CMC · California Mechanical Code

Where must outdoor air intakes be located and how far from contaminants?

The California Mechanical Code requires outdoor-air intakes to be placed at or beyond the minimum distances shown in Table 402.4.1 from listed contaminant sources (or to meet equivalent protection using ASHRAE 62.1 Appendix B). Measure the shortest distance from the intake to each contaminant source, watch special rules for plumbing vents, roof/snow, lab exhausts, and provide rain/snow/bird protections as required.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

Outdoor air intakes (including required natural-ventilation openings) must be sited so the shortest distance from the intake to any listed contaminant source is at least the minimum shown in Table 402.4.1 or as determined by the calculation methods in ASHRAE 62.1 Normative Appendix B. The general design requirement for outdoor intakes is set out in § 402.4 and the location/separation criteria are in § 402.4.1 and Table 402.4.1 of the California Mechanical Code.

Place intakes no closer to the listed contaminant sources than the distances in Table 402.4.1 (unless you use the ASHRAE Appendix B dispersion calculation and show equal or better performance).

Requirements in detail

Key concept — what distance is measured

The code measures the shortest distance from the intake opening to the specific potential outdoor contaminant source listed in Table 402.4.1. This governs location compliance. § 402.4.1.

How to comply

Comply by meeting either:
- the separation distances in Table 402.4.1, or
- the calculation methods in ASHRAE 62.1 Normative Appendix B with all other Section 402.4 requirements. § 402.4.1 (1)–(2).

Decision-relevant distances (code table)

Use this table to pick the required clearance for the contaminant source in your project. The distances are feet unless noted.

Potential contaminant source (object)	Minimum separation distance	Code reference
Class 2 exhaust/relief outlet	10 ft	Table 402.4.1; § 402.4.1
Class 3 exhaust/relief outlet	15 ft	Table 402.4.1; § 402.4.1
Class 4 exhaust/relief outlet	30 ft	Table 402.4.1; § 402.4.1
Cooling tower exhaust	25 ft	Table 402.4.1; § 402.4.1
Cooling tower intake or basin	15 ft	Table 402.4.1; § 402.4.1
Driveway, street, or parking place	5 ft	Table 402.4.1; § 402.4.1
Garage entry, automobile loading area, drive‑in queue	15 ft	Table 402.4.1; § 402.4.1
Garbage storage / dumpsters / pick‑up area	15 ft	Table 402.4.1; § 402.4.1
Plumbing vents terminating ≥ 3 ft above intake	3 ft	Table 402.4.1; § 402.4.1
Plumbing vents terminating < 3 ft above intake	10 ft	Table 402.4.1; § 402.4.1
Roof, landscaped grade, or other surface directly below intake	1 ft	Table 402.4.1; § 402.4.1
Thoroughfare with high traffic volume	25 ft	Table 402.4.1; § 402.4.1
Truck loading area or dock; bus parking / idling area	25 ft	Table 402.4.1; § 402.4.1
Vents, chimneys, flues from combustion appliances/equipment	15 ft	Table 402.4.1; § 402.4.1

Notes:

The table entries above are the direct transposition of Table 402.4.1 in the CMC; use it for initial siting decisions. § 402.4.1.
The CMC also requires that intakes be designed in accordance with Section 402.4 (general design) and follow subsequent subsections that address rain, snow, bird screening, etc.

Exceptions & special cases

Use ASHRAE 62.1 Appendix B dispersion calculations instead of the table distances when you can demonstrate equivalent or better protection; the code explicitly permits that. § 402.4.1 (2).
The Table 402.4.1 minimum distances do not apply where the surface below the intake is sloped > 45° from horizontal or where the surface is less than 1 inch wide (for example, a narrow sloping metal coping). § 402.4.1 — Exception.
Fuel‑burning equipment clearances are governed by the applicable NFPA standards (NFPA 54/ANSI Z223.1, NFPA 31, NFPA 211) rather than Table 402.4.1 distances; see § 402.4.1.2.
Laboratory fume hood exhaust has its own separation criteria and must comply with ANSI/ASSP Z9.5 per § 402.4.1.4.
Other intake design factors required by the code: manage rain entrainment (methods listed in § 402.4.2), prevent rain intrusion into outdoor equipment (§ 402.4.3), and manage snow entrainment where applicable (§ 402.4.4) — these are separate but related design obligations.

