Reverse-engineered Rinnai RXP199iN water heater Bluetooth protocol for Home Assistant

The ha-rinnai-rxp199in project provides a Home Assistant integration that reads data from Rinnai SENSEI RX/RXP tankless water heaters using their native Bluetooth interface. The integration operates locally without cloud connectivity or Wi-Fi modules. The underlying communication protocol was reverse-engineered from scratch and is documented in the repository. The integration is read-only and does not support control commands. Development and testing were performed against the RXP199iN model, though other RX/RXP models with the same Bluetooth module are expected to function. Claude Opus 5.5 is credited as a co-author on multiple commits, including those for connection layer rewrites, test dependency installation, and the implementation of low-flow lockout detection. The integration requires a dedicated Bluetooth adapter on the Home Assistant host and a manual bonding process involving physical access to the heater. Current limitations include the lack of control functionality and the requirement for a single persistent Bluetooth connection, which prevents simultaneous use of the Rinnai mobile application.