CMC · California Mechanical Code

Primary/secondary airflow, air balance and required primary outdoor‑air fraction

This explains how the California Mechanical Code requires you to calculate system ventilation efficiency (Ev), size outdoor‑air intakes (Vot = Vou/Ev), meet zone minimum primary‑air (Vpz‑min = Voz·1.5), and have the system professionally tested and balanced (air balance) — referencing **§ 403.5.1.2**, **§ 403.5.1.3**, and **§ 403.10**.

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Last reviewed: July 6, 2026

What the code requires — 2–4 sentences

The California Mechanical Code requires that system ventilation efficiency (how well outdoor air is delivered to the breathing zone) be determined using the simplified procedure or the alternate procedure, and that outdoor‑air intake and zone primary‑air requirements be sized accordingly (see § 403.5.1.2 and § 403.5.1.3). All mechanical ventilation systems must be tested and balanced to demonstrate the installation meets the design intent; testing must be done by an approved certified technician (see § 403.10).

The single most important rule: make the system ventilation efficiency calculation and meet the zone minimum primary‑air requirements before you set the outdoor‑air intake — and then have the system professionally tested and balanced.

Requirements in detail

Key defined terms (first use bolded)

  • Primary airflow (Vpz, Vps) — the air supplied from the air handler to a zone (zone primary = Vpz; system primary = Vps).
  • Secondary recirculation — recirculated air introduced to a zone after the primary airstream (relevant for some dual‑fan or fan‑powered systems).
  • Primary outdoor‑air fraction (Zpz) — the fraction of the zone primary airflow that must be outdoor air before any secondary recirculation is added (Zpz = Voz / Vpz).
  • System ventilation efficiency (Ev) — a measure used to convert the uncorrected outdoor‑air requirement into the outdoor‑air intake at the handler.
  • Air balance — testing and adjustment (by qualified technicians) required to show the installed system meets design airflow intent.

What to calculate and which sections control it

  • Use the simplified procedure (or the alternate procedure) to determine system ventilation efficiency (Ev) — § 403.5.1.2 directs you to the method, and § 403.5.1.3 gives the simplified Ev formulas.
  • After Ev is determined, compute the design outdoor‑air intake (Vot) from the uncorrected outdoor‑air requirement (Vou) using Vot = Vou / Ev. The outdoor intake sizing requirement is set out in § 403.5.2.
  • For VAV zones, the code establishes a zone minimum primary airflow (Vpz‑min) that must be used to compute the primary outdoor‑air fraction (Zpz); see § 403.5.1.4 and the related definitions and equations.
  • All systems must be tested, balanced and operated per § 403.10; single‑family residential is excepted.

Decision‑relevant values and formulas (quick reference)

Item Formula / Value Code Reference
Simplified Ev when occupant diversity D < 0.60 Ev = 0.88·D + 0.22 § 403.5.1.3
Simplified Ev when D ≥ 0.60 Ev = 0.75 § 403.5.1.3
Zone minimum primary airflow Vpz‑min = Voz · 1.5 § 403.5.1.4
Outdoor‑air intake (design) Vot = Vou / Ev § 403.5.2
Primary outdoor‑air fraction (per zone) Zpz = Voz / Vpz § 404.3.1(2)
Average outdoor air fraction (system) Xs = Vou / Vps § 404.2
Air balance requirement Systems must be tested and balanced by certified technician (AABC, NEBB, TABB or equivalent). § 403.10

Notes:

  • D = occupant diversity ratio (use Equation 403.5.1.1 to compute). Ev from the simplified procedure is allowed by § 403.5.1.2 and quantified in § 403.5.1.3.
  • If you use the alternate/more detailed method, see § 404.0 and associated formulas for zone/system efficiency and fractions.

