CMC · California Mechanical Code
How do I calculate ventilation rates for a ventilation zone?
To size ventilation for a zone, compute breathing‑zone outdoor air using Vbz = Rp × Pz + Ra × Az (use the Rp and Ra from Table 402.1), then divide Vbz by the zone distribution effectiveness Ez to get the outdoor air you must deliver to the zone (Voz); these steps and the Ez rules are in **§ 403.2** of the CMC.
Last reviewed: July 6, 2026
What the code requires — 2–4 sentences
In plain English: determine the outdoor air required in the breathing zone of each ventilation zone using a people-based term plus an area-based term, then adjust for how effectively the supplied air reaches the breathing zone. The fundamental equations and definitions are in § 403.2 (zone calculations), with the breathing-zone formula in § 403.2.1 and the zone supply requirement in § 403.2.3.
The simplest rule: provide at least Vbz = Rp × Pz + Ra × Az (breathing‑zone outdoor airflow) and then supply Voz = Vbz / Ez (zone outdoor airflow) — use the Rp/Ra values in Table 402.1 and Ez from Table 403.2.2.
Requirements in detail
Key defined terms (first use bolded)
- Breathing zone outdoor airflow (Vbz) — the outdoor airflow required in the breathing zone of the occupiable space(s) in a ventilation zone. See § 403.2.1.
- Zone outdoor airflow (Voz) — the outdoor airflow provided to the ventilation zone by the supply air distribution system. See § 403.2.3.
- Zone air distribution effectiveness (Ez) — factor that accounts for how well supply air reaches the breathing zone; determined from Table 403.2.2 (or by ASHRAE 62.1 for special systems). See § 403.2.2.
The core equations you must use
- Breathing-zone outdoor airflow:
- Vbz = Rp × Pz + Ra × Az — § 403.2.1. Rp and Ra are taken from Table 402.1.
- Convert breathing-zone requirement to zone supply:
- Voz = Vbz / Ez — § 403.2.3. Ez is taken from Table 403.2.2 or determined per §403.2.2 rules.
Decision‑relevant values & where to get them
| Item | What it is / how to measure | Typical units | Code Reference |
|---|---|---|---|
| Az | Net occupiable floor area of the ventilation zone | ft² (m²) | § 403.2.1 |
| Pz | Zone population (number of people during use) | people | § 403.2.1 |
| Rp | Outdoor air rate per person | CFM/person (use Table 402.1) | Table 402.1 referenced by § 403.2.1 |
| Ra | Outdoor air rate per area | CFM/ft² (use Table 402.1) | Table 402.1 referenced by § 403.2.1 |
| Vbz | Breathing‑zone outdoor airflow (calculated) | CFM (L/s) | § 403.2.1 |
| Ez | Zone air distribution effectiveness (from Table 403.2.2 or ASHRAE 62.1 for special systems) | dimensionless | § 403.2.2 |
| Voz | Zone outdoor airflow required at supply | CFM (L/s) | § 403.2.3 |
(If your occupancy category is covered by ASHRAE/ASHE 170, those rates override Table 402.1 as stated in § 403.2.)
Zone air distribution details that affect calculations
- Determine Ez from Table 403.2.2 (or per ASHRAE 62.1 for stratified or personalized ventilation systems). See § 403.2.2.
- For stratified systems the code requires design conditions (e.g., supply air at least 4°F cooler than average room temp and returns located not less than 9 feet above the floor) or use ASHRAE 62.1 to determine Ez. See § 403.2.2.1 and subsections.
Exceptions & special cases
- The zone calculation procedure in § 403.2 does not apply where ASHRAE/ASHE 170 ventilation rates are required; in those occupancies use ASHRAE/ASHE 170 rates. See § 403.2.
- The method in Chapter 4 (Sections 403.2 etc.) is not permitted for certain OSHPD spaces listed in Table 4‑A; check § 403.0 for that limitation.
