CMC · California Mechanical Code

Alternative procedure for multiple‑zone system ventilation efficiency (Ev)

Q: If Ev < 1.0 does that mean I need more outdoor air?

Not necessarily — Ev < 1.0 means the system does not deliver outdoor air perfectly to the breathing zone in the critical zone. You must use Ev in the outdoor‑air intake calculation (Vot = Vou / Ev, § 403.5.2) to size the intake so required breathing‑zone rates are met.

Q: Do I need to document assumptions for Er and Ez?

Yes. When using the secondary‑recirculation equations in § 404.3.2 , the designer must determine and justify Er (secondary recirculation fraction) and Ez (zone air distribution effectiveness) because Evz depends on these choices. file

If a single air handler serves multiple zones and you aren’t using the simplified method, California’s Mechanical Code requires computing system ventilation efficiency (Ev) by (1) finding the system outdoor‑air fraction Xs, (2) calculating each zone’s ventilation efficiency Evz using §404.3 rules for single‑supply or secondary‑recirculation systems, and (3) taking the lowest Evz as the system Ev per §404.1; use Ev to size the outdoor‑air intake.

Ask a question free See coverage

Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The California Mechanical Code provides an alternative procedure to calculate system ventilation efficiency Ev for multiple‑zone recirculating systems when the simplified method is not used. The system ventilation efficiency is equal to the lowest zone ventilation efficiency (Evz) among zones served by an air handler per § 404.1 . The procedure requires computing the system average outdoor‑air fraction (Xs) and each zone’s Evz using the formulas in § 404.2 and § 404.3, respectively .

The system Ev for a multi‑zone recirculating system is the minimum zone Ev (Ev = min(Evz)). See § 404.1.

Requirements in detail

Key defined terms (first mention bolded)

Ev — System ventilation efficiency (efficiency with which outdoor air reaching the air handler is delivered to the breathing zone in the most critical ventilation zone) § 404.1.
Evz — Zone ventilation efficiency (efficiency for an individual ventilation zone) § 404.3.
Xs — Average outdoor air fraction at the primary air handler (Xs = Vou / Vps) § 404.2.
Vou — Uncorrected outdoor‑air intake (determined per § 403.5.1).
Vps — System primary airflow at the condition analyzed (total primary air supplied at the air handler).
Zpz — Zone primary outdoor‑air fraction required in the primary airstream to a zone (Zpz = Voz / Vpz) § 404.3.1(2).

What to compute (high level)

Determine the uncorrected outdoor‑air intake (Vou) per § 403.5.1 (use occupant diversity, Rp, Ra, area and people rates) .
Determine the system primary airflow (Vps) at the analyzed condition (total primary air at the air handler) § 404.2 .
Compute Xs = Vou / Vps per § 404.2 .
For each zone, compute Evz using the appropriate sub‑section of § 404.3 (single supply or secondary‑recirculation) and then set Ev = minimum(Evz) per § 404.1 .

Equations and decision‑relevant dimensions (table)

Variable / decision dimension	What it means	How to calculate	Code Reference
Ev	System ventilation efficiency	Ev = minimum(Evz)	§ 404.1
Xs	Average outdoor air fraction (system)	Xs = Vou / Vps	§ 404.2
Evz (single supply)	Zone ventilation efficiency for single‑supply systems	Evz = 1 + Xs − Zpz, where Zpz = Voz / Vpz	§ 404.3.1 (Equations 404.3.1(1),(2))
Evz (secondary recirc)	Zone ventilation efficiency where secondary recirculation exists	Evz = (Fa + Xs·Fb − Zpz·Ep·Fc) / Fa with Fa, Fb, Fc and Ep defined in § 404.3.2	§ 404.3.2 (Equations 404.3.2(1)–(5))
Vou	Uncorrected outdoor‑air intake	Sum of people and area components using occupant diversity	§ 403.5.1 (Equation 403.5.1)
Vps	System primary airflow	Total primary air supplied by the air handler (condition analyzed)	Defined in § 404.2 and term definitions § 404.3
Vpz (Vpz‑min)	Zone primary airflow (and VAV minimum)	For VAV, use the lowest expected zone primary airflow; simplified procedure also establishes Vpz‑min = Voz × 1.5 per § 403.5.1.4	§ 404.3.1 and § 403.5.1.4 file

Which Evz formula to use

Use § 404.3.1 for single‑supply systems (all supply air to a zone is mixed primary + system recirculated air) — e.g., VAV single‑duct, constant‑volume reheat, multizone systems .
Use § 404.3.2 for systems with secondary recirculation (fan‑powered boxes, secondary return paths); this requires additional parameters Ep, Er, and Ez and the Fa/Fb/Fc fractions defined in § 404.3.2 .

