You can try this, works for electric and gas.
Not so many clever stuff in it, but all the point is to put that in HA.
It could access Internet to be used via the application but, as with all Broadlink devices, you can just put that in a closed VLAN/SSID that doesn’t access Internet.
To check if it’s working, I connect my android phone to that same SSID while I set it up, and after that, you’ll only need the HA server to acces through port 80 to it.
At $25 each, you can have one per room if your heating is electric !