- If you have a specialized distribution (stratified or personalized ventilation), the code lets you use ASHRAE 62.1 to determine Ez instead of the table in CMC; see § 403.2.2.1.
Common mistakes
- Using gross floor area instead of the required net occupiable floor area (Az) — § 403.2.1 defines Az.
- Forgetting to apply the Ez factor (mixing Vbz and Voz): designers calculate Vbz then divide by Ez to get the supply outdoor air required to the zone — see § 403.2.3.
- Pulling Rp/Ra from the wrong table or jurisdictional standard — the CMC points you to Table 402.1 for Rp and Ra; do not substitute values unless the code or AHJ permits. See § 403.2.1.
- Overlooking occupancy overrides: certain healthcare spaces and similar occupancies may require ASHRAE/ASHE 170; the CMC instructs you to use those rates where applicable § 403.2.
Worked example (concrete scenario using CMC example values)
The CMC includes an Appendix I worked example for an indoor horticultural cultivation space that directly demonstrates Equation 403.2.1. Using that Appendix I example:
Given (Appendix I example): net occupiable area Az = 800 ft², zone population Pz = 5 people. Appendix I uses Rp = 60 CFM/person and Ra = 1 CFM/ft² for that use. See I 801.1.1.
Step 1 — compute breathing-zone outdoor airflow (Vbz) per § 403.2.1:
- Vbz = Rp × Pz + Ra × Az
- Vbz = (60 CFM/person × 5 people) + (1 CFM/ft² × 800 ft²) = 300 + 800 = 1100 CFM.
Step 2 — determine zone supply (Voz) per § 403.2.3:
- Voz = Vbz / Ez. The code requires Ez from Table 403.2.2 (or by ASHRAE 62.1). If the system is a typical well‑mixed supply and Ez = 1.0 (assumption — confirm from Table 403.2.2 for your supply type), then Voz = 1100 / 1.0 = 1100 CFM. See § 403.2.3 and § 403.2.2.
Notes: the Appendix I example expressly demonstrates the Vbz arithmetic; the Ez value must be selected from Table 403.2.2 for your specific distribution type — if Ez < 1, Voz increases accordingly.
Related provisions
- § 403.0 — scope and limits (OSHPD exceptions).
- § 403.1 — general instruction to follow Sections 403.2–403.10 for design outdoor air intake flow.
- § 403.2.1 — breathing‑zone equation and definitions (Vbz = Rp·Pz + Ra·Az).
- § 403.2.2 — zone air distribution effectiveness (Ez), and stratified/personal ventilation rules.
- § 403.2.3 — Voz = Vbz / Ez.
- § 403.3 – § 403.5 — procedures for calculating outdoor-air intake for single-zone, 100% OA, and multiple-zone recirculating systems (use when sizing the outdoor air intake).
- § 403.10 — air balance and testing requirements for mechanical ventilation systems.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 403.0 High relevance — show source text
403.0 Ventilation Rates. [Not permitted for OSHPD 1, 2, 3, 4 & 5 spaces listed in Table 4-A]
403.1 General. The design outdoor air intake flow rate for a ventilation system shall be determined in accordance with Section 403.2 through Section 403.10.
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VENTILATION AIR
403.2 Zone Calculations. Ventilation zone parameters shall be determined in accordance with Section 403.2.1 through Section 403.2.3 for ventilation zones served by the ventilation system, except that the ventilation rates from ASHRAE/ASHE 170 shall be used for the occupancy categories, as applicable. [ASHRAE 62.1:6.2.1]
403.2.1 Breathing Zone Outdoor Airflow. The outdoor airflow required in the breathing zone ( Vbz ) of the occupiable space or spaces in a ventilation zone shall be not less than the value determined in accordance with Equation 403.2.1.
