After mainlining kernel patches for the CoolPi GenBook, I turned to the Eurotronic Comet WiFi thermostats — cheap WiFi heating valves with no documented API and no Home Assistant integration. Here is how I got them fully integrated locally.
The Problem
The Comet WiFi uses a DA16200 WiFi SoC and communicates via MQTT to Eurotronic cloud servers. No local API, no Tuya compatibility, no custom firmware path.
The Solution: DNS Redirect + Local MQTT
Redirect all mqtt*.eurotronic.io hostnames to a local Mosquitto broker via Pi-hole. The thermostats connect locally instead of to the cloud.
Pi-hole DNS Override
192.168.0.118 mqtt.eurotronic.io
192.168.0.118 mqtt1.eurotronic.io
192.168.0.118 mqtt2.eurotronic.io
192.168.0.118 mqtt3.eurotronic.io
192.168.0.118 mqtt4.eurotronic.io
192.168.0.118 mqtt5.eurotronic.ioImportant: Redirect ALL mqtt* variants — including all numbered servers. I initially missed mqtt3 and one thermostat bypassed my broker entirely.
MQTT Protocol
Topic structure: 02/PREFIX/MAC/S/register (commands) and 02/PREFIX/MAC/V/register (values). Key registers: A0 = target temp, A1 = current temp, A6 = battery. Temperatures are hex-encoded: value * 2, prefixed with #. Example: 21.0C = #2a.
HA YAML Climate Entities
Skip custom integrations — use HA built-in MQTT climate via YAML. Much more reliable than the HACS comet_wifi_integration which had QoS 2 issues.
climate:
- name: Wohnzimmer
temperature_command_topic: "02/000131E7/MAC/S/A0"
temperature_command_template: '{{ "#" + "%02x" % (value|float*2)|int }}'
temperature_state_topic: "02/000131E7/MAC/V/A0"
temperature_state_template: '{{ int(value[1:3],base=16)/2 }}'
current_temperature_topic: "02/000131E7/MAC/V/A1"
current_temperature_template: '{{ int(value[1:3],base=16)/2 }}'Polling
Thermostats are battery-conscious and rarely push data. Poll every 2 hours by publishing #0b to S/A0. Responses can take minutes.
Security Note
All Comet WiFi devices share the same MQTT credentials per installation. The original cloud broker was essentially open — local control is actually more secure.
Result
Three thermostats fully in Home Assistant with temperature control and battery monitoring — no cloud, no subscription. Developed in one session with Claude Code.
