Apologies if this appears twice - the original may be in the moderation queue but I've not seen anything!

I have a Rinnai (made by Atag) E32S condensing boiler. It claims to support OpenTherm in the manual and has terminals marked OpenTherm on the boiler connection block. Rinnai sell a controller called the RS100 which apparently allows it to be controlled in a modulating fashion and I assumed that it also used OpenTherm technology.

I bought and connected an Evohome OpenTherm bridge. Initially it flashed red as expected. I bound it to the controller and it started to give three red pulses (not sure what that means but I assumed it meant it wasn't talking to the boiler). I shut down power to everything and when it turned back on it was doing the green pulse as expected.

A signal test shows 5 flashes on the bridge, but there's no "Excellent" on the controller like there is with the BDR91s (though the help text suggests that's normal).

The boiler is behaving as if there's nothing controlling it other than its normal behaviours. It's only firing when the volt-free demand contacts are closed (from the valve microswitches), and sitting at the maximum temperature rather than modulating in any way. The manual for the RS100 controller suggest that when the boiler is being successfully controlled over the bus it will ignore its volt-free contacts so I assume that the boiler is not hearing the bridge at all.

I have the older, non-wifi version of the Evohome controller so my debugging opportunities are limited. I have access to a multimeter and oscilloscope - though the fact the bridge is powered at all suggests it's not a physical issue and I'm starting to wonder if my boiler doesn't actually talk OpenTherm at all?

Does anyone have any advice on debugging this or anything on Rinnai/Atag E-series boilers?