Vbz = Rp•Pz + Ra• Az (Equation 403.2.1)
Where: Az = zone floor area, the net occupiable floor area of the ventilation zone, ft [2] (m [2] ). Pz = zone population, the number of people in the ventilation zone during usage. Rp= outdoor airflow rate required per person (CFM/person) as determined from Table 402.1. Ra= outdoor airflow rate required per unit area (CFM/ft [2] ) as determined from Table 402.1.
[ASHRAE 62.1:6.2.1.1] 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, 1 cubic foot per minute per square foot = 5.08 [(L/s)/m [2] ] 403.2.2 Zone Air Distribution Effectiveness. The zone air distribution effectiveness ( Ez ) shall be determined in accordance with Table 403.2.2. [ASHRAE 62.1:6.2.1.2] 403.2.2.1 Stratified Air Distribution Systems. A stratified air distribution system shall be designed in accordance with Section 403.2.2.1.1 through Section 403.2.2.2.2, or the zone air distribution effectiveness ( Ez ) shall be determined in accordance with ASHRAE 62.1. [ASHRAE 62.1:6.2.1.2.1] 403.2.2.1.1 Supply Air. Cool air shall be at least 4°F (2°C) less than the average room air temperature. [ASHRAE 62.1:6.2.1.2.1.1] 403.2.2.1.2 Return Air. The return air openings or pathways shall be located not less than 9 feet (2743 mm) above the floor. [ASHRAE 62.1:6.2.1.2.1.2]
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.
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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).
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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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 § 403.2.1. 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 § 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 § 801.0 High relevance — show source text
I 801.0 Ventilation and Exhaust System.
I 801.1 General. The indoor air quality in indoor spaces using environmental controls for horticultural cultivation, and processing shall comply with Chapter 4 and Section I 801.1.1. The ventilation and exhaust system shall comply with Section I 801.1.2 through Section I 801.1.6. Indoor spaces used for horticultural cultivation shall have an independent ventilation system.
All mechanical ventilation and exhaust systems shall be tested, balanced, and operated to demonstrate to the owner or designated representative and Authority Having Jurisdiction that the installation and performance of the systems conform to 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.
I 801.1.1 Breathing Zone Outdoor Airflow Rate. The outdoor airflow required in the breathing zone ( V bz ) of the occupiable space or spaces in a ventilation zone
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APPENDIX I
shall be as shown in Table 402.1 and not less than the value determined in accordance with Equation I 801.1.1. Vbz = Rp - Pz + Ra - Az (Equation I 801.1.1)
Where:
Az = zone floor area, the net occupiable floor area of the ventilation zone, ft [2] .
Pz = zone population, the number of people in the ventilation zone during typical usage. Rp = outdoor airflow rate required per person (CFM/person) in accordance with the following, as applicable:
(1) 60 CFM/person for cultivation, production, and processing facilities or factories.
(2) 15 CFM/person for centers and dispensaries.
Ra = outdoor airflow rate required per unit area, 1 CFM/ft [2]
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] ]
Sample Calculation: Determine the outdoor airflow required in the breathing zone ( Vbz ) of an indoor space used for horticultural cultivation with an occupiable floor area of 800 square feet and a maximum of 5 people expected to occupy the zone.
Sample Solution: Vbz = Rp - Pz + Ra - Az Vbz = (60 CFM/person • 5 people) + (1 CFM • 800 SF) Vbz = 1100 CFM. The ventilation system shall be capable of providing not less than 1100 CFM of outdoor air.
I 801.1.2 Ventilation System Requirements. When activated by the gas detection system required by Section I 601.1, the mechanical purge ventilation system shall remain on until manually reset. The purge ventilation system ducting shall terminate outdoors in an approved location. The ventilation system shall be designed to operate at a negative pressure of 0.01 inches water column (0.002 kPa) in relation to the exhausted indoor space.
CMC § 2.5 High relevance — show source text
(3) Select louvers that are Class A when rated according to AMCA 511 and tested per the AMCA 500-L wind-driven rain test when subjected to a simulated rainfall of 3 inches (76 mm) per hour and a 29 mph (13 m/s) wind velocity. The maximum design core area velocity through the louver shall correlate to a Class A rating.