Exceptions & special cases

The alternative procedure of Chapter 404.0 is intended specifically for multiple‑zone recirculating systems and is to be used when the simplified method in § 403.5.1.3 is not used (see § 404.1) .
For VAV systems, the designer must use the lowest expected zone primary airflow at the analyzed condition for Vpz per § 404.3.1 and must also be aware of the minimum primary airflow requirement established by the simplified method (Vpz‑min = Voz × 1.5) in § 403.5.1.4 when applicable file.
When secondary recirculation exists, the designer must select a representative secondary recirculation fraction (Er) based on system configuration and determine zone air distribution effectiveness (Ez) per § 403.2.2 as required in § 404.3.2 file.

Common mistakes

Treating Ev as an average of Evz values instead of the minimum Evz — the code explicitly requires Ev = min(Evz) § 404.1 .
Using a wrong Xs (e.g., corrected Vou or wrong Vps) — Xs must be Vou (uncorrected) divided by Vps at the condition analyzed per § 404.2 .
Applying the single‑supply formula to systems with secondary recirculation (ignore Ep/Er/Ez) — instead use § 404.3.2 when secondary recirculation exists .
Forgetting to use the lowest zone primary airflow for VAV when determining Zpz (Vpz) — VAV requires using the lowest expected Vpz for the design condition analyzed § 404.3.1 .
Not documenting assumptions for Er and Ez when using the secondary‑recirculation equations — the code expects these to be determined by the designer and justified § 404.3.2 .

Worked example — concrete scenario

Scenario: A single air handler serves 3 ventilation zones (A, B, C). The system is single‑supply (no secondary recirculation). The designer has determined:

Zone outdoor air requirements: Voz,A = 300 cfm, Voz,B = 150 cfm, Voz,C = 100 cfm (from zone calculations).
Zone primary (design) airflows: Vpz,A = 1500 cfm, Vpz,B = 800 cfm, Vpz,C = 400 cfm (lowest expected primary flow for each zone at the condition analyzed) — used in Zpz per § 404.3.1 .
Uncorrected outdoor‑air intake previously calculated per § 403.5.1: Vou = 400 cfm (assumed for this example) .
System primary airflow at the handler: Vps = 2700 cfm (sum of primary airflows) § 404.2 .

Step 1 — Compute Xs (Equation 404.2):
Xs = Vou / Vps = 400 / 2700 = 0.148 (14.8%) § 404.2 .

Step 2 — Compute each zone Zpz = Voz / Vpz (Equation 404.3.1(2)) and then Evz = 1 + Xs − Zpz (Equation 404.3.1(1)) § 404.3.1 :

Zone A: Zpz = 300 / 1500 = 0.200 → Evz,A = 1 + 0.148 − 0.200 = 0.948.
Zone B: Zpz = 150 / 800 = 0.1875 → Evz,B = 1 + 0.148 − 0.1875 = 0.9605.
Zone C: Zpz = 100 / 400 = 0.250 → Evz,C = 1 + 0.148 − 0.250 = 0.898.

Step 3 — System Ev (Equation 404.1): Ev = minimum(Evz) = min(0.948, 0.9605, 0.898) = 0.898 (≈0.90) § 404.1 .

Step 4 — If you need the design outdoor‑air intake (Vot), use Vot = Vou / Ev per § 403.5.2 (note: this is outside Chapter 404 but directly related): Vot = 400 / 0.898 ≈ 445 cfm § 403.5.2 .

Document each assumption (Vou, Vps, Vpz values, Ez/Ep/Er if applicable) because results depend on those inputs and the code requires this procedure when the simplified method of § 403.5.1.3 is not used § 404.1 .

