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. The ESP8266 runs Espeasy with Ser2Net.
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. In the future we might want to look at  but for this we need to run at least 0.78, which probably means we have to reinstall Home Assistant, possibly in docker for easy updates. The_Niz will probably do this soon. We probably also need to reinstall Espeasy on the NodeMCU.