Exceptions & special cases

  • Single‑family residential systems are exempt from the formal air balance testing requirement in § 403.10.
  • The code permits using the alternate procedure in § 404.0 instead of the simplified § 403.5.1.3 formulas when that produces a more accurate system Ev (common for complex, secondary‑recirculation systems). § 403.5.1.2 explicitly allows either approach.
  • Secondary‑recirculation systems (fan‑powered boxes, dual‑fan dual‑duct, transfer‑fan arrangements) require the alternate equations in § 404.3.2 to evaluate zone ventilation efficiency and primary fractions; simplified single‑supply equations (§ 404.3.1) do not capture those configurations.

Common mistakes

  • Assuming Ev = 0.75 for every system without checking D. The simplified formula yields a lower Ev when D < 0.60 (Ev = 0.88·D + 0.22). Always compute D first. § 403.5.1.3.
  • Failing to enforce Vpz‑min = Voz · 1.5 for VAV zones — this undervalues the required primary flow and inflates Zpz, which can reduce calculated Ev or cause under‑ventilation when secondary recirculation is present. § 403.5.1.4.
  • Not performing a professional air balance after installation (or using unqualified staff). § 403.10 requires certification by AABC, NEBB, TABB, or equivalent.
  • Mixing up average outdoor‑air fraction Xs (system level) with Zpz (zone primary fraction) — they are different and appear in the Evz equations in § 404.3. Use the correct formula for the system type.

Worked example — concrete scenario

Scenario assumptions (example owner/designer numbers):

  • Three zones served by one air handler. Zone outdoor air requirements: Voz1 = 300 cfm, Voz2 = 400 cfm, Voz3 = 300 cfm (total Vou = 1,000 cfm).
  • System design primary airflow at the handler Vps = 4,000 cfm.
  • Estimated simultaneous occupants (system population) Ps = 30, sum of zone populations (design) ∑Pz = 50, so D = Ps / ∑Pz = 30/50 = 0.60.

Step 1 — pick Ev using simplified procedure:

  • Because D = 0.60, simplified rule gives Ev = 0.75 per § 403.5.1.3.

Step 2 — compute outdoor intake:

  • Vot = Vou / Ev = 1,000 cfm / 0.75 = 1,333 cfm as required at the handler intake. § 403.5.2.

Step 3 — check minimum zone primary air (VAV rule):

  • For Zone 1: Vpz‑min = Voz1 · 1.5 = 300 · 1.5 = 450 cfm. Use that Vpz when computing Zpz if Vpz would otherwise be lower. § 403.5.1.4.

Step 4 — compute primary outdoor‑air fraction for Zone 1:

  • If Vpz = 450 cfm, then Zpz = Voz1 / Vpz = 300 / 450 = 0.667 (66.7%). § 404.3.1(2).

Step 5 — compute average outdoor air fraction at handler:

  • Xs = Vou / Vps = 1,000 / 4,000 = 0.25. § 404.2.

Interpretation:

  • Zone 1 requires a high primary outdoor‑air fraction (66.7%) because its primary flow relative to its Voz is small; this tends to reduce zone ventilation efficiency (Evz) if Xs is low. If you instead used the alternate method in § 404.0, you would compute zone Evz (for a single‑supply system Evz = 1 + Xs – Zpz) and then the system Ev = minimum Evz — which may force a larger Vot or different Vps selection.

Finally:

  • After sizing, the installer must have the system tested and balanced by a qualified technician in accordance with § 403.10 to confirm Vps, Vpz, and intake flows match the design. Single‑family residences are excepted.

Commonly used related provisions (quick list)

  • § 403.5.1.2 — System ventilation efficiency: method selection (simplified vs alternate).
  • § 403.5.1.3 — Simplified procedure and Ev formulas.
  • § 403.5.1.4 — Zone minimum primary airflow (Vpz‑min = Voz·1.5).
  • § 403.5.2 — Outdoor‑air intake calculation (Vot = Vou / Ev).
  • § 404.0 / § 404.2 / § 404.3.1 / § 404.3.2 — Alternate procedure: average outdoor air fraction (Xs), zone Evz equations, and primary outdoor‑air fraction definitions. Useful for secondary‑recirculation systems.
  • § 403.10 — Air balance: testing, balancing and certification requirements (exceptions for single‑family).