Common mistakes

Assuming Table distances are the only option — you can use ASHRAE Appendix B dispersion calculations if documented, but many designers overlook this alternative. § 402.4.1 (2).
Measuring from the wrong point — the code requires the shortest distance from the intake opening to the contaminant source; measure horizontally/vertically as appropriate, not from equipment centerlines unless that is the shortest path. § 402.4.1.
Forgetting plumbing‑vent rules — if a plumbing vent is terminated ≥ 3 ft above the intake the required separation is only 3 ft; otherwise 10 ft applies. Table 402.4.1.
Ignoring roof and snow conditions — snow accumulation is explicitly treated as “surface directly below intake” at expected snow depth for the 1 ft clearance rule. § 402.4.1.3.
Treating exhaust outlets from other ventilation systems as irrelevant — separation for Class 2/3/4 exhausts applies between different systems, per § 402.4.1.1.

Worked example — rooftop intake near a cooling tower and plumbing vent

Scenario:

You have a rooftop outdoor air intake for an AHU and a cooling tower exhaust located on the same roof.
Horizontal distance from the intake opening to the cooling tower exhaust outlet = 20 ft.
A plumbing vent terminates 2 ft higher than the intake (i.e., less than 3 ft above the intake).

Apply the rules:

The code requires cooling tower exhaust to be at least 25 ft from the intake per Table 402.4.1. Your horizontal separation is 20 ft, which does not meet Table 402.4.1. § 402.4.1.
The plumbing vent is < 3 ft above the intake, so the required separation to that plumbing vent is 10 ft (Table 402.4.1). Measure the shortest distance — if that distance is less than 10 ft, it fails the table requirement. § 402.4.1.
Options to achieve compliance:
- Move the intake so it is at least 25 ft from the cooling‑tower exhaust and at least 10 ft from the plumbing vent (if measured shortest distance remains <3 ft vertical). (Simple and code‑direct.)
- Re‑route/terminate the plumbing vent so it is ≥ 3 ft above the intake — then the plumbing vent clearance becomes 3 ft (Table 402.4.1), which may allow the existing layout to pass. § 402.4.1.
- Use the ASHRAE 62.1 Appendix B dispersion calculation to demonstrate equivalent or better protection from the cooling tower plume at the existing 20 ft separation, and document the calculation and assumptions. If the Appendix B analysis shows acceptable concentrations at the intake, you may comply without moving the intake. § 402.4.1 (2).

Practical tip: document the chosen compliance path (table distances or Appendix B calculations), include plan views with measured shortest distances, and record any mitigations (vent termination height, filtered intake, hooding).

Related provisions (CMC)

§ 402.4 — General requirement that ventilation system outdoor air intakes be designed in accordance with § 402.4.1 through § 402.4.5.
§ 402.4.1 — Location and Table 402.4.1 separation criteria (primary controlling provision).
Table 402.4.1 — Air intake minimum separation distances (the numeric table this article relies on).
§ 402.4.1.1 — Separation for exhaust/relief outlets between ventilation systems (Class 2 & Class 3 rules).
§ 402.4.1.2 — Fuel‑burning appliance clearances reference (NFPA 54/31/211).
§ 402.4.1.3 — Roof/landscaped grade or other surface directly below intake (snow note).
§ 402.4.1.4 — Laboratory exhaust must comply with ANSI/ASSP Z9.5.
§ 402.4.2 – § 402.4.5 — Rain entrainment, rain intrusion, snow entrainment, bird screens and related intake protections/requirements.
§ 407.2.1 — Additional CMC guidance for some systems (example: certain minimum distances for intakes from exhaust outlets and equipment — consult where applicable).

Code references

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

CMC § 402.1. High relevance — show source text

Where:

Vbz = breathing zone outdoor airflow, per Table 402.1. Az = zone floor area, the net occupiable floor area of the ventilation zone. Hvs = vertical separation between the center of the top and bottom openings’ free operable area; in case of multiple horizontally spaced pairs of openings, use shortest distance encountered. As = openable area of smallest opening (top or bottom); in case of multiple horizontally spaced pairs of top-and-bottom openings, use aggregated areas. Al = openable area of largest opening (top or bottom); in case of multiple horizontally spaced pairs of top-and-bottom openings, use aggregated areas.

