Code Reference
MechanicalASHRAE 62.1-2022

Section 6.3.1/System Ventilation Efficiency

ASHRAE 62.1 Section 6.3.1 covers the system ventilation efficiency (Ev) calculation for multi-zone recirculating systems.

What this section requires

For multi-zone recirculating systems, the outdoor air intake rate (Vot) must be determined using the system ventilation efficiency (Ev) from Table 6.3.1 or calculated using the multiple spaces equation in Appendix A. The zone with the highest ratio of zone outdoor air fraction (Zp) to zone primary air fraction determines the system ventilation efficiency. Vot = Vou / Ev, where Vou is the uncorrected system outdoor air. Ev is always less than or equal to 1.0, meaning the system must provide more outdoor air than the sum of the zone requirements.

Why this section exists

In a multi-zone air handling system, outdoor air is mixed at the air handler and distributed to all zones simultaneously. Some zones need a high percentage of outdoor air (conference rooms) while others need less (storage areas). The system must provide enough outdoor air to satisfy the critical zone (highest percentage need), which means other zones receive more outdoor air than they need. The system ventilation efficiency quantifies this overventilation penalty.

What plan reviewers look for

Plan reviewers check that the ventilation calculation includes the system-level Ev calculation (not just zone-level Voz values). They verify that the critical zone is identified and that the system outdoor air intake (Vot) is used to size the outdoor air damper and intake. For VAV systems, they check that the calculation accounts for the turndown condition where reduced supply airflow increases the zone outdoor air fraction.

Common violations

System ventilation efficiency not calculated
The ventilation schedule shows zone outdoor air rates (Voz) for each zone but does not include the system-level calculation to determine the total outdoor air intake at the air handler. Simply summing the zone rates is incorrect for multi-zone systems.
Critical zone not identified
The Ev calculation does not identify which zone has the highest outdoor air fraction (Zp) and therefore controls the system outdoor air rate. The reviewer cannot verify the Ev value without this information.
VAV turndown not considered
For VAV systems, the calculation uses design (maximum) supply airflow but does not check the minimum airflow condition. At minimum airflow, the zone outdoor air fraction increases and the system Ev decreases, potentially requiring more outdoor air at the AHU.
Compliance tip
Include the full system-level ventilation calculation on the mechanical drawings: zone primary airflows, zone outdoor air fractions (Zp), system ventilation efficiency (Ev), and the resulting system outdoor air intake (Vot). For VAV systems, check Ev at both design and minimum airflow conditions.

Related ASHRAE 62.1 requirements

Section 6.2.1 covers zone-level outdoor air calculations. Table 6.3.1 provides Ev values. Appendix A provides the detailed multiple-spaces equation for calculating Ev. Section 6.2.5 covers demand control ventilation as an alternative for high-density zones.

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Related sections

403.3Outdoor Air RateIMC 20216.2.1Zone Outdoor Air RateASHRAE 62.1-20226.2.2Ventilation Rate TableASHRAE 62.1-2022

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