The air change effectiveness (ACE), is defined as the age of air that would occur throughout the room if the air was perfectly mixed, divided by the average age of air where occupants breathe. The age of air at a particular location is the average time elapsed since molecules of air at that location entered the building. Because the average age of air exiting the room is identical to the age of air that would occur throughout the room if the indoor air were perfectly mixed, the ACE is also the exhaust-air age divided by the average age of air where occupants breathe. In practical terms, the ACE equals the effective ventilation rate at the breathing zone divided by the ventilation rate that would occur throughout the building with perfect mixing and at the same outdoor air supply. A short-circuiting flow pattern between the air supply diffusers and return grilles, increases the room-air age and causes ACE to be less than unity. Perfect mixing results in an ACE of unity. Preferentially ventilating the breathing zone with outside air will cause the ACE to be greater than unity.

The age of air or its reciprocal, the effective local ventilation rate, is typically measured by using the tracer gas stepup technique. (Sandberg and Sjoberg, 1983 or ANSI/ASHRAE, 1997)

• Table 6: Air Change Effectiveness of Conventional Mixing-Type Air Supply Technologies.

Our research as shown that the conventional ceiling-based air supply technologies used in most US commercial buildings result in an ACE close to unity. In other words, there is not much short circuiting of air between supply diffusers and return grilles. The lowest values of ACE occur when warm air is supplied at the ceiling and the return grilles are also at the ceiling, and the ventilation system supplies 100% outside air. In this situation, the ACE can be as low as 0.69. (LBNL-40292)

Damper set to control minimum outside air supply. But it is very difficult to measure OA supply. In VAV systems, the OA supply varies with the total air supply. Field studies indicate that actual minimum ventilation rates are very different from code.