Related provisions

§ 404.0 — Alternative Procedure for Multiple‑Zone Systems Ventilation Efficiency (intro) .
§ 404.1 — System Ventilation Efficiency (Ev = minimum(Evz)) .
§ 404.2 — Average Outdoor Air Fraction (Xs = Vou / Vps) .
§ 404.3 — Zone Ventilation Efficiency, including § 404.3.1 (single supply) and § 404.3.2 (secondary recirculation) file.
§ 403.5.1 — Uncorrected outdoor‑air intake (Vou) and occupant diversity (D) — used to compute Vou referenced in § 404.2 .
§ 403.5.1.3 — Simplified procedure for system ventilation efficiency (use of Chapter 404 is for when this simplified method is NOT used) .
§ 403.5.1.4 — Zone minimum primary airflow for VAV systems (Vpz‑min = Voz × 1.5) — relevant to selecting Vpz for Zpz § 404.3.1 file.
§ 403.2.2 — Zone air distribution effectiveness (Ez) referenced by § 404.3.2 .

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.

2025 CALIFORNIA MECHANICAL CODE 75

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

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 § 403.9.2 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.

2025 CALIFORNIA MECHANICAL CODE 75

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

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 § 403.9 High relevance — show source text
Table 403.9 Airstreams or Sources

Description Air Class . . . . . . . . . . . 75

403.10 Air Balance . . . . . . . . . . . . . . . . . . . 75

404.0 Alternative Procedure for

Multiple-Zone Systems Ventilation Efficiency . . . . . . . . . . . 76

xxx

404.1 System Ventilation Efficiency . . . . . 76

404.2 Average Outdoor Air Fraction. . . . . 76

404.3 Zone Ventilation Efficiency . . . . . . . 76

405.0 Ventilation for Residential

Occupancies. . . . . . . . . . . . . . . . . . 77

405.1 General. . . . . . . . . . . . . . . . . . . . . . 77

405.2 Ventilation Air Rate. . . . . . . . . . . . . 77

405.3 Bathroom Exhaust . . . . . . . . . . . . . 77

405.4 Kitchen Exhaust . . . . . . . . . . . . . . . 77

405.5 Ventilation Openings . . . . . . . . . . . 77

406.0 Evaporative Cooling System for Health Care Facilities . . . . . . . . 78

407.0 Ventilation System Details . . . . . . . 78

407.1 General. . . . . . . . . . . . . . . . . . . . . . 78

407.2 Outdoor Air Intakes and

Exhaust Outlets . . . . . . . . . . . . . . . 78

407.3 Air Balance. . . . . . . . . . . . . . . . . . . 78

407.4 Air Circulation . . . . . . . . . . . . . . . . . 78

407.5 Variable Air Volume . . . . . . . . . . . . 79

407.6 Economizers. . . . . . . . . . . . . . . . . . 79

407.7 Unoccupied Turndown. . . . . . . . . . 79

407.8 Building Pressurization . . . . . . . . . 80

408.0 Filters . . . . . . . . . . . . . . . . . . . . . . . 80

408.1 General. . . . . . . . . . . . . . . . . . . . . . 80

408.2 Filters for Hospitals. . . . . . . . . . . . . 80

408.3 Filters for Skilled Nursing Facilities, Intermediate Care Facilities, and Correctional

Treatment Centers . . . . . . . . . . . . . 80

408.4 Filters for Outpatient Facilities . . . . 80

408.5 Filters for Recirculating

Room Units. . . . . . . . . . . . . . . . . . . 80
CMC § 403.3 High relevance — show source text
403.3 Single-Zone Systems. For ventilation systems where one or more air handlers supply a mixture of outdoor air and recirculated air to only one ventilation zone, the outdoor air intake flow ( Vot ) shall be determined in accordance with Equation 403.3. [ASHRAE 62.1:6.2.2]

Vot = Voz (Equation 403.3)

403.4 One Hundred Percent Outdoor Air Systems. For ventilation systems where one or more air handlers supply only outdoor air to one or more ventilation zones, the outdoor air intake flow ( Vot ) shall be determined in accordance with Equation 403.4. [ASHRAE 62.1:6.2.3]

Vot = ∑ all zones Voz (Equation 403.4)

403.5 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, 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.

2025 CALIFORNIA MECHANICAL CODE 73

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

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).
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.

76 2025 CALIFORNIA MECHANICAL CODE

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

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 § 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.

