ATAG Q25S with OpenTherm, DHW not working

[Xirr]

Cheers to all,

I'm new here and would love to get suggestions to a problem with the R8810 Opentherm receiver combined with and ATAG Q25S boiler.

The problem is that with the OpenTherm receiver the boiler doesn't switch over to and doesn't heat the DHW circuit.

I have an Evohome Wifi setup with a couple of HR92 thermostat valves. The Evohome controller was connected to the boiler via BDR91 wireless relay. Hot water (DHW) is in a separate storage tank that gets heated via a two way valve which the boiler switches over whenever the storage water temp drops below 65 degree. (=DHW control via the ATAG boiler, disabled in Evohome) That setup worked perfectly fine.

Now I've connected a R8810 OpenTherm wireless receiver instead of the BDR91. The heating circuit works fine, including modulation and all. However, the boiler doesn't switch over to DHW heating anymore. Removing the R8810 and replacing it with BDR91 was the only way to get the hot water working again.

I have not found reference in any installation manual (both ATAG and Honeywell) for any setting or parameter that would cause/remedy this behavior when the R8810 OpenTherm is used. There aren't really any setting related to DHW on the boiler and the DHW control on the Evohome controller is disabled. As the heating works fine the bus connection boiler/receiver must be working properly as well, I assume.

After a a long while of trying various settings it seems that when DHW demand is triggered the boiler prioritises that demand and switches to DHW mode immediately while connected with the BDR91, but ignores the DHW demand when the R8810 is connected.

Has anyone else had a similar problem?

Any suggestion how this could be resolved (short of buying the DHW kit)?

Any help or suggestions would be greatly appreciated!

(My setup is in Belgium but my French is atrocious, hence the question in this forum!)

Andre

[Bart]

I think that is a solo boiler, so the priority for DHW so I guess that is realized by wiring. When moving from BDR91 to OpenTherm, what wiring connections did you change?
Do you have drawings of the installation?
When DHW is requested, nothing at all happens?