I would like to understand the requirements for calculating the flush-out duration for a project which is served by multiple HVAC units. This project contains several buildings with isolated rooms which are served by its own AHU or FCU. Should each HVAC zone be calculated separately based on each HVAC unit and the area it serves? The capacities of each AHU/FCU vary as do the square footages of each space served by the different units, so it would seem that summing the total area of different, isolated HVAC zone in a building and dividing by the total cfm of multiple units was NOT the correct way to do this calculation.