ASHRAE Standard 62-1989 provides the multiple spaces method to accommodate the ventilation needs of buildings without excessive energy utilization. This is accomplished by utilizing unvitiated outdoor air in the return air from overventilated spaces. Some spaces are overventilated because the outdoor air fraction in the primary air, dictated by the critical space, is higher than that required for other noncritical spaces. In the derivation presented in the standard, the unvitiated outdoor air centrally mixes with the new outdoor air at the air-handling unit. Consequently, the derivations in the present standard will not accommodate fan-powered variable-air-volume systems where unvitiated plenum air is mixed locally at the terminal with the primary air. This restriction has been addressed in the literature. However, infiltration, exhaust, and interzonal transfer have been assumed not to exist in both the standard and all of the work presented in the literature to date. This deficiency is addressed in this paper. Unfortunately, a single equation could not be found to accommodate the added complexity introduced by the additional airflow paths and associated potential short-circuit paths. This paper demonstrates that the problem can be solved using spreadsheet templates. The spreadsheet reduces a complicated problem to one that is easily solved.

Units: SI