Volumetric airflow rates used to estimate required operable area are based on the following:
Dry-air density of 0.075 lbda/ft [3] (1.2 kgda/m [3] ) at a barometric pressure of 1 atm (101.3 kPa) and an air temperature of 70°F (21°C)
Temperature difference between indoors and outdoors of 1.8°F (1°C)
Gravity constant of 32.2 ft/s [2] (9.81m/s [2] )
Window discharge coefficient of 0.6

ical ventilation systems shall include controls, manual or automatic, that enable the fan system to operate wherever the spaces served are occupied. The system shall be designed to maintain minimum outdoor airflow as required by Section 403.0 under any load conditions.

402.4 Outdoor Air Intakes. Ventilation system outdoor air intakes shall be designed in accordance with Section 402.4.1 through Section 402.4.5. [ASHRAE 62.1:5.5]

402.4.1 Location. Outdoor air intakes (including openings that are required as part of a natural ventilation system) shall be located such that the shortest distance from the intake to any specific potential outdoor contaminant source listed in Table 402.4.1 shall be equal to or greater than the following:

(1) The separation distance in Table 402.4.1 or

(2) The calculation methods in ASHRAE 62.1 Normative Appendix B and shall comply with all other requirements of this section. [ASHRAE 62.1:5.5.1]

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VENTILATION AIR

TABLE 402.4.1

AIR INTAKE MINIMUM SEPARATION DISTANCE

[ASHRAE 62.1:TABLE 5-1]

OBJECT	MINIMUM DISTANCE (feet)
Class 2 air exhaust/relief outlet	10
Class 3 air exhaust/relief outlet	15
Class 4 air exhaust/relief outlet	30
Cooling tower exhaust	25
Cooling tower intake or basin	15
Driveway, street, or parking place	5
Garage entry, automobile loading area, or drive-in queue	15
Garbage storage/pick-up area,

CMC § 407.1 High relevance — show source text
407.1 General.

407.1.1 All supply-air, return air, and exhaust-air sys- tems shall be mechanically operated and such systems for areas listed in Table 4-A shall be operated continu- ously. Natural ventilation through windows or other openings such as louvers will be considered as supple- mental to the required mechanical ventilation systems.

Exceptions:

(1) Natural ventilation shall not be used in airborne infection isolation rooms and protective environment

rooms.

(2) Unoccupied turndown shall be permitted in accor- dance with Section 407.7.

407.1.2 Fans serving exhaust systems shall be located at the discharge end of the system. Ductwork within the building shall be under negative pressure. The ventilation rates shown in Table 4-A shall be considered as minimum acceptable rates and shall not be construed as precluding the use of higher ventilation rates if they are required to meet design conditions. 407.1.3 Services/Systems and Utilities. (Refer to Sec- tion 1224.4.1 of the California Building Code).

407.2 Outdoor Air Intakes and Exhaust Outlets.

407.2.1 Outdoor Air Intakes. Outdoor air intakes shall be located at least 25 feet (7.62 m) from exhaust outlets of ventilating systems, combustion equipment stacks, med- ical-surgical vacuum systems, cooling towers, and areas that may collect vehicular exhaust or other noxious fumes. Plumbing vents shall be located in relation to out- door air intakes per California Plumbing Code. The bot- tom of outdoor air intakes shall be located as high as practicable, but not less than 10 feet (3048 mm) above ground level. If installed above the roof, they shall be located 18 inches (457 mm) above roof level or 3 feet (914 mm) above a flat roof where heavy snowfall is anticipated. Outside air intakes located in a below grade areaway shall have the top of the areaway extend a minimum of 10 feet (3048 mm) above grade.

Exceptions:

(1) These dimensions may be reduced if it is demon- strated by the submission of details and calculations that location of intakes with respect to exhausts and

their orientation, or the use of special filters, pro- vides equal performance.

(2) The requirements regarding the bottom of outdoor air intakes and installation through the roof do not apply to skilled nursing facilities, intermediate-care facilities or nonsensitive areas in correctional treat-

ment centers.

407.2.2 Exhaust Outlets. Exhaust outlets shall be located a minimum of 10 feet (3048 mm) above adjoining grade and 10 feet (3048 mm) from doors, occupied areas, and operable windows.

