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404.1 General
This section presents an alternative procedure for calculating the system ventilation efficiency (Ev) where values in Table 403.5.2 are not used. The system ventilation efficiency shall equal the lowest zone ventilation efficiency among the ventilation zones served by the air handler in accordance with Equation 404.1. [ASHRAE 62.1 :A1.3]

Ev = minimum (Evz) (Equation 404.1)
404.2 Average Outdoor Air Fraction
The average outdoor air fraction (Xs) for the ventilation system shall be determined in accordance with Equation 404.2.
(Equation 404.2)
Xs = Vou/Vps
The uncorrected outdoor air intake (Vou) shall be determined in accordance with Section 403.5.3, and the system primary airflow (Vps) shall be determined at the condition analyzed. [ASHRAE 62.1:A1.1]
404.3 Zone Ventilation Efficiency
The zone ventilation efficiency (Evz) shall be the efficiency with which a system distributes outdoor air from the intake to an individual breathing zone, and 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 the air supplied to a 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. Examples of single supply systems include constant-volume reheat, singleduct VAV, single-fan dual-duct, and multizone systems.

Evz = 1+Xs-Zpz (Equation 404.3.1)

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 Section 403.5.1. [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 dual-duct and fan-powered mixing-box systems, and systems that include transfer fans for conference rooms.

Evz = (Fa+XsFb-ZpzEpFc)/Fa [Equation 404.3.2(1)]

The system air fractions Fa, Fb, and Fc shall be determined in accordance with Equation 404.3.2(2), Equation 404.3.2(3), and Equation 404.3.2(4). The zone primary air fraction (Ep) shall be determined in accordance with Equation 404.3.2(5). For single-zone and single-supply systems Ep shall equal to 1.0. The zone secondary recirculation fraction (Er) shall be determined by the designer based on system configuration. The zone air distribution effectiveness (Ez) shall be determined in accordance with Section 403.2.2. [ASHRAE 62.1:A1.2.2]
Where:
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.
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.
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.
Vdz - Zone discharge airflow: The expected discharge (supply) airflow to the zone that includes primary airflow and secondary recirculated airflow, cfm (m3/min).
Vpz - Zone primary airflow: Determine in accordance with Section 403.5.1.
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 secondary recirculation air. [ASHRAE 62.1:A3]

Related Code Sections

404.0 Ventilation Air, Multiple-Zone Systems
system ventilation efficiency shall equal the lowest zone ventilation efficiency among the ventilation zones served by the air handler in accordance ...
403.5 Ventilation Air, Multiple-Zone Recirculating Systems
For ventilation systems where one or more air handlers supply a mixture of outdoor air and recirculated air to more than one ventilation zone ...