i went down the rabbit hole a bit and there doesn't seem to be a homekit-enabled millivolt (not low voltage, which is 24V apparently) thermostat available. however, i did find this little project: https://zekesnider.com/using-an-ecobee-thermostat-with-a-mil... it seems you could patch any low-voltage thermostat to support millivolt in the same way (with a 24V transformer and a relay switch), but the ecobee3 lite has homekit support built-in.