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The outdoor air required to be supplied by each ventilation system shall be determined in accordance with Sections 403.3.1.1.2.1 through 403.3.1.1.2.3.4 as a function of system type and zone outdoor airflow rates.
403.3.1.1.2.1 Single Zone Systems
Where one air handler supplies a mixture of outdoor air and recirculated return air to only one zone, the system outdoor air intake flow rate (Vot) shall be determined in accordance with Equation 4-3.
(Equation 4-3)
403.3.1.1.2.2 100-Percent Outdoor Air Systems
Where one air handler supplies only outdoor air to one or more zones, the system outdoor air intake flow rate (Vot) shall be determined using Equation 4-4.
(Equation 4-4)
403.3.1.1.2.3 Multiple Zone Recirculating Systems
Where one air handler supplies a mixture of outdoor air and recirculated return air to more than one zone, the system outdoor air intake flow rate (Vot) shall be determined in accordance with Sections 403.3.1.1.2.3.1 through 403.3.1.1.2.3.4.
403.3.1.1.2.3.1 Primary Outdoor Air Fraction
The primary outdoor air fraction (Zp) shall be determined for each zone in accordance with Equation 4-5.
(Equation 4-5)
where:
Vpz  = Primary airflow: The airflow rate supplied to the zone from the airhandling unit at which the outdoor air intake is located. It includes outdoor intake air and recirculated air from that air-handling unit but does not include air transferred or air recirculated to the zone by other means. For design purposes, Vpz shall be the zone design primary airflow rate, except for zones with variable air volume supply and Vpz shall be the lowest expected primary airflow rate to the zone when it is fully occupied.
403.3.1.1.2.3.2 System Ventilation Efficiency
The system ventilation efficiency (Ev) shall be determined using Table 403.3.1.1.2.3.2 or Appendix A of ASHRAE 62.1.
SYSTEM VENTILATION EFFICIENCYa, b
Max (Zp) Ev
≤ 0.15 1
≤ 0.25 0.9
≤ 0.35 0.8
≤ 0.45 0.7
≤ 0.55 0.6
≤ 0.65 0.5
≤ 0.75 0.4
> 0.75 0.3
  1. Max (Zp) is the largest value of Zp calculated using Equation 4-5 among all the zones served by the system.
  2. Interpolating between table values shall be permitted.
403.3.1.1.2.3.3 Uncorrected Outdoor Air Intake
The uncorrected outdoor air intake flow rate (Vou) shall be determined in accordance with Equation 4-6.
(Equation 4-6)
where:
D  = Occupant diversity: the ratio of the system population to the sum of the zone populations, determined in accordance with Equation 4-7.
(Equation 4-7)
where:
Ps  = System population: The total number of occupants in the area served by the system. For design purposes, Ps shall be the maximum number of occupants expected to be concurrently in all zones served by the system.
403.3.1.1.2.3.4 Outdoor Air Intake Flow Rate
The outdoor air intake flow rate (Vot) shall be determined in accordance with Equation 4-8.
(Equation 4-8)

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