Section 120.1(b) of the code dictates the minimum amount of outside air that must be provided based upon the occupancy type of the space. This is a known quantity based upon your Zone Occupancy selection, and as a result you cannot go below this minimum. Changing the occupant density and the cfm/occupant at the Room level of the Tree, Occupant tab will have no impact, since the values taken from 120.1(b) are hardwired into the CEC CBECC engine. The only change you can make is to increase the outside air above the minimum, this is done by going into the Room level of the Building Tree, Occupant tab, and increasing the Occupant Fraction above the default of 50% (taken from 120.1(b)) to a higher number.
Note that changes to the ventilation numbers will impact your load calculations, since you are free to input any values you wish for these calculations.
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