2025 CALIFORNIA MECHANICAL CODE 73

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

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 § 402.2.1.6 High relevance — show source text
402.2.1.6(A)(1) Single Openings . . . . . . . . . . . . . . . 71

Table Minimum Openable Areas:

402.2.1.6(A)(2) Two Vertically

Spaced Openings . . . . . . . . . . . . . . 71

402.3 Mechanical Ventilation . . . . . . . . . . 71

402.4 Outdoor Air Intakes. . . . . . . . . . . . . 71

Table 402.4.1 Air Intake Minimum

Separation Distance . . . . . . . . . . . . 72

403.0 Ventilation Rates . . . . . . . . . . . . . . 72

403.1 General. . . . . . . . . . . . . . . . . . . . . . 72

403.2 Zone Calculations. . . . . . . . . . . . . . 73

403.3 Single-Zone Systems . . . . . . . . . . . 73

403.4 One Hundred Percent Outdoor

Air Systems . . . . . . . . . . . . . . . . . . 73

403.5 Multiple-Zone Recirculating Systems . . . . . . . . . . . . . . . . . . . . . 73

403.6 Design for Varying Operating Conditions. . . . . . . . . . . . . . . . . . . . 74

403.7 Exhaust Ventilation. . . . . . . . . . . . . 74

403.8 Dynamic Reset . . . . . . . . . . . . . . . . 75

403.9 Air Classification and

Recirculation. . . . . . . . . . . . . . . . . . 75

Table 403.9 Airstreams or Sources

Description Air Class . . . . . . . . . . . 75

403.10 Air Balance . . . . . . . . . . . . . . . . . . . 75

404.0 Alternative Procedure for

Multiple-Zone Systems Ventilation Efficiency . . . . . . . . . . . 76

xxx

404.1 System Ventilation Efficiency . . . . . 76

404.2 Average Outdoor Air Fraction. . . . . 76

404.3 Zone Ventilation Efficiency . . . . . . . 76

405.0 Ventilation for Residential

Occupancies. . . . . . . . . . . . . . . . . . 77

405.1 General. . . . . . . . . . . . . . . . . . . . . . 77

405.2 Ventilation Air Rate. . . . . . . . . . . . . 77

405.3 Bathroom Exhaust . . . . . . . . . . . . . 77

405.4 Kitchen Exhaust . . . . . . . . . . . . . . . 77

405.5 Ventilation Openings . . . . . . . . . . . 77

406.0 Evaporative Cooling System for Health Care Facilities . . . . . . . . 78

407.0 Ventilation System Details . . . . . . . 78

407.1 General. . . . . . . . . . . . . . . . . . . . . . 78

407.2 Outdoor Air Intakes and
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.

76 2025 CALIFORNIA MECHANICAL CODE

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

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 § 120.1 Medium 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 § 110.2 Medium relevance — show source text
Compliance with the cooling-only perfor-
mance is required as defined in Notes b and c of Table 110.2-I.
h. Water-to-water heat pumps with a capacity less than 135,000 Btu/h are included in Table 110.2-B, Heat Pumps—Minimum Efficiency Requirements.
i. Source leaving liquid temperature.
1. The cooling evaporator liquid flow rate used for the heating rating for a reverse cycle air-to-water heat pump shall be the flow rate determined during the full-load cool-
ing rating.
2. The cooling evaporator liquid flow rate for the simultaneous cooling and heating and heat recovery liquid cooled chilling packages rating shall be the liquid flow rates
from the cooling operation full-load rating.
3. For heating-only fluid-to-fluid chiller packages, the evaporator flow rate obtained with an entering liquid temperature of 54°F and a leaving liquid temperature of 44°F
shall be used.
j. NA means the requirements are not applicable.|a. The size category is the full-load net refrigeration cooling mode capacity, which is the capacity of the evaporator available for cooling of the thermal load external to the chill-
ing package.
b. For air source heat pumps, compliance with both the 47°F and 17°F heating source outdoor air temperature (OAT) rating efficiency is required for heating.
c. Heating full-load rating conditions are at standard rating conditions defined in AHRI 550/590 (I-P), Table 4, which includes the impact of defrost for air source heating ratings.
d. For units that operate in both cooling and heating, compliance with both the cooling and heating efficiency is required.