Exception: Airborne infection isolation rooms shall comply with Section 414.1.
CMC § 407.2.2 High relevance — show source text
Plumbing vents shall be located in relation to out-_ door air intakes per California Plumbing Code. The bot- tom of outdoor air intakes shall be located as high as practicable, but not less than 10 feet (3048 mm) above ground level. If installed above the roof, they shall be located 18 inches (457 mm) above roof level or 3 feet (914 mm) above a flat roof where heavy snowfall is anticipated. Outside air intakes located in a below grade areaway shall have the top of the areaway extend a minimum of 10 feet (3048 mm) above grade.

Exceptions:

(1) These dimensions may be reduced if it is demon- strated by the submission of details and calculations that location of intakes with respect to exhausts and

their orientation, or the use of special filters, pro- vides equal performance.

(2) The requirements regarding the bottom of outdoor air intakes and installation through the roof do not apply to skilled nursing facilities, intermediate-care facilities or nonsensitive areas in correctional treat-

ment centers.

407.2.2 Exhaust Outlets. Exhaust outlets shall be located a minimum of 10 feet (3048 mm) above adjoining grade and 10 feet (3048 mm) from doors, occupied areas, and operable windows.

Exception: Airborne infection isolation rooms shall comply with Section 414.1.

407.2.2.1 Hazardous Exhaust Outlets. Haz- ardous exhaust outlets from airborne infection iso- lation rooms, bronchoscopy and sputum collection exhaust, hazardous drug compounding, morgues, autopsy rooms and laboratory chemical fume hoods shall discharge a minimum of 10 feet (3048 mm) above the adjacent roof surface and a minimum of 30 feet (9144.mm) from outdoor air intakes, build- ing openings and areas normally accessible to the public.

407.3 Air Balance.

407.3.1 The ventilation systems shall be designed and balanced to provide the general air balance relationship to adjacent areas, shown in Table 4-A. The ventilation systems shall be balanced in accordance with the latest edition of standards published by the Associated Air Bal- ance Council (AABC), the National Environmental Bal- ancing Bureau (NEBB), or the Testing, Adjusting and Balancing Bureau (TABB). Air balance tolerancing val- ues shall not result in noncompliance of the minimum required pressurization per Table 4-A.

407.4 Air Circulation.

407.4.1 Design of the ventilation system shall provide air movement that is generally from clean to less clean areas.

407.4.1.1 All air distribution devices and supply air outlets shall meet the requirements of ASHRAE 170, Section 6.7.2 and Table 6-2.

407.4.1.2 Supply outlets and return and exhaust air inlets shall be located to prevent short-circuiting.

CMC § 402.4.1 High relevance — show source text

402.4.1 Location. Outdoor air intakes (including openings that are required as part of a natural ventilation system) shall be located such that the shortest distance from the intake to any specific potential outdoor contaminant source listed in Table 402.4.1 shall be equal to or greater than the following:

(1) The separation distance in Table 402.4.1 or

(2) The calculation methods in ASHRAE 62.1 Normative Appendix B and shall comply with all other requirements of this section. [ASHRAE 62.1:5.5.1]

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VENTILATION AIR

TABLE 402.4.1

AIR INTAKE MINIMUM SEPARATION DISTANCE

[ASHRAE 62.1:TABLE 5-1]

OBJECT	MINIMUM DISTANCE (feet)
Class 2 air exhaust/relief outlet	10
Class 3 air exhaust/relief outlet	15
Class 4 air exhaust/relief outlet	30
Cooling tower exhaust	25
Cooling tower intake or basin	15
Driveway, street, or parking place	5
Garage entry, automobile loading area, or drive-in queue	15
Garbage storage/pick-up area, dumpsters	15
Plumbing vents terminating at least 3 feet above the level of the outdoor air intake	3
Plumbing vents terminating less than 3 feet above the level of the outdoor air intake	10
Roof, landscaped grade, or other surface directly below intake	1
Thoroughfare with high traffic volume	25
Truck loading area or dock, bus parking/idling area	25
Vents, chimneys, and flues from combustion appli- ances and equipment	15

For SI units: 1 foot = 304.8 mm

402.4.1.1 Exhaust/Relief Outlets. Separation criteria for Class 2 and Class 3 exhaust/relief outlets apply to the distance from the outdoor air intakes for one ventilation system to the exhaust and relief outlets for any other ventilation system. [ASHRAE 62.1:5.5.1.1]

402.4.1.2 Fuel-Burning Equipment. The minimum distances relative to fuel-fired appliances shall be as required by NFPA 54/ANSI Z223.1 for fuelgas-burning appliances and equipment, NFPA 31 for oil burning appliances and equipment, and NFPA 211 for other combustion appliances and equipment.

