Our building is fully occupied by the same company, so all the floors have similar open space layout, except for enclosed managers or small meeting rooms.

There´s an outside air intake in each AHU level. Each floor is served by one AHU, that provides a mixture of outside air with return air, on a constant volume. There are no VAV´s, no dampers and no different zones in our ventilation plenum.

Since the floors are mostly open space, have no critical zones (bigger meeting rooms, in the last floor, are served by different AHUs), and are all of the same type (office space), is it wrong to consider it a single-zone system, and measure the intakes only in the AHU level? Or should I consider it a multizone and go through all the VRP calculations?