403.5 Multiple-Zone Recirculating Systems — 2025 California Mechanical Code

For ventilation systems where one or more air handlers supply a mixture of outdoor air and recirculated air to more than one ventilation zone, the outdoor air intake flow ( Vot ) shall be determined in accordance with Section 403.5.1 through Section 403.5.2. [ASHRAE 62.1:6.2.4]

403.5.1 Uncorrected Outdoor Air Intake. The uncorrected outdoor air intake ( Vou ) flow shall be determined in accordance with Equation 403.5.1. [ASHRAE 62.1:6.2.4.1]

(Equation 403.5.1) Vou = D ∑ all zones ( Rp•Pz ) + ∑ all zones ( Ra•Az )

403.5.1.1 Occupant Diversity. The occupant diversity ratio ( D) shall be determined in accordance with Equation 403.5.1.1 to account for variations in population within the ventilation zones served by the system.

(Equation 403.5.1.1) D = Ps / ∑ all zones Pz

Where the system population ( Ps ) is the total population in the area served by the system.

VENTILATION AIR

Exception: Alternative methods to account for occupant diversity shall be permitted, provided that the resulting ( Vou ) value is not less than that determined in accordance with Equation 403.5.1.

[ASHRAE 62.1:6.2.4.1.1]

403.5.1.2 System Ventilation Efficiency. The system ventilation efficiency ( Ev ) shall be determined in accordance with Section 403.5.1.3 for the

simplified procedure or Section 404.0 for the alternate procedure. These procedures also establish zone minimum primary airflow rates for VAV systems.

[ASHRAE 62.1:6.2.4.2]

403.5.1.3 Simplified Procedure for System Ventilation Efficiency. System ventilation efficiency ( Ev ) shall be determined in accordance with Equation 403.5.1.3(1) or Equation 403.5.1.3(2).

[ASHRAE 62.1:6.2.4.3 – 6.2.4.3.1]

[Equation 403.5.1.3(1)]

Ev = 0.88 • D + 0.22 for D < 0.60

Ev = 0.75 for D ≥ 0.60 [Equation 403.5.1.3(2)]

403.5.1.4 Zone Minimum Primary Airflow. For each zone, the minimum primary airflow ( Vpz-min ) shall be determined in accordance with Equation 403.5.1.4. [ASHRAE 62.1:6.2.4.3.2]

Vpz-min = Voz• 1.5 (Equation 403.5.1.4)

403.5.2 Outdoor Air Intake. The design outdoor air intake flow ( Vot ) shall be determined in accordance with Equation 403.5.2. [ASHRAE 62.1:6.2.4.4]

Vot = Vou / Ev (Equation 403.5.2)