[ASHRAE 62.1:5.5.1.2]

402.4.1.3 Roof, Landscaped Grade, or Another Surface Directly Below Intake. Where snow accumulation is expected, the surface of the snow at the expected average snow depth shall be considered to be a surface directly below an intake.

CMC § 150.0 High relevance — show source text
Filters and heat/energy cores behind access panels, access doors, or grilles located not more than 10 feet above a walking surface inside a space specified above comply with this requirement. Exception to Section 150.0(o)1Civa: Systems that require servicing from inside the attic shall have the following:
1. A fault indicator display (FID) meeting the requirements of Reference Appendix JA17.
2. An attic access door located in a wall or, where attic access is provided through a ceiling, an attic access hatch that includes an integrated ladder.
3. A walkway from the attic access door to the HRV/ERV. b. IAQ system component accessibility: Fans, motors, heat exchangers, filters and recovery cores shall meet all applicable requirements of California Mechanical Code Section 304.0 accessibility for service. c. Outdoor air intake design: Outdoor air intakes shall comply with California Mechanical Code Section 402.4.1.
d. Outdoor air intake location and accessibility: To provide access for cleaning, outdoor air intakes shall be accessible. Air intakes located not more than 10 feet above a walking surface comply with this requirement. If located on roofs, they shall meet the requirements of California Mechanical Code Section 304.3.1.

Exception to Section 150.0(o)1Civd: Outdoor air intake serving equipment with an FID meeting the requirements of Reference Appendix JA17.

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SINGLE-FAMILY RESIDENTIAL BUILDINGS— MANDATORY FEATURES AND DEVICES

D. Air filtration. Air filtration shall conform to the specifications in Section 150.0(m)12. Compliance with ASHRAE 62.2 Sections 6.7 (Minimum Filtration) and 6.7.1 (Filter Pressure Drop) shall not be required.

E. Reserved.

F. Reserved.

G. Local mechanical exhaust. A local mechanical exhaust system shall be installed in each kitchen and bathroom. Systems shall be rated for airflow in accordance with ASHRAE 62.2 Section 7.1. i. Nonenclosed kitchens shall have a demand-controlled mechanical exhaust system meeting the requirements of Section 150.0(o)1Giii. ii. Enclosed kitchens and all bathrooms shall have either one of the following alternatives a or b: a. A demand-controlled mechanical exhaust system meeting the requirements of Section 150.0(o)1Giii. b. A continuous mechanical exhaust system meeting the requirements of Section 150.0(o)1Giv. iii. Demand-controlled mechanical exhaust. A local mechanical exhaust system shall be designed to be operated as needed.

a. Control and operation. Demand-controlled mechanical exhaust systems shall be provided with at least one of the following controls:
1. A readily accessible occupant-controlled ON-OFF control.
2. An automatic control that does not impede occupant ON control. b. Ventilation rate and capture efficiency. The system shall meet or exceed either the minimum airflow in accordance with Table 150.0-E or the minimum capture efficiency in accordance with Table 150.0-E, and Table 150.0-G. Capture efficiency ratings shall be determined in accordance with ASTM E3087 and listed in a product directory approved by the Energy Commission.
CMC § 35.68 High relevance — show source text
52|China Lake Naf|35.68|–117.68|California| |747020|0.50|Lemoore Reeves NAS|36.33|–119.95|California| |747185|0.46|Imperial|32.83|–115.58|California| |747187|0.46|Palm Springs Thermal AP|33.63|–116.17|California| |747188|0.48|Blythe Riverside Co Arpt|33.62|–114.72|California|

iii. Required Mechanical Ventilation Rate [ASHRAE 62.2:4.1.2] The Required Mechanical Ventilation Rate ( Q fan ) shall be calculated using Equation 150.0-F. (Equation 150.0-F) Q fan = Q tot – Φ ( Q inf × A ext )

where:

