OpenTherm gateway has been put between the heater and the thermostat. This way we are able to read the OpenTherm communication between them, and control the heater using the gateway. It is a board with a PIC controller that already came pre-flashed. It can be used with a FTDI cable or you can put an ESP8266 based NodeMCU board on it and communicate with that over Wifi. We went for the latter.
Integration with home assistant
We installed py-otgw-mqtt on Jarvis to have the OpenTherm stuff available via MQTT. It is running as user otgw-mqtt. A /etc/init.d/otgw-mqtt has been made so that the service can be stopped and started through init, and is has been set to autostart.