Code references

Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:

  • CMC § 403.10 High relevance — show source text

    403.10 Air Balance. All mechanical ventilation systems shall be tested, balanced, and operated to demonstrate that the installation and performance of the systems are in accordance with the design intent. All testing and balancing shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies.

    Exception: For single family residential, compliance with Section 403.10 shall not be required.

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    VENTILATION AIR

    404.0 Alternative Procedure for Multiple-Zone Sys- tems Ventilation Efficiency.

    404.1 System Ventilation Efficiency. 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] This section presents an alternative procedure for calculating the system ventilation efficiency ( Ev ) for multiple zone recirculating systems that shall be used when Section 403.5.1.3 is not used.

    Ev = minimum ( Evz ) (Equation 404.1)

    404.2 Average Outdoor Air Fraction. The average outdoor air fraction ( X s) for the ventilation system shall be determined in accordance with Equation 404.2.

    Xs =Vou / Vps (Equation 404.2)

    Where the uncorrected outdoor air intake ( Vou ) is determined in accordance with Section 403.5.1, 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 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.

  • CMC § 403.5.1.1 High relevance — show source text

    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.

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    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)

    403.6 Design for Varying Operating Conditions. Ventilation systems shall be designed to be capable of providing not less than the minimum ventilation rates required in the breathing zone where the zones served by the system are occupied, including all full- and part-load conditions. The minimum outdoor air intake flow shall be permitted to be less than the design value at part-load conditions. [ASHRAE 62.1:6.2.5 – 6.2.5.1]

  • CMC § 403.2.2. High relevance — show source text

    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.

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    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 =

  • CMC § 404.3.2 High relevance — show source text

    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.

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    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.

  • CMC § 120.1 High relevance — show source text

    50 percent of the peak primary airflow; or b. The design zone outdoor airflow rate as specified by Section 120.1(c)3. ii. The volume of primary air in the deadband shall not exceed the design zone outdoor airflow rate as specified by Section 120.1(c)3. iii. The first stage of heating consists of modulating the zone supply air temperature setpoint up to a maximum setpoint no higher than 95ºF while the airflow is maintained at the dead band flow rate. iv. The second stage of heating consists of modulating the airflow rate from the dead band flow rate up to the heating maximum flow rate. v. Control sequences of operation for reheat zones shall be in accordance with ASHRAE Guideline 36. B. For each zone without DDC, the volume of primary air that is reheated, re-cooled, or mixed air supply shall not exceed the larger of the following: i. 30 percent of the peak primary airflow; or ii. The design zone outdoor airflow rate as specified by Section 120.1(c)3.

    Exception 1 to Section 140.4(d): Zones with special pressurization relationships or cross-contamination control needs.

    Exception 2 to Section 140.4(d): Zones served by space-conditioning systems in which at least 75 percent of the energy for reheating, or providing warm air in mixing systems, is provided from a site-recovered or site-solar energy source.

    Exception 3 to Section 140.4(d): Zones in which specific humidity levels are required to satisfy non-covered process loads. Computer rooms or other spaces where the only process load is from IT equipment may not use this exception.

    Exception 4 to Section 140.4(d): Zones with a peak supply-air quantity of 300 cfm or less.

    Exception 5 to Section 140.4(d): Systems serving healthcare facilities.

    (e) Economizers.

    1. Each cooling air handler that has a design total mechanical cooling capacity over 33,000 Btu/hr or chilled-water cooling systems without a fan or that use induced airflow that has a cooling capacity greater than the systems listed in Table 140.4C, shall include either: A. An air economizer capable of modulating outside-air and return-air dampers to supply 100 percent of the design supply air quantity as outside air; or B. A water economizer capable of providing 100 percent of the expected system cooling load at outside air temperatures of 50°F dry-bulb and 45°F wet-bulb and below. Exception 1 to Section 140.4(e)1: Where special outside air filtration and treatment, for the reduction and treatment of unusual outdoor contaminants, makes compliance infeasible. Exception 2 to Section 140.4(e)1: Where the use of outdoor air for cooling will affect other systems, such as humidification, dehumidification or supermarket refrigeration systems, so as to increase overall building LSC. Exception 3 to Section 140.4(e)1: Systems serving hotel/motel guestrooms. Exception 4 to Section 140.4(e)1: Where comfort cooling systems have the cooling efficiency that meets or exceeds the cooling efficiency improvement requirements in Table 140.4-F. Exception 5 to Section 140.4(e)1: Fan systems primarily serving computer rooms. See Section 140.9(a) for computer room economizer requirements.
  • CMC § 62.1 High relevance — show source text