Q fan = required mechanical ventilation rate, cfm (L/s). Q tot = total required ventilation rate, cfm (L/s) from Equation 150.0-B. Q inf = effective annual average infiltration rate, cfm (L/s) from Equation 150.0-E. A ext = 1 for single-family detached homes, or the ratio of exterior envelope surface area that is not attached to garages or other dwelling units to total envelope surface area for attached dwelling units not sharing ceilings or floors with other dwelling units, occupiable spaces, public garages, or commercial spaces. Φ = 1 for balanced ventilation systems and Q inf /Q tot otherwise. iv. Requirements for balanced and supply-only ventilation systems. a. IAQ filter and HRV/ERV accessibility: System air filters and HRV/ERV heat/energy recovery cores shall be located such that they are accessible for service from within occupiable spaces, basements, garages, balconies, and mechanical closets. Filters and heat/energy cores behind access panels, access doors, or grilles located not more than 10 feet above a walking surface inside a space specified above comply with this requirement. Exception to Section 150.0(o)1Civa: Systems that require servicing from inside the attic shall have the following:
1. A fault indicator display (FID) meeting the requirements of Reference Appendix JA17.
2. An attic access door located in a wall or, where attic access is provided through a ceiling, an attic access hatch that includes an integrated ladder.
3. A walkway from the attic access door to the HRV/ERV. b. IAQ system component accessibility: Fans, motors, heat exchangers, filters and recovery cores shall meet all applicable requirements of California Mechanical Code Section 304.0 accessibility for service. c. Outdoor air intake design: Outdoor air intakes shall comply with California Mechanical Code Section 402.4.1.
d. Outdoor air intake location and accessibility: To provide access for cleaning, outdoor air intakes shall be accessible. Air intakes located not more than 10 feet above a walking surface comply with this requirement. If located on roofs, they shall meet the requirements of California Mechanical Code Section 304.3.1.

Exception to Section 150.0(o)1Civd: Outdoor air intake serving equipment with an FID meeting the requirements of Reference Appendix JA17.

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CMC § 405.4.1 High relevance — show source text
instructions.

405.4.1 Exhaust Rate. For intermittent-controlled operations, the exhaust rate shall be not less than 100 ft [3] /min (47.2 L/s) for range hoods or 300 ft [3] /min (142 L/s) for mechanical exhaust fans including downdraft appliances. For continuous operated ventilation, the exhaust rate shall be not less than 50 ft [3] /min (23.6 L/s). 405.5 Ventilation Openings. Occupiable spaces shall be provided with a readily accessible ventilation opening openable to the outdoors. The opening shall be not less than 5 square feet (0.464 m [2] ) or 4 percent of the occupied floor area. The openable area shall be based on free, unobstructed area through the opening.

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VENTILATION AIR

406.0 Evaporative Cooling System for Health Care Facilities. [OSHPD 1, 2, 3, 4 & 5] Direct evaporative cool- ing systems where the air directly contacts the wetted surface or spray shall be limited in health facilities to nonpatient areas such as laundry rooms, and boiler or machinery rooms. Similar rooms with high heating-producing equipment will be consid- ered when specifically approved by the enforcing agency. The evaporative pads shall be a synthetic type. Filters shall be required in accordance with Tables 4-B and 4-C except utility rooms, i.e.: boiler or machinery rooms.

407.0 Ventilation System Details. [OSHPD 1, 1R, 2, 3, 4 & 5]

407.1 General.

407.1.1 All supply-air, return air, and exhaust-air sys- tems shall be mechanically operated and such systems for areas listed in Table 4-A shall be operated continu- ously. Natural ventilation through windows or other openings such as louvers will be considered as supple- mental to the required mechanical ventilation systems.

Exceptions:

(1) Natural ventilation shall not be used in airborne infection isolation rooms and protective environment

rooms.

(2) Unoccupied turndown shall be permitted in accor- dance with Section 407.7.

407.1.2 Fans serving exhaust systems shall be located at the discharge end of the system. Ductwork within the building shall be under negative pressure. The ventilation rates shown in Table 4-A shall be considered as minimum acceptable rates and shall not be construed as precluding the use of higher ventilation rates if they are required to meet design conditions. 407.1.3 Services/Systems and Utilities. (Refer to Sec- tion 1224.4.1 of the California Building Code).

