Yesterday I received the parts for the opentherm gateway. It's a small board that lets me connect the heater and thermostat to my network. With this I should be able to control the heater and thermostat and read the temperature with Home Assistant.
Today I soldered the kit and flashed the NodeMCU that connects the board to the wifi network. It already works standalone and I can read the values through the network. Now I only need to connect the board with the heater and thermostat.