According to the choice of ventilation system, the type and size of the device is selected. In a decentralised system, there is a small equal pressure air handling unit in each apartment, providing air exchange only for that apartment.
For central ventilation there is one common air handling unit, sized according to the number of apartments. Together with the apartment VAV boxes, it provides independent ventilation for each apartment as required. A Central ventilation system can also be used for ventilation of office buildings, schools, etc.