407.2 Outdoor Air Intakes and Exhaust Outlets.

407.2.1 Outdoor Air Intakes. Outdoor air intakes shall be located at least 25 feet (7.62 m) from exhaust outlets of ventilating systems, combustion equipment stacks, med- ical-surgical vacuum systems, cooling towers, and areas that may collect vehicular exhaust or other noxious _fumes.
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 § 2.1 High relevance — show source text
Exception: The glazed areas shall not be required where artificial light and a local exhaust system are provided. The minimum local exhaust rates shall be 50 cubic feet per minute (25 L/s) for intermittent ventilation or 20 cubic feet per minute (10 L/s) for contin- uous ventilation in accordance with the California Mechanical Code, Chapter 4 . Exhaust air from the space shall be exhausted directly to the outdoors.

R325.2.1 Bathroom exhaust fans. Each bathroom containing a bathtub, shower or tub/shower combination shall be mechanically ventilated for purposes of humidity control in accordance with the California Mechanical Code, Chapter 4; and the California Green Building Standards Code, Chapter 4, Division 4.5.

Note: Window operation is not a permissible method of providing bathroom exhaust for humidity control.

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BUILDING PLANNING

R325.3 Ventilation. Ventilation air rates shall be in compliance with the California Mechanical Code.

R325.4 Opening location. Outdoor intake and exhaust openings shall be located in accordance with Sections R325.4.1 and R325.4.2.

R325.4.1 Intake openings. Mechanical and gravity outdoor air intake openings shall be located not less than 10 feet (3048 mm) from any hazardous or noxious contaminant, such as vents, chimneys, plumbing vents, streets, alleys, parking lots and loading docks.

For the purpose of this section, the exhaust from dwelling unit toilet rooms, bathrooms and kitchens shall not be considered as hazardous or noxious.

Exceptions:
1. The 10-foot (3048 mm) separation is not required where the intake opening is located 3 feet (914 mm) or greater below the contaminant source.
2. Vents and chimneys serving fuel-burning appliances shall be terminated in accordance with the applicable provisions of the California Mechanical Code.
3. Clothes dryer exhaust ducts shall be terminated in accordance with the California Mechanical Code.
R325.4.2 Exhaust openings. Exhaust air shall not be directed onto walkways.

R325.5 Outside opening protection. Air exhaust and intake openings that terminate outdoors shall be protected with corrosionresistant screens, louvers or grilles having an opening size of not less than [1] / 4 inch (6 mm) and a maximum opening size of [1] / 2 inch (13 mm), in any dimension. Openings shall be protected against local weather conditions. Outdoor air exhaust and intake openings shall meet the provisions for exterior wall opening protectives in accordance with this code.

R325.6 Interior stairway illumination. Interior stairways shall be provided with an artificial light source to illuminate the landings and treads. The light source shall be capable of illuminating treads and landings to levels of not less than 1 footcandle (11 lux) as measured at the center of treads and landings. There shall be a wall switch at each floor level to control the light source where the stairway has six or more risers.

Exception: A switch is not required where remote, central or automatic control of lighting is provided.
CMC § 402.4.1 High relevance — show source text
Exception: The minimum separation distance in Table 402.4.1 shall not apply where outdoor surfaces below the air intake are sloped more than 45 degrees (0.79 rad) from horizontal or where such surfaces are less than 1 inch (25.4 mm) in width.

[ASHRAE 62.1:5.5.1.3]

402.4.1.4 Laboratory Exhaust. Separation criteria for fume hood exhaust shall be in compliance with ANSI/ASSP Z9.5. [ASHRAE 62.1:5.5.1.4]

402.4.2 Rain Entrainment. Outdoor air intakes that are part of the mechanical ventilation system shall be designed to manage rain entrainment in accordance with one or more of the following:

(1) Limit water penetration through the intake to 0.07 oz/(ft [2] - h) [21.5 g/(m [2] - h)] of inlet area when tested using the rain test apparatus described in UL 1995.

(2) Select louvers that limit water penetration to a maximum of 0.01 oz/ft [2] (3.05 g/m [2] ) of louver free area at the maximum free area intake velocity through the louver. This water penetration rate shall be determined when subjected to the water penetration test in AMCA 500-L or equivalent. Manage the water that penetrates the louver by providing a drainage area or moisture removal devices.

