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Todays heating boilers usually talk 'OpenTherm' protocol, but there are a lot of slightly older (but still very good and efficient) devices installed, which cannot be controlled through OpenTherm. For Junkers, this heating equipment needs to be controlled through one of their thermostats, eg TR21 or TR100. According to Junkers, these thermostats are required in order to maximize the efficiency and economy of the complete setup. You could control such a heating boiler with a simple on/off thermostat, but that would not be optimal from the point of energy consumption, as well as comfort.

Looking in a bit more detail to what the TR21 (and similar) is doing, it's not very complicated : instead of a simple on/off signal, the thermostat provides a voltage which varies between 0v and 24V. The bigger the gap between the wanted room temperature and the actual temperature, the higher the voltage. This voltage then drives the heating gas burner, which can 'modulate' (= adapt) its output power from 30% to 100%. The lower the output, the higher the efficiency.

So in order to control this Junkers heater from a microcontroller, we need to end up with a similar 0-24V output as the TR21