The Efficient Home Forum

I have purchased the wifi boiler control and hot water controllers (Connected Thermostat Pack ATP921R3100 and ATF500DHW Hot Water Kit) and am now trying to wire up these to the boiler.

A few questions / looking for advice:

1 - am I better wiring the controllers up using the BDR91 relays that come with the kits, or via an opentherm bridge? It seems that the opentherm bridge also controls the water temperature so that may be a better option?
2 - my boiler has a weather compensation system, so if I do connect up using the opentherm bridge would I be better off disconnecting the weather compensation?
3 - does the opentherm bridge control both hot water and heating, so only one connection to the boiler needed, or would I still need the relay for the hot water?
4 - on various forums a number of vittodens customers seem to say that the hot water heating doesn't work very well as the boiler doesn't output maximum heat to the unvented cylinder without a separate control unit installed within the boiler to tell it not to limit the water temperature. Does opentherm overcome this?

Thanks in advance.

Jon

Sorry Jon, looks like your post got missed!

In answer to your questions for future readers...

1. OpenTherm is a much more capable way of controlling evohome and will always be my preference over ON/OFF or TPI control with a BDR91 relay.

2. Weather Compensation and OpenTherm do not work together normally. It is either OpenTherm or WC.

3. The R8810 OpenTherm Bridge when used with evohome will tell the boiler what to do. The BDR91 still needs to operate the valve that controls hot water and we recommend using 'hot water priority' when using OpenTherm.

4. evohome will instruct the boiler what to do and just like WC, the flow temp will be reduced to increase efficiency and matched to the internal temperature load.

It should be noted that there is a 'rapid learning phase' during the first 7-14 days of using evohome with OpenTherm and the R8810 will instruct the boiler to fire at higher than normal flow temps while the learning process gets underway. This can seem like OpenTherm is not working, but this is perfectly normal and lower modulation temperatures will happen after the rapid learning phase has ended.

I hope that helps.

Thanks,

Richard

Thanks Richard

Some follow-up questions on 3 and 4

3 - I don't have any external valves for the hot water. The boiler has its own separate feeds/returns for hot water and heating. I'm hoping opentherm can call for either water or heating as I'm not sure where I would wire the BDR into.

4 - comments I refer to are in the link below. Maybe 'try it and see' will be the best way forward, with a call to Viessman if the boiler doesn't feed hot water on 'full blast' when needed.

https://www.diynot.com/diy/threads/vies ... 416/page-4

Jon

Hi Jon,

The comments you refer to on DIYnot are from early last year when no one really knew how OpenTherm worked (including the very clued up Dan Robinson who is a friend of mine).

As for comment 3, this will be a call to Viessmann technical to find this out. 'Overviews' on how the equipment works I am happy to help with but individual situations/circumstances I do not get involved with - this is above my pay grade and what manufacturers are for.

Thanks,

Richard

I have just specified a Viessmann 100W boiler with the intention of using it with Evohome and an Opentherm bridge. It has been installed and is awaiting commissioning and the installation of Evohome (by Dan Robinson, I hope). This all seemed fine except I have just read a thread on Automatedhome.com which suggests that there is an incompatibility between the Viessmann Opentherm implementation and Evohome. The issue reported is that the Evohome always calls for 100% boiler output (flow temperature not defined by either master or slave) if a zone is outside 1.5C rain / shine, summer or winter causing persistent overshoots.

The thread is here: http://www.automatedhome.co.uk/vbulleti ... ots/page11

As a newbie to Evohome is this a big issue and should I be ditching the Opentherm bridge in favour of Viessmann weather compensation and a straightforward BR91 switch?

It would seem to me having read the thread that while one could argue about whether Viessmann's implementation is correct fundamentally the Evohome controller is failing to do what it says on the tin by failing to demand an appropriate flow temperature for the load (determined by learning or external air temperature received from the internet or whatever). Isn't that why one has bought a modulating boiler and an opentherm controller: to avoid overshoots and wasted gas?

Also, is this an issue that Honeywell is likely to address through a software update and is the Opentherm bridge bought now updatable?

I would be grateful for any advice.

Thank you.

Sorry, perhaps I should have posted in the technical discussion but actually it is more of a practical question on the best set up to go for with the Viessmann now, while one waits for Honeywell and Viessmann get their acts together.

Francis

Update on my setup - it all works!

Viesmann Vittodens 100W boiler - heating controlled by opentherm bridge, as well as existing viesmann weather comp sensor.

Hot water controlled through BDR91 acting as a relay switch wired into the Viessmann Cylinder Demand box and boiler trigger input.

Existing Viessmann Vitotrol controller completely removed.

When the heating fires it gets full blast heating to get the house up to temperature, then modulates down to a normal radiator heat.

When the hot water fires it gets full blast to heat the cylinder quickly.

Just two issues to fix now

1 - remove the phantom hot water relay (I managed to pair two in the setup but I only have one)
2 - fix the cylinder overflow as it is leaking water outside

A problem fir a rainy day methinks