(4) Use rain hoods sized for no more than 500 feet per minute (2.5 m/s) face velocity with a downward-facing intake such that all intake air passes upward through a horizontal plane that intersects the solid surfaces of the hood before entering the system.
(5) Manage the water that penetrates the intake opening by providing a drainage area or moisture removal devices. {ASHRAE 62.1:5.5.2} 402.4.3 Rain Intrusion. Air-handling and distribution equipment mounted outdoors shall be designed to prevent rain intrusion into the airstream when tested at design airflow and with no airflow, using the rain test apparatus described in UL 1995. [ASHRAE 62.1:5.5.3] 402.4.4 Snow Entrainment. Where climate dictates, outdoor air intakes that are part of the mechanical ventilation system shall be designed as follows to manage water from snow that is blown or drawn into the system:
(1) Access doors to permit cleaning of wetted surfaces shall be provided.
(2) Outdoor air ductwork or plenums shall pitch to drains designed in accordance with the requirements of ASHRAE 62.1. [ASHRAE 62.1:5.5.4]
402.4.5 Bird Screens. Outdoor air intakes shall include a screening device designed to prevent penetration by a 0.5 inch (13 mm) diameter probe. The screening device material shall be corrosion resistant. The screening device shall be located, or other measures shall be taken, to prevent bird nesting within the outdoor air intake. [ASHRAE 62.1:5.5.5]
403.0 Ventilation Rates. [Not permitted for OSHPD 1, 2, 3, 4 & 5 spaces listed in Table 4-A]
403.1 General. The design outdoor air intake flow rate for a ventilation system shall be determined in accordance with Section 403.2 through Section 403.10.
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VENTILATION AIR
403.2 Zone Calculations. Ventilation zone parameters shall be determined in accordance with Section 403.2.1 through Section 403.2.3 for ventilation zones served by the ventilation system, except that the ventilation rates from ASHRAE/ASHE 170 shall be used for the occupancy categories, as applicable. [ASHRAE 62.1:6.2.1]
403.2.1 Breathing Zone Outdoor Airflow. The outdoor airflow required in the breathing zone ( Vbz ) of the occupiable space or spaces in a ventilation zone shall be not less than the value determined in accordance with Equation 403.2.1.
Vbz = Rp•Pz + Ra• Az (Equation 403.2.1)
CMC § 403.2.2. Medium 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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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 § 403.10 Medium 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 § 1 Medium relevance — show source text
Appendix G Example Calculation of Outdoor Air Rate. Appendix G gives an example of how to calculate the required outdoor air rate under Chapter 4.
Appendix H Professional Qualifications. Appendix H provides a baseline of knowledge for installers, inspectors, or employers working with and around systems covered within the scope of this code to maintain health and safety.
Appendix I Indoor Horticultural Facilities. Appendix I addresses indoor spaces using environmental controls for horticultural cultivation and processing of plants for human ingestion, inhalation, and topical application. The appendix includes recommendations pertaining to fire protection and suppression systems, carbon dioxide detection systems, flammable solvents, ventilation and exhaust systems, particulate and odor control, and fumigation.
Appendix J Clean Air Delivery. Appendix J provides criteria for an increased level of protection for occupant health by delivering and monitoring clean air in occupied areas of certain buildings. This appendix addresses Demand Control Ventilation (DCV) for occupancy groups A, B, E, and I and recommends that each occupiable zone have CO 2 sensors which communicate with building mechanical systems to adjust airflow rates and maintain acceptable CO 2 levels. Additionally, this appendix contains provisions for mechanical system controls pertaining to sensor calibration, intervals for collected data, and automatic remediation actions to increase the amount of outdoor air provided to each occupiable zone. The CO 2 sensors addressed in this appendix are intended to give notifications that can assist with awareness of unhealthy levels and give insight to building occupants as to when mitigation action is necessary.
xviii 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
SAMPLE LEGISLATION FOR ADOPTION OF THE UNIFORM MECHANICAL CODE
The Uniform Codes are designed to be adopted by jurisdictions through an ordinance. Jurisdictions wishing to adopt the 2024 Uniform Mechanical Code as an enforceable regulation governing mechanical systems by reference should ensure the legal basis under which adoption and implementation are included in the ordinance.
