The Efficient Home Forum

Having spend hours trawling this forum I've learnt a huge amount (thanks to all) but I remain really confused about the situation with regards combining Evohome with Viessmann Vitodens combi boilers. Aspects of this topic have cropped up in a number of threads, most starting from a few years ago. I'd be really grateful for advice on the situation as it is today.

Unless Viessmann have changed the firmware in the new models of Vitodens boilers, I understand from this forum that: -
- When an OpenTherm device is plugged into the Vitodens boiler it sets it's flow temp at max (weather compensation off) and relies upon the OpenTherm device to regulate flow temp.
- The Evohome system does not send a flow temp to the boiler so the boiler is left to run at max flow temp.
- The Evohome system demands 100% from the boiler if any one of the HR92 TRVs is more than1.5degC below it's target temp; it is only once all HR92s are within 1.5degC of target temp the Evohome system modulates the boiler down (flow temp remains at max).
Based upon this It seems much of the benefit of OpenTherm is being lost.

My situation:
Detached house built 1999, 3 bedrooms, three bathrooms. I live alone, h/w demand only surges when family and friends visit. Most of the time I'm heating one of three rooms in the house at anyone time. In the new year I'll be installing a new combi boiler along with an Evohome system with HR92 TRVs (had Evohome in my last house and loved it).
My preferred boiler is the Vitodens 111-W 25W (great hot water flow rate), failing that either a Vitodens 100W 35kW or 200W 32kW.
My goal is to get maximum efficiency from my system.

Questions:
(a) Is my understanding above of the Vitodens and Evohome situation correct? If so what benefit am I getting from having Evohome connected via the R8810 OpenTherm?

(b) Should I forget OpenTherm and fit a BD91? At least this way the Vitodens uses its weather compensation to manage flow temp. The downside presumably is there is no boiler modulation.

(c) What is the difference between the Vitodens 100W and 200W? Is it just the modulation range?

In anticipation, many thanks, and thanks anyway for all that has already been shared on here.

I don't know about your specific boiler but I can tell you about our situation.
I have a vitodens 200-w which is not opentherm compatible, as far as I know. The viessmann thermostat installed in our home uses kmbus to communicate with the boiler.
The boiler does have a on/off contact but using this seems indeed to require to set the boiler in a mode which ignores its outdoor sensor and so its functionality of choosing a heating curve and circulation pump power is not active which is a real shame and a dealbreaker for me.

So I opted for cobbled-together solution of turning the boiler on and off using the viessmann API, using our home automation system to read out the BD91 and command the boiler over API. This means the viessmann thermostat stays and the boiler keeps using its weather dependent settings.
This is far from ideal with the viessmann api being spotty at times but works at the moment.

David38 wrote:Having spend hours trawling this forum I've learnt a huge amount (thanks to all) but I remain really confused about the situation with regards combining Evohome with Viessmann Vitodens combi boilers. Aspects of this topic have cropped up in a number of threads, most starting from a few years ago. I'd be really grateful for advice on the situation as it is today.

Unless Viessmann have changed the firmware in the new models of Vitodens boilers, I understand from this forum that: -
- When an OpenTherm device is plugged into the Vitodens boiler it sets it's flow temp at max (weather compensation off) and relies upon the OpenTherm device to regulate flow temp.
- The Evohome system does not send a flow temp to the boiler so the boiler is left to run at max flow temp.
- The Evohome system demands 100% from the boiler if any one of the HR92 TRVs is more than1.5degC below it's target temp; it is only once all HR92s are within 1.5degC of target temp the Evohome system modulates the boiler down (flow temp remains at max).
Based upon this It seems much of the benefit of OpenTherm is being lost.

My situation:
Detached house built 1999, 3 bedrooms, three bathrooms. I live alone, h/w demand only surges when family and friends visit. Most of the time I'm heating one of three rooms in the house at anyone time. In the new year I'll be installing a new combi boiler along with an Evohome system with HR92 TRVs (had Evohome in my last house and loved it).
My preferred boiler is the Vitodens 111-W 25W (great hot water flow rate), failing that either a Vitodens 100W 35kW or 200W 32kW.
My goal is to get maximum efficiency from my system.

