ESPAtherma is an embedded firmware application (PlatformIO/Arduino) for monitoring and basic control of Daikin Altherma heat pump systems via an ESP32 or ESP8266 microcontroller. In production, it connects to the heat pump's X10A serial port, queries specific register addresses (reverse-engineered Daikin I and S protocols), converts raw bytes into human-readable values using configurable label definitions, and publishes the data over MQTT to a Home Assistant instance. It also provides Home Assistant MQTT auto-discovery for seamless integration. The architecture is a single-threaded loop: setup reads model-specific definition files and MQTT/WiFi config, then repeatedly queries registers, converts data, and publishes to MQTT. Key entry points are `src/main.cpp`, `include/comm.h` (serial query), and `include/homeassistant.cpp` (discovery payload generation). External dependencies include PubSubClient, ArduinoJSON, and board-specific libraries (M5Unified for M5Stick). The project also includes reverse-engineering scripts, serial log dumps, and a heat pump emulator, making it a hybrid of embedded application and protocol research. However, the primary usage and codebase character are that of an IoT monitoring/control application.