After a few detours, I've managed to configure a ESP8266 MQTT gateway and a DHT22 sensor with HA as my controller.
The next step is to try and get HVAC control working from HA. I've put the example sketch on an Arduino Uno and it's loading just fine - a new entry has appeared in mysensors.json. The Arduino Serial monitor shows that registration happens successfully.
The issue is that I can't see any indication in HA of the HVAC device. I was hoping that it would just appear in the same way as the DHT sensor.
I feel like I've missed some sort of fundamental configuration to make HASS pay attention to climate control. Does anyone have any recommended steps for troubleshooting?