The following sample ordinance is a guide for drafting an ordinance for adoption that addresses key components regulations and resolutions.
ORDINANCE NO.
An ordinance of the [JURISDICTION] adopting the 2024 edition of the Uniform Mechanical Code, regulating and controlling the design, construction, quality of materials, erection, installation, alteration, repair, location, relocation, replacement, addition to, use or maintenance of mechanical systems in the [JURISDICTION]; providing for the issuance of permits and collection of fees therefor; repealing Ordinance No. of the
[JURISDICTION] and all other ordinances and parts of the ordinances in conflict therewith.
The [GOVERNING BODY] of the [JURISDICTION] does ordain as follows:
Section 1 Codes Adopted by Reference. That certain documents, three (3) copies of which are on file in the office of the [JURISDICTION'S KEEPER OF RECORDS] and the [JURISDICTION], being marked and designated as the 2024 Uniform Mechanical Code, including Appendix Chapters [FILL IN THE APPENDIX CHAPTERS BEING ADOPTED], as published by the International Association of Plumbing and Mechanical Officials, be and is hereby adopted as the Code of the [JURISDICTION], in the State of [STATE NAME] regulating and controlling the design, construction, quality of materials, erection, installation, alteration, repair, location, relocation, replacement, addition to,
CMC § 62.1 Medium relevance — show source text
Controls shall be designed to coordinate operation of the natural and mechanical ventilation systems. [ASHRAE 62.1:6.4.3] D. Naturally ventilated spaces shall also include a mechanical ventilation system designed in accordance with Section 120.1(c)3. Exception 1 to Section 120.1(c)2D: Spaces not served by a space-conditioning system. Exception 2 to Section 120.1(c)2D: Spaces where natural ventilation openings complying with Section 120.1(c)2 are either permanently open or have controls that prevent the openings from being closed during periods of expected occupancy. 3. Mechanical ventilation. Occupiable spaces shall be ventilated with a mechanical ventilation system capable of providing an outdoor airflow rate to the zone ( V z ) no less than Equation 120.1-F as described below:
(Equation 120.1-F) V z = The larger of R p × P z or R a × A z
Where:
R p = 15 cubic feet per minute of outdoor airflow per person P z = The expected number of occupants. For spaces without fixed seating, the expected number of occupants shall be the expected number specified by the building designer or the default occupancy density in Table 120.1-A times the occupiable floor area of the zone, whichever is greater. For spaces with fixed seating, the expected number of occupants shall be determined in accordance with Section 1004.6 of the California Building Code. R a = The area-based minimum ventilation airflow rate in Table 120.1-A. A z = The net occupiable floor area of the ventilation zone in square feet.
Exception 2 to Section 120.1(c)3: Transfer air. The rate of outdoor air required by Section 120.1(c)3 may be provided with air transferred from other ventilated space if: A. Use of transfer air is in accordance with Section 120.1(g); and B. The outdoor air that is supplied to all spaces combined, is sufficient to meet the requirements of Section 120.1(c)3 for each space individually. 4. Exhaust ventilation. The design exhaust airflow shall be determined in accordance with the requirements in Table 120.1-B. Exhaust makeup air shall be permitted to be any combination of outdoor air, recirculated air, or transfer air.
