404.3 Zone Ventilation Efficiency — 2025 California Mechanical Code

The zone ventilation efficiency ( Evz ) shall be determined in accordance with Section 404.3.1 or Section 404.3.2. [ASHRAE 62.1:A1.2] 404.3.1 Single Supply Systems. For single supply systems, where all of the air supplied to each ventilation zone is a mixture of outdoor air and system-level recirculated air, zone ventilation efficiency ( Evz ) shall be determined in accordance with Equation 404.3.1(1). Examples of single supply systems include constant-volume reheat, single-duct VAV, single-fan dual-duct, and multizone-systems.

Evz = 1+ Xs – Zpz [Equation 404.3.1(1)]

Where the average outdoor air fraction for the system ( Xs ) shall be determined in accordance with Equation 404.2 and the primary outdoor air fraction for the zone ( Zpz ) shall be determined in accordance with Equation 404.3.1(2).

Zpz = Voz/Vpz [Equation 404.3.1(2)]

For VAV systems, Vpz is the lowest zone primary airflow value expected at the design condition analyzed.

[ASHRAE 62.1:A1.2.1] 404.3.2 Secondary-Recirculation Systems. For secondary-recirculation systems where the supply air or a portion thereof to each ventilation zone is recirculated air (air that has not been directly mixed with outdoor air) from other zones, zone ventilation efficiency ( Evz ) shall be determined in accordance with Equation 404.3.2(1). Examples

of secondary-recirculation systems include dual-fan dualduct and fan-powered mixing-box systems, and systems that include transfer fans for conference rooms.

[Equation 404.3.2(1)] Evz = ( Fa+Xs•Fb–Zpz•Ep•Fc )/ Fa

Where system air fractions Fa, Fb, and Fc are determined in accordance with Equation 404.3.2(2), Equation 404.3.2(3), and Equation 404.3.2(4), as applicable.

Fa = Ep + (1– Ep )• Er [Equation 404.3.2(2)]

Fb = Ep [Equation 404.3.2(3)]

Fc = 1–(1– Ez )•(1– Er )•(1– Ep ) [Equation 404.3.2(4)]

Where the zone primary air fraction ( Ep ) shall be determined in accordance with Equation 404.3.2(5), zone secondary recirculation fraction ( Er ) shall be determined by the designer based on system configuration, and zone air distribution effectiveness ( Ez ) shall be determined in accordance with Section 403.2.2. [ASHRAE 62.1:A1.2.2]

Ep = Vpz / Vdz [Equation 404.3.2(5)]

Where:

Az = Zone floor area: The net occupiable floor area of the ventilation zone, ft [2] (m [2] ).

D = Occupant diversity: The ratio of the system population to the sum of the zone populations. Ep = Primary air fraction: The fraction of primary air in the discharge air to the ventilation zone. Er = Secondary recirculation fraction: In systems with secondary recirculation of return air, the fraction of secondary recirculated air to the zone that is representative of average system return air rather than air directly recirculated from the zone. Ev = System ventilation efficiency: the efficiency with which the system distributes air from the outdoor air intake to the breathing zone in the ventilationcritical zone, which requires the largest fraction of outdoor air in the primary airstream. Evz = Zone ventilation efficiency: The efficiency with which the system distributes air from the outdoor air intake to the breathing zone in any particular ventilation zone.

Ez = Zone air distribution effectiveness: A measure of the effectiveness of supply air distribution to the breathing zone. Ez is determined in accordance with Section 403.2.2.

Fa = Supply air fraction: The fraction of supply air to the ventilation zone from sources or air outside the

zone.

76 2025 CALIFORNIA MECHANICAL CODE

VENTILATION AIR

Fb = Mixed air fraction: The fraction of supply air to the ventilation zone from fully mixed primary air.

Fc = Outdoor air fraction: The fraction of outdoor air to the ventilation zone from sources of air outside

the zone.

Ps = System population: the simultaneous number of occupants in the area served by the ventilation sys tem.

Pz = Zone population: see Section 403.2.1. Ra = Area outdoor air rate, CFM/ft [2] . See Section 403.2.1.

Rp = People outdoor air rate, CFM/person. See Section 403.2.1.

Vbz = Breathing zone outdoor airflow, CFM. See Section 403.2.1.

Vdz = Zone discharge airflow, CFM. The expected discharge (supply) airflow to the zone that includes primary airflow and secondary recirculated airflow, CFM.

Vot = Outdoor air intake flow, CFM. See Section 403.3, Section 403.4, and Section 403.5.2.

Vou = Uncorrected outdoor air intake, CFM. See Section 403.5.1.

Voz = Zone outdoor airflow, CFM. See Section 403.2.3. Vps = System primary airflow, CFM. The total primary airflow supplied to all zones served by the system from the air-handling unit at which the outdoor air intake is located.

Vpz = Zone primary airflow, CFM. The zone primary airflow to the ventilation zone, including outdoor air and recirculated air.

Xs = Average outdoor air fraction: At the primary air handler, the fraction of outdoor air intake flow in the system primary airflow. Zpz = Primary outdoor air fraction: The outdoor air fraction required in the primary air supplied to the ventilation zone prior to the introduction of any secondary recirculation air. [ASHRAE 62.1:A3]

For SI Units: 1 square foot = 0.0929 m [2], 1 cubic foot per minute =

0.0283 m [3] /min, 1 cubic foot per minute = 0.4719 L/s, 1 cubic foot per

minute per square foot = 5.08 [(L/s)/m [2] ]