Viessmann, OpenTherm and smart thermostats

[jelockwood]

I am currently researching a choice of new boiler and intend to use a Tado Smart thermostat system. I am looking for a sealed system boiler, that is not a combi system nor to have an expansion tank. I also want to get a matching i.e. same manufacturer hot water tank.

This seems to limit choices to Vaillant, Worcester Bosch and Viessmann as most others do not do matching hot water tanks.

Vaillant recently-ish changed their controller from the compatible VR65 to the incompatible VR66 and hence their proprietary eBus implementation with the result Tado is no longer compatible. (Tado is pretty unique in supporting eBus with various boiler manufacturers.) Whilst there is the VR33 module to translate Vaillaint's eBus to standard OpenTherm Vaillant will regard the use of this as invalidating your warranty even though they make this part. (It is intended for the Dutch market which requires boilers to use OpenTherm.) Worcester Bosch also use eBus but as far as I am aware have not changed their eBus standard so is still supported by Tado. Viessmann use OpenTherm.

I had therefore been leaning towards Worcester Bosch but recently discovered that Viessmann have a matching hot water thank and also sell a matching solar hot water system and seem unique in this later aspect. I had not originally considered fitting solar hot water but do intend to fit solar power tiles but my roof layout would make it easy and feasible to fit both.

However here on this site I did come across a potential issue discussed in this thread - https://www.automatedhome.co.uk/vbul...-W-overshoots/

It basically boils down (groan! ) to the fact that if you use OpenTherm then the built in controls are disabled but the boiler does not send the Central Heating Max heat value and that the Honeywell Evohome does not itself have the ability to set the same value with the result the boiler can unnecessarily run in max heat until the thermostat learns the environment.

I asked Tado about this and I thought the answer might be of interest to other people here. I would also like to hear from anyone who is using Tado with a Viessmann boiler. (It should be noted Viessmann actually resell Tado.)

Here is what Tado said :-

Thanks for consulting with us.

When using OpenTherm (or any other digital control interface) we will make the heating demand as a modulating command, (from 0% to 100% as required). The 100% will be affected by the limit that is established in the boiler. So, when we're talking about a boiler model that can adjust the temperature with a knob in the front, for example, this knob will be limiting our request (our 100% is the max it has set in the knob).

We don't have any known issue with Viessmann boilers, however we don't have that many connected yet.

What we can do in Opentherm is setting a max flow temperature (as a one time setting, once you've done the installation), but if you would need to change this often then it wouldn't be possible.

Now as additional information to the above remember that I said that with Viessmann when OpenTherm is activated this disables the built-in controls. Also as per the older thread I linked to above Viessmann wasn't and likely still isn't sending the Central Heating Max Heat value to the smart thermostat and hence causing a problem with the Honeywell Evohome.

However the potential good news is that Tado say their product allows setting this value during installation i.e. when you add the boiler to the setup on your account and this differs from Honeywell which don't offer that option.

Does anyone have any advice on what value to set? I believe the consensus from the above thread was 80c?

[Somebody]

Do you have a S or Y plan system with Hot Water? I was considering going OpenTherm with Evohome when replacing my boiler but I was advised that hot water prioritisation is a factor to consider.

Specifically "You do need to understand that an OpenTherm system operates quite differently to on/off switching system and therefore traditional heating system configurations (S & Y Plan) are not ideal for modulation system conversions.

S & Y Plan systems were designed in a time where water temperature for heating and hot water was the same. With OpenTherm, space heating can be at a very different temperature (maybe 30 Deg C) to what is then required for hot water reheat (70+ Deg C). There are other factors too, but this is the main reason why S & Y Plan systems are bad system design if you are considering going over to OpenTherm."

[jelockwood]

I will be ripping out the entire existing system and starting from scratch so the fact I currently have a Y plan setup may not be relevant. I do see your point though.

[GreenGuy]

If you use Viessmann with Opentherm make sure that Viessmann can supply and support the whole system, I'm using a Viessmann with Opentherm and Honeywell Evohome and they don't play well together. I had to build and programme the Opentherm Gateway to get proper DHW control.

[jelockwood]

If you use Viessmann with Opentherm make sure that Viessmann can supply and support the whole system, I'm using a Viessmann with Opentherm and Honeywell Evohome and they don't play well together. I had to build and programme the Opentherm Gateway to get proper DHW control.

Yes I linked to and mentioned one of the issues between Viessmann and Evohome which is why I was enquiring about people's experiences if any of using Tado instead.

I am also hoping the response from Tado showing they do allow once during the setup a manual value for the Max heat setting unlike Evohome would avoid that issue.

[marc_w]

I asked Tado about this and I thought the answer might be of interest to other people here. I would also like to hear from anyone who is using Tado with a Viessmann boiler. (It should be noted Viessmann actually resell Tado.)

I can confirm this, Tado support remotely changed my system's maximum flow temperature after my request (it took two working days).
I have Viessmann 100w boiler and after changing connection of my extention kit from switched live to Opentherm interface of my boiler, boiler was operating at up to 88°C on the flow. I was unable to set the maximum on the boiler itself, as is discussed in detail in the link you posted.

Does anyone have any advice on what value to set? I believe the consensus from the above thread was 80c?

I requested max temperature of 75°C as recommended by Viessmann

I have always found Tado support helpful. Previously, when I changed my setup by adding the extention kit, they sent me wiring instructions and also changed all the settings remotely so all I had to do was to disconnect my Tado thermostat wires and connect extention kit instead. This also meant that heating suddenly gone off in the middle of the day while my partner was at home and the was at work - but I don't really have a problem with that.
So I can really recommend Tado, very happy with them and their products.

But Viessmann are little bit disappointing, my boiler was installed in Oct 2019 and it is shocking that Viessman still haven't done anything about it.