Viessmann, Opentherm, Weather-Compensation, Evohome

18 Sep 2016
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United Kingdom
Hi Everyone,

I've been searching online and came across this website which seemed to have knowledgeable contributors so thought I'd join to ask some questions about a planned new central heating system. Apologies for the long post whilst I set the scene.

My house consists of a lounge, a family room/office, an open plan kitchen/dining room, utility and WC downstairs. Upstairs are 4 bedrooms, an en-suite and bathroom. From the layout, it makes sense to zone the system; especially since my wife works from home a few days a week and our weekly schedule varies a lot.

My new system will consist of a system boiler, an unvented hot water tank, and 12 radiators (4 of which are towel rail type and the rest standard). In future, I plan to include solar thermal on the hot water tank so am requesting a twin coil solar cylinder installed ready for when I have the cash to get it installed. I am planning to get either a Viessmann, Worcester Bosch, or Vaillant. Preference at the moment is for Viessmann or Worcester Bosch as they have 10 year guarantees and good customer service. I've read that Viessmann support OT however comments online suggest only certain versions. Another brand is Intergas (which is apparently fully OT compliant) but none of the plumbers I've had around know much about them.

Before I click "go" on a system, I want to ensure that all of the components can work together in harmony with an Evohome Controller. To my list of questions:

1. Is the use of Opentherm compatible with zoning? I presumed this would be the best configuration but some comments online appear to suggest zoning and opentherm are not compatible (Bear in mind there is specific system design criterion for 'system boilers' using OpenTherm with evohome - no motorised zone valve on heating side of the system and HR92's fitted to all radiators is one such requirement).

2. Is the use of Opentherm compatible with a boiler with Weather Compensation (WC), will the two systems "fight" each other? I've read Opentherm is equivalent to WC for internal temperature compensation but is it that simple and would the Evohome use online external temperature to adjust OT control?

3. Is the Evohome "learning" feature of how long a zone takes to warm up/cool down compatible with Opentherm?

4. If the boiler also has WC, will the Evohome Wi-Fi Connected Controller be able to "learn" the time-to-heat/cool a zone whilst also using Opentherm as the warm up/cool down times will be affected by the boilers own independent weather compensation and the response will not always be consistent?

5. Do I need the ATF500DHW Hot Water Kit to control my hot water or does the OpenTherm Bridge R8810 allow for both central heating and water control?

6. Will the Evohome Wi-Fi Connected Controller allow for water temperature control - if so what hardware is required? Once my solar thermal is fitted, I want to maximise the solar heating and programme the boiler to only top-up the water heating to specific temperatures at specific times.

7. Which boiler/s would you recommend that would be compatible with my control system? My plumber has suggested a Vitodens 200-W (which seems a good boiler as it has wider control over the OT control range?, but also has WC), yet I've read online its not compatible with Evohome OT. The Vitodens 100W seems to work according to some online forums but has a compromised OT control range.

Thanks and looking forward to here your thoughts!
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You my friend have more money than sense. For a wee gaff like yours that's complete overkill!!
Have you any idea how long the small savings you'll make will take to buy all that 'so called' technoligy. Save your money & buy a bigger house.
If the same new boiler, Viessmann Vitodens 200, the same unvented cylinder, the same radiators, and importantly the same energy performance rated house.

On/OFF room thermostats (ErP class I), most mechanical room stats

TPI room thermostats (ErP class IV), most digital room stats.

Modulating controls by weather compensation (ErP class VI), need an outdoor sensor.


OpenTherm is a protocol.

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