A hybrid ventilation system combines both mechanical and natural ventilation modes. The natural ventilation mode, especially, causes some challenges for analysis tools since the varying nature of naturally driven flow means that transient calculations should be used to predict the flow fields. Analysis tools based on CFD have some advantages for detailed investigations since, in addition to evaluating the flow and temperature fields, it is possible to use CFD methods to calculate air exchange efficiency and other flow indices. But doing this for natural and hybrid ventilation means that new models and procedures are necessary to meet the demands of the transient flow phenomena. Some ideas are presented in this paper and are applied to air exchange in a room caused by different driving forces. By use of the proposed ideas and procedures it is possible to calculate the air exchange efficiency for any hybrid ventilated space.