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ASHRAE Standard 62.1 – 2004: Ventilation for Acceptable Indoor Air Quality contains a generalized form of the Multiple Spaces Equation that includes consideration of air delivered through secondary air paths such as fan-powered boxes or transfer air fans. The Multiple Spaces Equation describes the recycling of unvitiated ventilation air in a recirculating system. This equation has a variable, Er, that describes the extent to which the secondary air comes from the zone in question, as opposed to coming from “average system return air”. Er had not been measured, so the standard was unable to provide users with exact guidance about what values to use. One of the objectives of RP-1276 was to measure values of Er in a typical building. A second objective was to experimentally determine whether the Multiple Spaces Equation is valid. Tracer gas experiments were carried out on a building in Omaha to study Er and to validate the Multiple Spaces Equation. The study of Er showed that its theoretical range is from zero to infinity. Its range in the test building is from 0.74 to 1.01 as built, but could range from 0.13 to 1.15 if the fan-powered boxes were located in different, but realistic locations. These results contradict the standard, which describes Er as having a range from 0 to 1. The experimental validation of the Multiple Spaces Equation shows that it is valid. The corollary to this result is that passive interzonal mixing can not be relied upon to satisfy the ventilation requirements of Standard 62.1 – 2004 in a typical building.