e. For heat recovery heating chilling package applications where there is simultaneous cooling and heating, compliance with the heating performance heat recover COPHR is
only required at one of the four heating AHRI 550/590 (I-P) standard ratings conditions of Low, Medium, Hot-Water 1 or Hot-Water 2. Compliance with the cooling-only perfor-
mance is required as defined in Notes b and c of Table 110.2-I.
f. For liquid source heat recovery chilling packages that have capabilities for heat rejection to a heat recovery condenser and a tower condenser, the COPHR applies to operation
at full load with 100 percent heat recovery (no tower rejection). Units that only have capabilities for partial heat recovery shall meet the requirements of Table 110.2-D, Water
Chilling Packages—Minimum Efficiency Requirements.
g. For heat recovery heating chilling package applications where there is simultaneous cooling and heating, compliance with the heating performance heat recover COPHR is
only required at one of the four heating AHRI 550/590 (I-P) standard ratings conditions of Low, Medium, Hot-Water 1 or Hot-Water 2. Compliance with the cooling-only perfor-
mance is required as defined in Notes b and c of Table 110.2-I.
h. Water-to-water heat pumps with a capacity less than 135,000 Btu/h are included in Table 110.2-B, Heat Pumps—Minimum Efficiency Requirements.
i. Source leaving liquid temperature.
1. The cooling evaporator liquid flow rate used for the heating rating for a reverse cycle air-to-water heat pump shall be the flow rate determined during the full-load cool-
ing rating.
2. The cooling evaporator liquid flow rate for the simultaneous cooling and heating and heat recovery liquid cooled chilling packages rating shall be the liquid flow rates
from the cooling operation full-load rating.
3.
CMC § 0.746 Medium relevance — show source text
For SI units: 1 horsepower = 0.746 kW
- Average full-load efficiencies shall be established in accordance with 10 CFR 431.
the space is in occupied-standby mode, and when using the Ventilation Rate Procedure, shall meet the following within 5 minutes of all rooms in that zone entering occupied-standby mode.

(1) Active heating set point shall be setback at least 1°F (0.6°C).

(2) Active cooling set point shall be setup at least 1°F (0.6°C).

(3) All airflow supplied to the zone shall be shut off whenever the space temperature is between the active heating and cooling set points.

Exception: Multiple zone systems without automatic zone flow control dampers.

[ASHRAE 90.1:6.5.3.9]

E 503.5.7 Hydronic System Design and Control. Hydronic system design and control shall be in accordance with Section E 503.5.7.1 and Section E 503.5.7.7.

E 503.5.7.1 Boiler Turndown. Boiler systems with design input of 1 000 000 Btu/h (293 kW) or more shall comply with the turndown ratio in accordance with Table E 503.5.7.

The system turndown requirement shall use multiple single-input boilers, one or more modulating boilers, or a combination of single-input and modulating boilers.

Boilers shall comply with the minimum efficiency requirements in Table E 503.7.1(6).

[ASHRAE 90.1:6.5.4.1]

2025 CALIFORNIA MECHANICAL CODE 439

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

APPENDIX E

TABLE E 503.5.7.2

PUMP FLOW CONTROL REQUIREMENTS

[ASHRAE 90.1: TABLE 6.5.4.2]

BOILER SYSTEM DESIGN INPUT, Btu/h MINIMUM TURNDOWN RATIO

≥1 000 000 and≤5 000 000 3 to 1

>5 000 000 and≤10 000 000 4 to 1

>10 000 000 5 to 1

»

TABLE E 503.5.7

BOILER TURNDOWN

[ASHRAE 90.1: TABLE 6.5.4.1]

For SI units: 1000 British thermal units per hour = 0.293 kW

E 503.5.7.2 Hydronic Variable Flow Systems. Chilled- and hot-water distribution systems that include three or more control valves designed to modulate or step open and close as a function of load shall be designed for variable fluid flow and shall be capable of and configured to reduce pump flow rates to not more than the larger of 25 percent of the design flow rate or the minimum flow required by the heating/cooling equipment manufacturer for the proper operation of equipment.

