So...

I have a WB Greenstar Classic 42 CDi ErP, which I've "persuaded" into giving me access to electronic data via
a) https://en.robbshop.nl/ems-ot-opentherm-converter-nefit
plugged into
b) http://ihormelnyk.com/opentherm_adapter
plugged into an ESP8266

Which seems to be working! (Previously I have had an ESP8266 monitoring demand via relays attached to 2x zone valves and some temperature sensors.

There's a few oddities though, maybe someone has been down this path before?

1) The 'return' temperature value seems to return always -76.80. I'm fairly sure this is not a translation error, the raw data coming back is 0x401cb334 (the last part being the value - b334 - which is definitely negative). This is a bit disappointing since I was keen to know the CH return temp to ascertain if I was condensing or not.

2) Similarly, the CH pressure value seems to be a constant '25.50', which is fairly obviously either not bars. Maybe it's bar*10?

3) Not sure I understand what 'modulation ratio' is telling me - it can be 'flame on, 0%', or values up to about 26%. Higher numbers seem to tally with more gas usage not less, which is the opposite to what I expected.

I was a bit surprised that the opentherm boiler setpoint (Tset) can be written, but not read. Indeed when I made an error attempting to read it, it just returned 0 (rather than an error - it's actually not in the spec) and set it to 0 (so no heat). Also setting a value above Tsetmax seems not to clip it to TSetMax, but to ignore you (so no heat). Not a biggie but it took me a while.

Other than that, I was happy to see I can actually combine it with my existing 2-zone system (1x evohome, 1x "normal" system), in that the boiler temperature can be below Tset, but if there is no zone calling for heat, it keeps the CH 'off' (which happily means I think the failure mode where my ESP interface fails will continue to work based on the last ch setpoint).

What I'm hoping is to use the current weather temperature plus which zones are enabled to adjust Tset. Zone 1 is our house, and Zone 2 is a separate small flat, so it's difficult since any temp fixed on the boiler is going to be a compromise of being too low for the house (on it's own), but too high for the flat (on it's own). I'm assuming that me setting the Tset value is broadly comparable to twiddling the 'CH temperature' dial (which sets TSetMax for OpenTherm, and I assume TSet equivalent when it's not OpenTherm)

It's already pleasing to see temp, CH and flame data, I've a much better idea of what's going on, if only I could get at the return temps