Inadequate mixing between the outdoor air and the return air before entering the air conditioning coil has been a concern of the heating, ventilation and air-conditioning (HVAC) industry because it generally results in poor indoor air quality (IAQ). Advances in computational fluid dynamics (CFD) and high speed computers make it possible, today, that the design and operation of a HVAC system can be simulated both reliably and economically. Computer simulation of some current designs based on recommended procedures suggests that the outdoor and return airstreams are poorly mixed even when the existing guidelines are followed.