherm should stay away from Viessmann boilers (I will be in the future, and it looks like ATAG is very good at out of the box Opentherm interoperability from what I am reading on here)
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I am aware that this thread is moving along a wide possible gamut of subjects!
Bruce_Miranda - Yes I think you are right about the control of HW by Evohome under my existing arrangement.
I assumed that if I now shift to a new OT boiler (Atag) I will have to bring the HW within the Evohome system rather than my make-do use of elements of the old time clock etc.
Question - I also assumed that I may need to change my UFH heating arrangements too? I currently use a sequence of Honeywell DT92E wireless room 'stats and corresponding receivers which all meet via a Emmeti wiring and relay centre and then power is then sent towards to the individual UFH valve heads and power to boiler and local UFH pump as required. I assume I may need to install the "Honeywell HCC80R Evohome Underfloor Zoning Controller RF" instead, or is this really only a duplication of what I already have in a sleeker format?
Thanks
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A HCC80R allows you to zone a single UFH manifold. Whether you need it of not completely depends on how your UFH is configured. I have UFH in one 100sqm open plan zone. So there was little point of a HCC80R, I just use a single thermostat and a BDR91. Also when zoning a UFH you will need to have intimate knowledge of the pipe runs and the valve placements on the manifold.
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Originally posted by StephenC View Post
Viessmann have no idea themselves about Opentherm, that is quite clear, and at best are designed with vendor lock in, so anyone considering Opentherm should stay away from Viessmann boilers (I will be in the future, and it looks like ATAG is very good at out of the box Opentherm interoperability from what I am reading on here)
Some of the manufacturers (Intergas for example) told me that they have run boiler bench tests with Evohome, using an Opentherm connection, with Honeywell representatives present.
As far as my Atag is concerned, Opentherm is controlling 100% to the T-Set (calculated Flow) temperature - not the max flow temperature. I have absolutely no idea how T-Set is calculated but it is set via an installer profile. The boiler has no manual control of the actual flow temperature. As I write, the boiler is on 70C Flow, 61C Return and 70C T-Set. One zone (3 large radiators) is on heat as the 5C window open temperature reduction kicked in without me noticing. I think my wife was feeding the birds when I was in the shower.
My only comment about Opentherm is that it seems, at least to me, to be slow to get the boiler into the full condensing mode but, having never owned a modulating condensing boiler before, I can only assume that under Opentherm control closing the gas valve reduces the effective boiler size from 23.2kW down to 5kW as a priority. Most certainly, at the moment, the boiler is just purring with little obvious external plume.
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"Which OpenTherm boilers work well with Evohome?"... Well, it's not strictly an "OpenTherm boiler", but I've just had a Vaillant EcoFit Pure 625 25kW system boiler installed, and (with the addition of a VR33 module), it seems to work perfectly. In particular, it still obeys the flow temperature limit set at the boiler, which seems to be an issue with other makes. The VR33 is not officially supported on UK boilers, and cannot be bought from UK distributors, but they're easy enough to get from European suppliers.
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