    [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)

    403.6 Design for Varying Operating Conditions. Ventilation systems shall be designed to be capable of providing not less than the minimum ventilation rates required in the breathing zone where the zones served by the system are occupied, including all full- and part-load conditions. The minimum outdoor air intake flow shall be permitted to be less than the design value at part-load conditions. [ASHRAE 62.1:6.2.5 – 6.2.5.1]

    403.6.1 Short-Term Conditions. Where it is known that peak occupancy will be of short duration, ventilation will be varied or interrupted for a short period of time, or both, the design shall be permitted to be based on the average conditions over a time period ( T ) determined by Equation 403.6.1.

    T = 3 v/Vbz (Equation 403.6.1)

    Where:

    T = averaging time period, minutes.

    v = the volume of the ventilation zone where averaging is being applied, ft [3] (m [3] ).

    Vbz = the breathing zone outdoor airflow calculated in accordance with Equation 403.2.1 and design value of the zone population ( Pz ), cubic foot per minute (CFM).

    For SI Units: 1 cubic foot = 0.0283 m [3], 1 cubic foot per minute =

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

    Acceptable design adjustments based on this optional provision including the following:

    (1) Zones with fluctuating occupancy: The zone population ( Pz ) shall be permitted to be averaged over time ( T ).

    (2) Zones with intermittent interruption of supply air: The average outdoor airflow supplied to the breathing zone over time ( T ) shall be not less than the breathing zone outdoor airflow ( Vbz ) calculated using Equation 403.2.1.

    (3) Systems with intermittent closure of the outdoor air intake: The average outdoor air intake over time ( T ) shall be not less than the minimum outdoor air intake ( Vot ) calculated using Equation 403.3, Equation 403.4, or Equation 403.5.1 as applicable.

  • CMC § 403.9.2 High relevance — show source text

    403.9.2 Class 2 Air. Recirculation of Class 2 air within the space of origin shall be permitted. Recirculation or transfer of Class 2 air to other Class 2 or Class 3 spaces shall be permitted, provided that the other spaces are used for the same or similar purpose or task and involve the same or similar pollutant sources as the Class 2 space. Transfer of Class 2 air to toilet rooms shall be permitted. Recirculation or transfer of Class 2 air to Class 4 spaces shall be permitted. Class 2 air shall not be recirculated or transferred to Class 1 spaces.

    Exception: When using any energy recovery device, recirculation from leakage, carryover, or transfer from the exhaust side of the energy recovery device shall be permitted but shall not be counted as outdoor air. Exhaust air transfer ratio of Class 2 air shall not exceed 10 percent of the outdoor air intake flow. {ASHRAE 62.1:5.18.3.2 – 5.18.3.2.5}

    403.9.3 Class 3 Air. Recirculation of Class 3 air within the space of origin shall be permitted. Class 3 air shall not be recirculated or transferred to any other space.

    Exception: When using any energy recovery device, recirculation from leakage, carryover, or transfer from the exhaust side of the energy recovery device shall be permitted but shall not be counted as outdoor air. Exhaust air transfer ratio of Class 3 air shall not exceed 5 percent of the outdoor air intake flow. {ASHRAE 62.1:5.18.3.3 – 5.18.3.3.2}

    403.9.4 Class 4 Air. Class 4 air shall not be recirculated or transferred to any space or recirculated within the space of origin. [ASHRAE 62.1:5.18.3.4]

    403.10 Air Balance. All mechanical ventilation systems shall be tested, balanced, and operated to demonstrate that the installation and performance of the systems are in accordance with the design intent. All testing and balancing shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies.

