Rinnai water heater power and communication interface/protocol
I am interested in exact same answer and I have exact same setup. The reason is their Rinnai's wifi module is complete mess due to bad WIFI and bad mobile app implementation.
If I could reverse engineer their protocol, I would like to replace their system with Raspberry Pi 4 or ESP32 design. I thought it was simple UART serial but it looks like more customized type of protocol.
yes, I think it is half duplex or something really slow. when you pull heater stats, it is horribly slow.
see here:
https://github.com/ayavilevich/rinnai-control-panel-sigrok-pd
It may be PLC communication and they have used sigrok on Rinnai control panel. I am assuming WIFI module uses the same PLC protocol.