Measurement and analysis of vitiation of secondary air in air distribution systems (RP-1276)

Appendix A of ANSI/ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality, describes the recycling of unvitiated ventilation air in a recirculating air-handling system. Equation A-2 considers air delivered through the primary air path (central air distribution system) and secondary air paths, such as fan-powered boxes or transfer-air fans. It contains a variable, E_r, that describes the extent to which the secondary air comes from the zone in question, as opposed to coming from “average system return air.” This paper describes the development of an equation that can be used to quantify E_r, and shows the results of the first experimental measurements of E_r. For these measurements, tracer gas experiments were carried out on an office building in Omaha, Nebraska. The study showed that the theoretical range of E_r is from zero to infinity. Its range in the test building is from 0.74 to 1.01 as built, but could range from 0.14 to 1.13 if the fan-powered boxes had been located in different, but realistic, locations. These findings contradict the standard, which defines E_r as ranging from 0 to 1.