Can I set the natural ventilation depending of temperature and CO2 ppm?

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Which Ventilation method are you using ? The Airflow network or the simple Ventilation Design Flow rate etc.. ?

In any case I would say EMS is the answer to all those type of controls. You can just set your Ventilation schedule using Schedule: Constant.

Using EMS, set the temperature of the zone and the C02 concentration as sensors. The component to be actuated is the schedule value. Write your program to execute the control you want. There's quite a lot of literature on EMS in the Input/Output reference and the example files .