Evohome, Vaillant 637 s plan and opentherm

[Pghstochaj]

Hi everybody

I note that this is a fairly quiet part of the forum so I am not holding out much hope, but it bugs me so I need to try!

My set up is as follows:
- Vaillant Ecotek 637 plus system boiler
- Unvented hot water tank
- S plan set up with one BDR91 doing the central heating circuit via an actuated valve and a second BDR91 doing the hot water system via a second actuated valve controlled by a hot water kit with the thermocouple in the thermowell on the tank.
- I have two control panels, one controlling downstairs rads and the hot water, and the other controlling the upstairs rads. I guess both are bound with the heating BDR91.
- Each rad has a HR92.

Reading up, there are simpler ways of setting it up but, it works fine as set up and I am happy with it. I use a lot of gas so any fractional improvement is of benefit.

My boiler is about to end its 7 year warranty. I want to explore whether my system can use opentherm. I know as standard, that Vaillant boilers in the UK cannot work with opentherm but I am willing to get the VR33 to modify the boiler. My question is, is it possible to have opentherm for this S plan design, and if so, how does the boiler know to go to a higher temperature for water reheat vs. heating flow temperatures. Also, how does it get wired, do I remove the actuated valve for the heating system? I looked a while back and couldn't find any obvious circuit drawings to understand how it would be set up. I think I would get a R8810 to replace the BDR91 on the heating valve and remove the valve (?), but retain the hot water BDR91 with valve, but if so, how does the boiler know to increase temp for hot water.

Thanks very much for anybody that can point me in the right direction. It would be fine to say "you can't do it" as it can put a stop to me looking into it and wasting my time!

[Richard]

The evohome system was designed so that OpenTherm could still be used with really basic boilers, hence it has its own hot water control using the hot water kit.

Basic operation when using evohome on OpenTherm is space heating = lower modulating flow temp & hot water demand = high flow temp.

'S Plan' is not a great design for OpenTherm and PDHW (Priority Domestic Hot Water) is a much better hydronic design for evohome, using a 3 port diverter or an NO (heating) and NC (hot water) valve (instead of two NC valves)- basically so you have somewhere the water can go at all times.

There are a few customers of ours who over the years have used the VR33 with evohome successfully on a Vaillant boiler (the VR33 is made by Vaillant and works with their boilers in the Netherlands?).

I cannot give you any more details, as I have not done this myself.

[Pghstochaj]

Thanks very much for the reply. Hopefully somebody that has done it will see this post.

I guess the bit I don't understand is how the boiler knows to do high temperature for hot water demand versus modulated temperature/flow for heating demand. I can't find any wiring schematics for it (I guess I could if I searched in Dutch!).

Looking at the bridge for heating:

I would remove the current central heating valve or make it a NO valve.
I would disconnect the BDR91 from the valve and boiler.
I would install the VR33 and connect the Evohome opentherm bridge to the VR33.

The boiler is now set up for opentherm on the heating?

However, does the BDR91 for the hot water stay as it is and the boiler can differentiate between a hot water run signal (and sets the flow temperature accordingly) versus the opentherm command for heating? That's the bit that I don't understand.

Again, thanks for your post and hopefully somebody that has done this will be able to see this post.

[sotafo]

Yes, you can integrate OpenTherm with your S-plan system using the VR33 module for your Vaillant boiler. Replace the heating BDR91 with the R8810 OpenTherm bridge to control the heating. Keep the hot water BDR91 and valve as is. The boiler will modulate its temperature based on demand through OpenTherm, but it should prioritize higher temps for hot water when needed.