Questions:
(a) Is my understanding above of the Vitodens and Evohome situation correct? If so what benefit am I getting from having Evohome connected via the R8810 OpenTherm?

(b) Should I forget OpenTherm and fit a BD91? At least this way the Vitodens uses its weather compensation to manage flow temp. The downside presumably is there is no boiler modulation.

(c) What is the difference between the Vitodens 100W and 200W? Is it just the modulation range?

In anticipation, many thanks, and thanks anyway for all that has already been shared on here.

AFAIK, the situation = the same.

Why so hung up on Viessmann and evohome. If you have evohome, there are boilers that work better with it (Intergas and Alpha are my recommendations) and everything else is a compromise imho? I do like Viessmann, but this functionality issue is a pain in the backside tbh.

The reason I've leaned towards the Vitodens 111-W is its hot water flow rate and in addition its modulation range. I have looked at Intergas and Alpha combi boilers but can't see they offer the hot water flow rate, certainly not that combined with the modulation range. However if using EvoHome with a Viessman disables a lot of the modulation benefits then all I might as well focus on is hot water flow rate

Well there is more details on the 100 series and evohome on OpenTherm here - viewtopic.php?f=5&t=477

I like the 111, but its a beast!

Thanks, I've spend some time slowly re-reading through that thread. I think the conclusion is unless Viessmann have changed their firmware in the latest models I'll have to go to Alpha or Intergas if I want the system to run efficiently.

David

hey
what did you do in the end
having some issues with mine, over here
posting.php?mode=edit&f=5&p=13006

I've decided to go with the Vitodens 111W.

I contacted technical support at Viessman about the problems I had heard with the Vitodens 111W and Opentherm with the following enquiry:
"Although the Vitodens 111W is Opentherm I have read that in Opentherm mode the boiler fixes the flow temp at maximum 85degC. As a result room temperature can overshoot and presumably this is also an inefficient way of running the boiler.
The articles I read were from 3-4 years ago and I understand at that time Viessman confirmed the situation. Has the firmware/boiler design since been changed or does the boiler still operate this way? Please advise on the best way to integrate the 111W with the Evohome controller."

The response I got was:
"The issue was never a Viessmann one,
An Opentherm device when first Installed is a learning device, so it usually pushes the Boiler to its limit to set a range of operation before it eventually then modulates itself down once the Boiler limits have been assessed,
Opentherm is supposed to be:-
• OpenTherm is a comm

• • unication protocol between the boiler and the room thermostat.
    • The communication protocol describes the way digital information is exchanged between central heating appliances and room thermostats.
    • OpenTherm is a multi point to point communication protocol (read and/or write) for use in residential heating.
    • With OpenTherm thermostat and a modulating boiler the boiler capacity is adjusted according to the heat demand.
    • Modulating results in longer on periods of the burner but with a lower water temperature.
    • Saves energy as the boiler operates on a higher efficiency with a lower water temperature.
    • Increased comfort feeling due to more stable and less fluctuating water temperature in the radiators.
    • More environmental friendly due to less boiler starts and stops.

However, we found that not all manufacturers of Opentherm controls offer the same functions, some simply provide Heating control while others, Heating and Hot Water control, which can cause issues with Flow temperature, in particular Hot Water as it can no longer be adjusted on the Boiler panel due to the Opentherm taking over the operational controls rendering the boiler panel redundant.

In light of this, the new Vitodens range of Boilers released over the past 18 months or so operate as below :-
• Set DHW temperature can be adjusted via OpenTherm thermostat (If OpenTherm thermostat doesn’t support DHW, S1 internal DHW setpoint temperature is used)
This internal setpoint is the 50 degree factory setting or whatever the boiler was set at prior to opentherm connectivity."


The response was not wonderfully helpful, it comes across to me as being a mixture of boiler plate and fogging the issue. However, I interpret the last paragraph as meaning Viessman have updated the firmware in the current Vitodens 111W models.

Regards, David