ecoTEC plus 624 (SW=0414/HW=7401) — is the write path just not there on this firmware?

[lukejameson]

So I've got a Vaillant ecoTEC plus 624 system boiler (VU GB 246/3-5, 2008, LPG) and I've been trying to get flow temp control going over eBUS. Reads all work fine, but every write I try just doesn't land.

ebusctl info: MF=Vaillant;ID=BAI00;SW=0414;HW=7401, loads vaillant/bai.308523.inc. Running ebusd 26.1 in a HA addon with an eBUS Adapter Shield C5.

What I want is simple: CH around 60, DHW around 75-80 (indirect cylinder, 20kW coil). I know #1572 is a 624 with SW=0518 / HW=7401 doing exactly this with SetMode writes. Same hardware, newer firmware.

Here's what I've tried:

read -h 08B5100100 → 0101

The config has SetMode (B510) as uw with hcmode + flowtempdesired + hwctempdesired + hwcflowtempdesired, but the boiler only ever sends back the single hcmode byte.

read SetModeOverride → ERR: invalid position in decode: off
(dies straight after hcmode)

writing SetMode with the format from #1572, "auto;45;-;-;0;0;1;0;0;0" → empty, nothing changes, FlowTempDesired stays 75.

write -h 08B509050E39006004 (FlowTempDesired) → ERR: element not found

write -h 08B509040EF20301 (HeatingSwitch d.23) → ERR: element not found

Also noticed the controller inputs in my loaded config (HeatingSwitch d.23, StoragereleaseClock d.25, TimerInputHc) are all marked r, while other boiler configs like bai.0010008045.inc have HeatingSwitch as r;w. So it's not just SetMode being awkward, it's everything.

So the question is really: is SW=0414 just read-only? Was eBUS on this generation monitoring/diagnostics only, with the write support added in later firmware? Because #1572 is the same HW (7401) on 0518 and it works there.

Any pointers welcome.