[ASHRAE 62.1:6.5.1]
66 2025 CALIFORNIA ENERGY CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS
TABLE 120.1-A—MINIMUM VENTILATION RATES Col2 Col3 Col4 Col5 OCCUPANCY CATEGORY MINIMUM OCCUPANT LOAD
DENSITY (persons/1,000 ft2)AREA-BASED MINIMUM
VENTILATION_ Ra_ (cfm/ft2)AIR CLASS NOTES Educational Facilities Educational Facilities Educational Facilities Educational Facilities Educational Facilities Daycare (through age 4) 14 0. CMC § 62.1 Medium relevance — show source text
[ASHRAE 62.1:6.2.1.1] 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, 1 cubic foot per minute per square foot = 5.08 [(L/s)/m [2] ] 403.2.2 Zone Air Distribution Effectiveness. The zone air distribution effectiveness ( Ez ) shall be determined in accordance with Table 403.2.2. [ASHRAE 62.1:6.2.1.2] 403.2.2.1 Stratified Air Distribution Systems. A stratified air distribution system shall be designed in accordance with Section 403.2.2.1.1 through Section 403.2.2.2.2, or the zone air distribution effectiveness ( Ez ) shall be determined in accordance with ASHRAE 62.1. [ASHRAE 62.1:6.2.1.2.1] 403.2.2.1.1 Supply Air. Cool air shall be at least 4°F (2°C) less than the average room air temperature. [ASHRAE 62.1:6.2.1.2.1.1] 403.2.2.1.2 Return Air. The return air openings or pathways shall be located not less than 9 feet (2743 mm) above the floor. [ASHRAE 62.1:6.2.1.2.1.2]
403.2.2.1.3 Stratification. The zone shall not contain any devices that mechanically mix the air, and shall be protected from impinging airstreams from adjacent ventilation zones.
[ASHRAE 62.1:6.2.1.2.1.3] 403.2.2.2 Personalized Ventilation Systems. A personalized ventilation system shall be designed in accordance with the following subsections, or the zone air distribution effectiveness ( Ez ) shall be determined in accordance with ASHRAE 62.1.
[ASHRAE 62.1:6.2.1.2.2] 403.2.2.2.1 Personalized Air. The personalized air shall be distributed in the breathing zone and designed such that the velocity is
equal to or less than 50 feet per minute (0.25 m/s) at the head/facial region of the occupant.
[ASHRAE 62.1:6.2.1.2.2.1] 403.2.2.2.2 Return Air. The return air openings or pathways shall be located more than 9 feet (2.8 m) above the floor. [ASHRAE 62.1:6.2.1.2.2.2]
403.2.3 Zone Outdoor Airflow. The zone outdoor airflow ( Voz ) provided to the ventilation zone by the supply air distribution system shall be determined in accordance with Equation 403.2.3. [ASHRAE 62.1:6.2.1.3]
Voz = Vbz / Ez (Equation 403.2.3)
Frequently asked questions
How do I find Rp and Ra values?
Rp and Ra are specified in Table 402.1 referenced by § 403.2.1; the code requires you use those table values unless a different standard (e.g., ASHRAE/ASHE 170) applies.
What if my system is stratified or uses personalized ventilation?
Use the design rules in § 403.2.2.1 (temperature and return location requirements) or determine Ez per ASHRAE 62.1 as allowed by § 403.2.2.
Do I always divide Vbz by Ez?
Yes — to get the zone outdoor airflow supplied by the system you must apply Voz = Vbz / Ez as required in § 403.2.3.
What if my zone is sometimes unoccupied or occupancy fluctuates?
The CMC provides procedures (in later sections of Chapter 4) to address averaging for fluctuating occupancy or short-term peaks; check those provisions and the AHJ for allowable averaging times. The zone‑calculation core, however, remains Vbz = Rp·Pz + Ra·Az.
Where do I document the calculation for plan review?
Show Az, Pz, the Rp and Ra values taken from Table 402.1, the computed Vbz, the Ez used (with its code source — Table 403.2.2 or ASHRAE 62.1), and the resulting Voz; include the equation references § 403.2.1 and § 403.2.3.
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