BOILER SYSTEM DESIGN INPUT, Btu/h	MINIMUM TURNDOWN RATIO
≥1 000 000 and≤5 000 000	3 to 1
>5 000 000 and≤10 000 000	4 to 1
>10 000 000	5 to 1

CMC § 25.4 Medium relevance — show source text

|
For multiple zone units, either outdoor air damper or zone damper modulate open to satisfy the zone ventilation
requirements.| |Step 2: Simulate signal well below the CO2 setpoint or follow manufacturers recommended procedures.|Step 2: Simulate signal well below the CO2 setpoint or follow manufacturers recommended procedures.|Step 2: Simulate signal well below the CO2 setpoint or follow manufacturers recommended procedures.|Step 2: Simulate signal well below the CO2 setpoint or follow manufacturers recommended procedures.| |
For single zone units, outdoor air damper modulates to the design minimum value.|
For single zone units, outdoor air damper modulates to the design minimum value.|
For single zone units, outdoor air damper modulates to the design minimum value.|
For single zone units, outdoor air damper modulates to the design minimum value.| |
For multiple zone units, either outdoor air damper or zone damper modulate to satisfy the reduced zone ventilation
requirements.

|
For multiple zone units, either outdoor air damper or zone damper modulate to satisfy the reduced zone ventilation
requirements.

|
For multiple zone units, either outdoor air damper or zone damper modulate to satisfy the reduced zone ventilation
requirements.

|
For multiple zone units, either outdoor air damper or zone damper modulate to satisfy the reduced zone ventilation
requirements.

| |Step 3: System returned to initial operating conditions.|Y/N|Y/N|Y/N|

B. Testing Results.	PASS / FAIL	Col3
Step 1: Simulate a high CO2 load (check box complete).
Step 2: Simulate a low CO2 load (check box complete).

For SI units: 1 inch = 25.4 mm

506 2025 CALIFORNIA MECHANICAL CODE

APPENDIX E

CERTIFICATE OF ACCEPTANCE MECH-6A	Col2
Demand Control Ventilation Systems Acceptance (Page 3 of 3)	Demand Control Ventilation Systems Acceptance (Page 3 of 3)
Project Name/Address:	Project Name/Address:
System Name or Identification/Tag:	System Location or Area Served:

C.	PASS/FAIL Evaluation (check one):
	PASS: AllConstruction Inspection responses are complete andTesting Results responses are “Pass.”
	FAIL: AnyConstruction Inspection responses are incomplete_OR_ there is one or more “Fail” responses inTesting Results section. Provide explanation below. Use and attach additional pages if necessary.

2025 CALIFORNIA MECHANICAL CODE 507

Frequently asked questions

When must I use this alternative procedure instead of the simplified method?

Use the Chapter 404 alternative procedure when you are designing a multiple‑zone recirculating system and you do not use the simplified method in § 403.5.1.3. The code explicitly directs use of § 404.0–404.3 in that case § 404.1.

How do I choose between the single‑supply and secondary‑recirculation Evz formulas?

If all supply to a zone is a mixed stream of primary + system recirculated air (no secondary recirculation paths), use § 404.3.1. If the system includes secondary recirculation (fan‑powered boxes, secondary return paths), use § 404.3.2, which requires Ep, Er and Ez inputs. See § 404.3. file

What is Xs and why does it matter?

Xs is the average outdoor‑air fraction at the air handler (Xs = Vou / Vps). It scales zone Evz values and directly affects system Ev; must be calculated at the condition analyzed per § 404.2.

If Ev < 1.0 does that mean I need more outdoor air?

Not necessarily — Ev < 1.0 means the system does not deliver outdoor air perfectly to the breathing zone in the critical zone. You must use Ev in the outdoor‑air intake calculation (Vot = Vou / Ev, § 403.5.2) to size the intake so required breathing‑zone rates are met.

Do I need to document assumptions for Er and Ez?

Yes. When using the secondary‑recirculation equations in § 404.3.2, the designer must determine and justify Er (secondary recirculation fraction) and Ez (zone air distribution effectiveness) because Evz depends on these choices. file

Ask about the CMC

Get cited, plain-English answers on the California Mechanical Code for your project — any code section, any scenario.

Start Free Trial

Related in the CMC

How do I calculate ventilation rates for a ventilation zone?

Single-zone, 100% outdoor-air, and multiple-zone procedures (when to use each)

Design for varying operating conditions and averaging time provisions

Ventilation rates and calculation procedures

California Mechanical Code