The following four major problem areas, common to all-air systems, are easily overcome with properly designed dedicated outside air (OA) systems working in parallel with sensible cooling equipment.
Ventilation air distribution in all-air variable-air volume (vav) systems. Engineers cannot be sure where the ventilation air, in all-air systems, is distributed once brought into the building and mixed with return air at the air-handling unit (ahu). Proper distribution is a function of vav box minimum settings, space sensible loads, local exhaust and exfiltration, short circuiting paths, and interzonal air transfer (Mumma, Lee 1998). Complete knowledge of all of these factors at design is not possible, much less in real time. Therefore engineers cannot really defend compliance with ASHRAE Standard 62-1999 (ASHRAE 1999) when using all-air vav systems. On the contrary, dedicated 100% OA systems place the proper ventilation air quantities into every space.