ESPAltherma is an ESP32/ESP8266 firmware that monitors Daikin Altherma (and ROTEX/HOVAL Belaria) heat pumps via their serial port (X10A). It queries register values, converts them to JSON, and publishes via MQTT with native Home Assistant MQTT auto-discovery. Optional relay control (on/off thermostat) and smart-grid (SG-Ready) features are supported. The firmware is built with PlatformIO and the Arduino framework, targeting multiple ESP32/ESP8266 variants (M5StickC, NodeMCU, etc.). Architecturally, the firmware is modular: serial communication (comm.h), MQTT handling (mqtt.h), Home Assistant discovery streaming (homeassistant.cpp), value conversion (converters.h), and model-specific register definitions (include/def/). The definition files are extensive, covering dozens of heat pump models and multiple languages, and are the result of significant reverse-engineering of Daikin's I and S protocols. The system uses a LabelDef array pattern to map register addresses to human-readable labels, and a central Converter class to interpret raw bytes. While primarily an application, the repository has a strong data/research character. It includes protocol documentation, a consolidated register list, discrepancy analysis across models, and tools to decode Daikin service files (ktb_decoder.py, ldd_decoder). The analyze_definitions.py script automates the consolidation of definition files and generation of a generic set. This hybrid nature makes the repo valuable both as a ready-to-deploy firmware and as a reference for anyone working with Daikin heat pump protocols.