    Exception: For single family residential, compliance with Section 403.10 shall not be required.

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    VENTILATION AIR

    404.0 Alternative Procedure for Multiple-Zone Sys- tems Ventilation Efficiency.

    404.1 System Ventilation Efficiency. 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] This section presents an alternative procedure for calculating the system ventilation efficiency ( Ev ) for multiple zone recirculating systems that shall be used when Section 403.5.1.3 is not used.

    Ev = minimum ( Evz ) (Equation 404.1)

    404.2 Average Outdoor Air Fraction. The average outdoor air fraction ( X s) for the ventilation system shall be determined in accordance with Equation 404.2.

  • CMC § 404.2 High relevance — show source text

    Xs =Vou / Vps (Equation 404.2)

    Where the uncorrected outdoor air intake ( Vou ) is determined in accordance with Section 403.5.1, 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 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]

  • CMC § 403.2.1. High relevance — show source text

    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] ]

    405.0 Ventilation for Residential Occupancies.

    405.1 General. Rooms or occupied spaces within residential occupancies, where the occupants are nontransient, shall be designed to have mechanical ventilation and exhaust air in accordance with Section 405.2 through Section 405.5.

    405.1.1 Natural Ventilation. Where approved by the Authority Having Jurisdiction, natural ventilation shall be permitted for Climate Zone 1, Climate Zone 2, or for thermally conditioned buildings for less than 876 hours

    per year.

    405.2 Ventilation Air Rate. The required mechanical ventilation outdoor air rate ( Qtot ) shall be as calculated in accordance with Equation 405.2.

    Exception: For existing buildings and where permitted by the Authority Having Jurisdiction, the total mechanical ventilation ( Qtot ) is not required where Qtot is calculated to be less than 15 ft [3] /min (7.08 L/s).

    Qtot = 0.03 Afloor + 7.5 ( Nbr + 1) (Equation 405.2)

    Where:

    Qtot = Total required ventilation outdoor air rate, CFM (L/s) Afloor = Floor area, ft [2] (m [2] ) Nbr = Number of bedrooms more than 1

    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

  • CMC § 1.8 Medium relevance — show source text

    For SI units: °C = (°F-32)/1.8

    • NR = Not Required

    432 2025 CALIFORNIA MECHANICAL CODE

    ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

    APPENDIX E

    (1) Unit controls shall have the mechanical cooling capacity control interlocked with the air economizer controls such that the outdoor air damper is at the 100 percent open position when mechanical cooling is on, and the outdoor air damper does not begin to close to prevent coil freezing due to minimum compressor run time until the leaving air temperature is less than 45°F (7°C).

    (2) DX units with a rated capacity no less than 65 000 Btu/h (19 kW) that control the capacity of the mechanical cooling directly based on occupied space temperature shall have not less than two stages of mechanical cooling capacity.

    (3) Other DX units, including those that control space temperature by modulating the airflow to the space, shall comply with the requirements of Table E 503.5.3. [ASHRAE 90.1:6.5.1.3]

    been previously cooled, either by mechanical cooling or by economizer systems.

    (4) Other simultaneous operation of heating and cooling systems to the same zone.

    Exceptions:

    (1) Zones for which the volume of air that is reheated, recooled, or mixed is less than the larger of the following:

    (a) For systems without DDC, 30 percent of the zone design peak supply.

    (b) For systems with DDC, the minimum primary airflow rate required to meet the Simplified Procedure ventilation requirements of Chapter 4 or ASHRAE 62.1 for the zone, permitted to be the average airflow rate as allowed by Chapter 4 or ASHRAE 62.1.

    (c) Any higher rate that can be demonstrated, to the satisfaction of the Authority Having Jurisdiction, to reduce overall system annual energy use by offsetting reheat/recool energy losses through a reduction in outdoor air intake for the system.

    (d) The airflow rate required to be in accordance with applicable codes or accreditation standards, such as pressure relationships or minimum air change rates.

