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Is it mandatory to have a Smarther air conditioner with Netatmo in each room I want to control in case of fan coil systems?

Yes. In an installation using a cooling fan for air conditioning, it is necessary to install a Smarther AC (+ HVAC actuator) in each room where fan coil control is required. The Smarther AC is required to measure the room temperature and send it to the HVAC actuator.