Outdoor air flow and primary supply air flow

This is one of the weak points of VAV systems. In order to guarantee proper ventilation at all conditions, the amount of outdoor air that needs to be brought in at the air-handling unit can be significantly higher than what's required if you just sum up the ventilation rate required for each zone. This is why decoupled ventilation systems are now preferred, especially for schools. A dedicated outdoor air system handles delivers only what is needed for ventilation to each zone, typically at neutral air temperature. Space sensible cooling and heating is provided by a recirculating VAV system (dual-duct), fan coil units, radiant panels, chilled beams, etc.