    (2) Zones with DDC that comply with the following:

    (a) The airflow rate in dead band between heating and cooling does not exceed the larger of the following:

    (i) The minimum primary airflow rate required to meet the Simplified Procedure ventilation requirements of Chapter 4 or ASHRAE 62.1 for the zone, permitted to be the average airflow rate as allowed by Chapter 4 or ASHRAE 62.1.

    (ii) A higher rate that can be demonstrated, to the satisfaction of the Authority Having Jurisdiction, to reduce overall system annual energy use by offsetting reheat/recool energy losses through a reduction in outdoor air intake.

    (iii) The airflow rate required with applicable codes or accreditation standards, such as pressure relationships or minimum air change rates.

    (b) The airflow rate that is reheated, recooled, or mixed shall be less than 50 percent of the zone design peak supply rate.

    (c) The first stage of heating consists of modulating the zone supply air temperature set

    TABLE E 503.5.3

    DX COOLING STAGE REQUIREMENTS FOR

    MODULATING AIRFLOW UNITS

    [ASHRAE 90.1: TABLE 6.5.1.3]

  • CMC § 62.1 Medium relevance — show source text

    (c) Any higher rate that can be demonstrated, to the satisfaction of the Authority Having Jurisdiction, to reduce overall system annual energy use by offsetting reheat/recool energy losses through a reduction in outdoor air intake for the system.

    (d) The airflow rate required to be in accordance with applicable codes or accreditation standards, such as pressure relationships or minimum air change rates.

    (2) Zones with DDC that comply with the following:

    (a) The airflow rate in dead band between heating and cooling does not exceed the larger of the following:

    (i) The minimum primary airflow rate required to meet the Simplified Procedure ventilation requirements of Chapter 4 or ASHRAE 62.1 for the zone, permitted to be the average airflow rate as allowed by Chapter 4 or ASHRAE 62.1.

    (ii) A higher rate that can be demonstrated, to the satisfaction of the Authority Having Jurisdiction, to reduce overall system annual energy use by offsetting reheat/recool energy losses through a reduction in outdoor air intake.

    (iii) The airflow rate required with applicable codes or accreditation standards, such as pressure relationships or minimum air change rates.

    (b) The airflow rate that is reheated, recooled, or mixed shall be less than 50 percent of the zone design peak supply rate.

    (c) The first stage of heating consists of modulating the zone supply air temperature set

    TABLE E 503.5.3

    DX COOLING STAGE REQUIREMENTS FOR

    MODULATING AIRFLOW UNITS

    [ASHRAE 90.1: TABLE 6.5.1.3]

    RATING CAPACITY,
    Btu/h    		 MINIMUM NUMBER
    OF MECHANICAL
    COOLING STAGES    		 MINIMUM
    COMPRESSOR
    DISPLACEMENT*


    ≥65 000 and <240 000    		 3 ≤35% of full load

    ≥240 000    		 4 ≤25% full load

    For SI units: 1000 British thermal units = 0.293 kW

    • For mechanical cooling stage control that does not use variable compressor displacement the percent displacement shall be equivalent to the mechanical cooling capacity reduction evaluated at the full load rating conditions for the compressor.

    E 503.5.4 Economizer Heating System Impact. HVAC system design and economizer controls shall be such that economizer operation does not increase the building heating energy use during normal operation.

    Exception: Economizers on variable air valve (VAV) systems that cause zone level heating to increase due to a reduction in supply air temperature. [ASHRAE 90.1:6.5.1.4] E 503.5.4.1 Economizer Humidification Sys- tem Impact. Systems with hydronic cooling and humidification systems designed to maintain inside humidity at a dew-point temperature more than 35°F (2°C) shall use a fluid economizer where an economizer is required in accordance with Section E 503.5 through Section E 503.5.4.1. [ASHRAE 90.1:6.5.1.5] E 503.5.5 Simultaneous Heating and Cooling Limitation, Zone Controls. Zone thermostatic controls shall prevent the following:

    (1) Reheating.

    (2) Recooling.

