Opentherm control of system boiler
Viessmann system boiler - 4 pipe installation (one heating circuit & one hot water circuit to a hot water cylinder)
Opentherm control (via EU wireless extender) seems to be working well and flow temperature optimization seems to have the boiler modulating nicely.
I can set/schedule the hot water temperature via the Tado app, but when I turn off the hot water via Tado app the boiler (via its own diagnostics software) still shows hot water as ‘on’ and always at a commanded temperature of 55C
Where does this 55C value come from? Can I not turn off ‘hot water’ with Tado?
In the Tado app the ‘hot water control’ room shows two devices … the wireless receiver (the zone controller for all the other ‘rooms’) and the smart thermostat.
I am not sure I want the smart thermostat to be a second zone controller (I just want it to act as a wireless temp sensor) but this is where my understanding ends.
Any thoughts on where the ‘on’ ‘55C’ command is coming from?
Thanks
Answers
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A guess regarding the 55C lock. This could be a lower set locked temperature inside the boiler's base tables, to prevent the HW tank from acquiring Legionella on its walls, which happens at 48C and below. It is mandatory in NHS clinics and hospitals and the boiler probably defaults to that setting when informed it is serving a HW cylinder. Your Viessman service engineer should have access to those base tables when conducting a service and, if you sign a waiver, they can remove that lower temperature lock.
Your other query confuses me. I dont understand how your HW cylinder communicates its temperature with the boiler and the Tado controller. If, as I suspect, the boiler is receiving that data from the feed and return temp sensors on the HW loop and the Tado gets that relayed on. If not, I still dont get why a Tado Stat is involved. Perhaps time to contact support and get them involved?
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Thanks for that. Yes, I’m not sure why the smart thermostat needs to be able to control the hot water but it does for some reason - probably a hangover from when the V3+ system was first designed.
Regarding the cylinder - there is a NTC thermistor in the tank which sends the temperature to the boiler.
Viessmann support say that the 55C temperature demand is not theirs and that it’s coming from Tado via the OpenTherm interface. Tado support are still looking at it.
At the moment I can’t ever turn off the hot water so all I can do is turn it down to the minimum temp in Tado which is 30C0 -
So. Tado seem not to accept that their software doesn’t work properly over Opentherm when controlling hot water from a system boiler.
Whatever it is doing, it is apparently too ‘imbedded in the V3+ software” and can’t be changed.
I turn OFF the hot water in Tado but Tado actually turns in ON @55C. The boiler manufacturer, Viessmann confirms this behaviour.
It’s taken over 40 emails with Tado support to convince them that a system boiler with a hot water cylinder is not actually a combi boiler. Escalating the issue results in me repeatedly being told that the boiler isn’t actually heating to 55C despite the software indicating that fact - that I have a cylinder full of hot water and hot water coming out of the taps when the hot water is actually turned off doesn’t seem to convince them there is a problem.
On a positive note, the central heating control over OpenTherm is working really nicely.
Don’t buy this system if you want to control a system boiler over OpenTherm. Support is all but useless.1 -
@Rob. Would you be open to getting the tech team to looking at this? This 55C phenomenon is appearing with Viessman boilers. Is this linked in some way to the some firmware driven loop - either on the boiler side, or the Tado control side - which may have been designed to ensure that HW tanks are maintained at a minimum of 55c to prevent Legionella?
I know this is a policy that hospitals and clinics apply.
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Yes, I’ve spent a lot of time trying to resolve this, not just for myself. Tado support don’t seem interested in understanding what is happening0
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Hello @quidsinken
I had a look at your support chat. I don't think it's fair to say that "Tado support don’t seem interested in understanding what is happening". They've explained to you that your tado° Smart Thermostat is already controlling the hot water.They've checked the device data. Turning the hot water off always seems to work.
The boiler might still display the 55°C, but that is not something we can change as it is not a tado° device issue.
They've also double checked with tado° engineers and they too confirmed that everything has been working as expected.
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Thank you.
I’m mindful of being unfair when criticising people who are in a position to help so I hear what you say.
It hasn’t been explained to me that the smart thermostat is controlling my hot water; but you will have seen from the support history that I began by questioning the involvement of the smart thermostat but to no end.
Either way, it’s a moot point as the support team tell me that the water is not being heated and that it’s just an indication behaviour that they have no interest in changing. Were that the case then perhaps ‘fair enough’.
I have an advantage in that I live here and know from first hand experience that the hot water heats to 55C despite it being turned OFF in Tado. It is fair to say that no one at Tado, as yet, acknowledged that fact. This is the disinterest to which I referred.
Viessmann, who make the boiler, have interrogated the device and tell me that any commands they issue either remotely or via the boiler front panel are being overwritten by the OpenTherm interface to which Tado is connected.
When I am told by Tado support that they have looked at the installation that the boiler doesn’t even fire and that I am mistaken then my frustration is, I think, reasonable.
I like the product and what it is trying to do. This is my third installation and I have invested quite a bit of money in seeing it work as touted.
I’d be very grateful if you would suggest a constructive way forward? Thanks again.0 -
So - just in case anyone else is thinking about using Tado with a Viessmann system boiler. Tado will not turn off the hot water. Turning off the hot water in Tado only turns off control of the hot water temperature (which then defaults to 55C)
From Tado support:
Viessmann is using a slightly different way of controlling the hot water than the tado° V3+ devices do. The Viessmann boiler is waiting for a hot water stop signal, not just the normal "no demand" signal but a signal that actually would prevent the hot water generation completely.
Unfortunately, the tado° V3+ items will never send that kind of signal, because it would also interfere with the hot water on demand for combi boilers and the tado° hardware currently has no way of reliably telling when it is connected to a combi boiler or not.0 -
It seems like the boiler is innocent in all this.
Viessmann (Germany) say: “ When operating with OpenTherm, the boiler is given temperature setpoints and command bits by the OT thermostat (= master!) so that it knows exactly what to do. Our internal logic then no longer applies”0 -
@Emcee. Is this problem on the bug list?
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Hot water frost protection (when away) doesn’t work either - heats the hot water to a constant 55C0
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Not as far as I can see.
@quidisinken, please stay in touch with Chris. I see you wrote again a couple days ago. He should be in touch shortly.1 -
I share the frustration of @quidsinken . I am experiencing a similar issue with a Sime boiler and Tado X wired thermostat that overrides the domestic hot water temperature setting in the boiler to 60 degrees. If I try to decrease it to 45, it goes back to 60. Tado support was quite unhelpful with me as well actually denying the issue stating that "Tado X wired thermostat cannot control domestic hot water". Luckily, my installer managed to talk with a member of Italian Tado support team via the professional hotline who acknowledged the issue and stated that developers are working on a fix, but that it is not easy as every boiler behaves differently when it comes to OpenTherm connection. But it is really frutstrating as a customer spending several hundreds euro/pounds to install a system and facing issues to be dismissed like @quidsinken and me. I was lucky to have an installer who went the extra mile and tried to understand the issue and eventuallumet a professional support member. They promised to keep my installer posted on the progress of the fix and I will pass by the information here.
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