(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]
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.

72 2025 CALIFORNIA MECHANICAL CODE

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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 § 1.9 High relevance — show source text
1. Natural ventilation shall not be required in kitchens where a local exhaust system is installed in accordance with the California Mechanical Code .
2. Required ventilation openings shall be permitted to open into a thermally isolated sunroom or roofed porch, deck, or patio where not less than 40 percent of the roofed area perimeter is open to the outdoor air.
3. Required ventilation openings shall be permitted to open into a thermally isolated sunroom provided there is an openable area between the adjoining room and the sunroom of not less than one-tenth of the floor area of the interior room and not less than 20 square feet (1.9 m [2] ). The minimum openable area of the sunroom to outdoor air shall be based on the total floor area of the adjoining room and the sunroom. 5. The windows, doors, louvers and other approved closeable openings not required by Section R319 may open into a passive solar energy collector for ventilation required by this section. The area of ventilation openings to the outside of the passive solar energy collector shall be increased to compensate for the openings required by the interior space.
R325.1.3 Adjoining rooms. For the purpose of determining light and ventilation requirements, rooms shall be considered to be a portion of an adjoining room where not less than one-half of the area of the common wall is open and unobstructed and provides an opening of not less than one-tenth of the floor area of the interior room and not less than 25 square feet (2.3 m [2] ).

R325.2 Bathrooms. Bathrooms, water closet compartments and other similar rooms shall be provided with aggregate glazing area in windows of not less than 3 square feet (0.3 m [2] ), one-half of which shall be openable.

Exception: The glazed areas shall not be required where artificial light and a local exhaust system are provided. The minimum local exhaust rates shall be 50 cubic feet per minute (25 L/s) for intermittent ventilation or 20 cubic feet per minute (10 L/s) for contin- uous ventilation in accordance with the California Mechanical Code, Chapter 4 . Exhaust air from the space shall be exhausted directly to the outdoors.

R325.2.1 Bathroom exhaust fans. Each bathroom containing a bathtub, shower or tub/shower combination shall be mechanically ventilated for purposes of humidity control in accordance with the California Mechanical Code, Chapter 4; and the California Green Building Standards Code, Chapter 4, Division 4.5.

Note: Window operation is not a permissible method of providing bathroom exhaust for humidity control.

3-72 2025 CALIFORNIA RESIDENTIAL CODE

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

BUILDING PLANNING

R325.3 Ventilation. Ventilation air rates shall be in compliance with the California Mechanical Code.

R325.4 Opening location. Outdoor intake and exhaust openings shall be located in accordance with Sections R325.4.1 and R325.4.2.

R325.4.1 Intake openings. Mechanical and gravity outdoor air intake openings shall be located not less than 10 feet (3048 mm) from any hazardous or noxious contaminant, such as vents, chimneys, plumbing vents, streets, alleys, parking lots and loading docks.

For the purpose of this section, the exhaust from dwelling unit toilet rooms, bathrooms and kitchens shall not be considered as hazardous or noxious.

Frequently asked questions

Can I use a local filter or hood instead of meeting the table distances?

Yes — the code permits reduced distances if you demonstrate by details and calculations (or use special filters/orientation) that an alternate location provides equal or better performance; alternatively you can use the ASHRAE 62.1 Appendix B dispersion method. § 402.4.1.

Does Table 402.4.1 apply to intakes on the same building roof as the contaminant source?

Yes — the table distances apply regardless of location; measure the shortest distance from intake to the source. If snow accumulation is expected, treat the snow surface at expected average depth as the surface directly below the intake per § 402.4.1.3.

What if an exhaust from another ventilation system is close to my intake?

Separation criteria for exhaust/relief outlets (Class 2 and Class 3) apply between systems; check § 402.4.1.1 and the appropriate class distance in Table 402.4.1.

Are rooftop intake heights specified?

The code emphasizes horizontal (shortest) distances in Table 402.4.1; other CMC provisions and related codes (e.g., § 407.2.1 and plumbing code provisions) address minimum vertical terminations/locations for specific equipment — consult those sections for height rules.

Do I need bird screens and rain protection at the intake?

Yes. The CMC requires bird screening to block a 0.5‑inch probe and lists methods to control rain entrainment (see § 402.4.2 and § 402.4.5).

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