    (3) Mixing or simultaneously supplying air that has been previously mechanically heated and air that has

  • CMC § 511.2.3 Medium relevance — show source text

    511.2.3 Exhaust Fan Operation. A hood exhaust fan(s) shall continue to operate after the extinguishing system has been activated unless fan shutdown is required by a listed component of the ventilation system or by the design of the extinguishing system. The hood exhaust fan shall start upon actuation of the extinguishing system if the exhaust fan and all cooking equipment served by the fan have been shut down, unless fan shutdown is required by a listed component of the ventilation system or by the listing of the extinguishing system. The exhaust fan shall be provided with a means so that the fan is activated when any heat-producing cooking appliance under the hood is turned on. [NFPA 96:8.2.3.1 – 8.2.3.3]

    511.3 Makeup Air. The makeup air quantity shall prevent negative pressures in the commercial cooking area(s) from exceeding 0.02 inch water column (0.005 kPa). Where the fire-extinguishing system activates, makeup air supplied internally to a hood shall be shut off.

    For compensating hoods, where a Type I or Type II hood has an internal discharge of makeup air, the makeup air flow shall not exceed 10 percent of the exhaust airflow, the exhaust airflow shall be the net exhaust from the hood in accordance with Section 508.5.1.2 through Section 508.5.1.5. The total hood exhaust shall be determined in accordance with Equation 511.3.

    E NET = E HOOD – MA ID (Equation 511.3)

    Where:

    E NET = net hood exhaust, CFM (L/s)

    E HOOD = total hood exhaust, CFM (L/s)

    MA ID = makeup air, internal discharge, CFM (L/s)

    For SI units: 1 cubic foot per minute = 0.0283 m [3] /min, 1 cubic foot per

    minute = 0.4719 L/s

    511.3.1 Air Balance. Design plans for a facility with a commercial kitchen ventilation system shall include a schedule or diagram indicating the design outdoor air balance. The design outdoor air balance shall indicate the exhaust and replacement air for the facility and the net exfiltration where applicable. The total replacement airflow rate shall equal the total exhaust airflow rate and the net exfiltration.

    511.4 Common Duct (Manifold) Systems. Master kitchen exhaust ducts that serve multiple tenants shall include provision to bleed air from outdoors or from adjacent spaces into the master exhaust duct where required to maintain the necessary minimum air velocity in the master exhaust duct.

    [NFPA 96:8.4.1]

    511.4.1 Connections. Bleed air ducts shall connect to the top or side of the master exhaust duct. [NFPA 96:8.4.2] 511.4.2 Fire Damper. The bleed-air duct shall have a fire damper at least 12 inches (305 mm) from the exhaust duct connection. [NFPA 96:8.4.3]

    511.4.3 Construction and Clearance. The bleed-air

Frequently asked questions

What is the single simplest thing I must do to comply?

Compute system ventilation efficiency using the simplified procedure in § 403.5.1.3 (or use the alternate method in § 404.0 when appropriate), size the outdoor intake with Vot = Vou / Ev, and have the system tested and balanced per § 403.10.

When can I use the simplified Ev formula instead of the full alternate method?

The simplified formulas in § 403.5.1.3 are permitted by § 403.5.1.2; use the alternate § 404.0 procedure for more complex or secondary‑recirculation systems where the simplified approach would be inaccurate.

Who can perform the air balance?

A technician certified by AABC, NEBB, TABB, or an equivalent approved agency must perform testing and balancing required by § 403.10 (single‑family homes excepted).

How do I find the required zone minimum primary airflow?

Use Vpz‑min = Voz · 1.5 as specified in § 403.5.1.4 when designing VAV zone minimums; this number affects the primary outdoor‑air fraction Zpz.

If Ev computes low, does that always mean I need a bigger outdoor‑air intake?

Yes — a lower Ev (system ventilation efficiency) increases Vot = Vou / Ev, so you will need a higher outdoor intake flow to deliver the same breathing‑zone ventilation. § 403.5.2.

Are residential HVAC systems exempt from anything here?

Yes — the air balance testing requirement of § 403.10 does not apply to single‑family residential systems.

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