CMC · California Mechanical Code
Requirements for environmental (domestic) exhaust ducts
For a homeowner: bathroom, kitchen and dryer exhaust ducts must be metal and airtight, terminate outdoors with a backdraft damper, and dryers need at least 4" rigid metal duct, max typical length 14 ft (adjusted for extra elbows) — follow §§ 504.0–504.4 and the listed exceptions.
Last reviewed: July 6, 2026
What the code requires — plain English
Environmental (domestic) exhaust ducts must be constructed and installed per Chapter 6 and be airtight as approved by the Authority Having Jurisdiction — see § 504.0 and § 504.1. Exhaust systems for environmental air must be independent of other exhaust systems (unless specifically allowed) and must terminate outdoors with required backdraft protection and clearance distances — see § 504.2 and termination rules referenced in § 502.2.1. Specific rules apply to domestic cooking exhaust (materials, smooth interior, UL tape/seal) and to clothes dryer ducts (material, diameter, length, makeup air, termination) — see § 504.3 and § 504.4.
Most important rule: Exhaust ducts for environmental/domestic uses must terminate outdoors, be independent/airtight as required, and use the materials, sizes, and installations required in § 504.0 – § 504.4.
Requirements in detail
Scope / general construction
- If a specific requirement is not in Chapter 5, ducts are built in accordance with Chapter 6 (materials, gauges, construction) and must be airtight as approved by the AHJ — § 504.1.
- Environmental air ducts that also form part of an approved smoke-control system have special treatment per the code (see the referenced sections in Chapter 5).
Backdrafts, termination and independence
- Exhaust ducts shall terminate outside and be equipped with backdraft dampers or motorized dampers that shut when not in use, except where the exhaust fan is required to run continuously or other listed exceptions apply — § 504.1.1.
- Environmental exhaust systems must be independent of other exhaust systems; combined systems must operate at negative pressure and must meet termination requirements in § 502.2.1. Clothes dryer systems are generally independent except where § 504.4 provides allowances. § 504.2.
Domestic cooking exhaust (range hoods / downdrafts)
Fans and equipment must comply with listed standards (UL 507, UL 858, UL 923, etc.) as applicable. All domestic kitchen exhaust ducts used with range/cooktop hoods must be metal, have smooth interior surfaces, and be fastened/sealed with duct mastic or metal tapes meeting UL 181A. Hoods must discharge outdoors via single-wall duct and not terminate in attics or crawl spaces — § 504.3.
Exception: Under-slab domestic downdraft ducts may be approved Schedule 40 PVC when installed and backfilled per the listed conditions and terminate above grade with a backdraft damper — § 504.3 (exception).
Clothes dryer exhaust (domestic)
Key numeric limits and installation rules are stated in § 504.4 and its subsections. See table below for decision-relevant values.
| Item | Value / Limit | Code Reference |
|---|---|---|
| Minimum duct diameter for Type 1 domestic dryer | 4 in (102 mm) nominal | § 504.4.2. |
| Minimum duct material & construction for Type 1 dryer | Rigid metal, smooth interior, thickness not less than 0.016 in (0.406 mm) | § 504.4.2. |
| Maximum combined horizontal + vertical length (domestic dryers) | 14 ft (4,267 mm) inclusive of two 90° elbows; subtract 2 ft for each 90° elbow in excess of two | § 504.4.2.1. |
| Transition ducts (connection to appliance) | Listed transition ducts ≤ 6 ft (1,829 mm) permitted; must not be concealed; installed per manufacturer | § 504.4.2.2. |
| Dryer exhaust power ventilator | When listed (UL 705) and installed per instructions, longer ducts permitted per manufacturer's limits | § 504.4.2.3. |
| Makeup air for dryer in closet | 100 in² (0.065 m²) opening or other approved means when closet intended for dryer | § 504.4.1(1). |
| Makeup air for Type 2 dryers (commercial) | 1 in² per 1,000 Btu/h of total dryer input rating | § 504.4.1(2). |
| Termination and termination accessories | Ducts must terminate outdoors per § 502.2.1; screens are NOT permitted at dryer termination; backdraft damper required; do not terminate into attic/crawl/concealed spaces | § 504.4 and § 502.2.1. |
| Fastening & joining | Do not assemble dryer exhaust with screws that extend into the duct; join so the male end is inserted in direction of airflow | § 504.4. |
Duct leakage, airtightness and material rules
- Where Chapter 5 does not specify, Chapter 6 construction rules apply (material, thickness, installation). The ducts must be airtight as approved by the AHJ — § 504.1. For duct construction and material categories see Chapter 6 (e.g., metallic duct requirements, UL listings) — § 602.x references.
- Duct leakage testing and allowable leakage is addressed elsewhere (Chapter 6 / § 603.9.2); environmental ducts used in air distribution may be subject to testing and maximum leakage rules — see § 603.9.2 for permitted leakage and test procedures.
Exceptions & special cases
- Backdraft dampers are not required where the exhaust fan is required to operate continuously, or where the exhaust duct does not discharge into a common exhaust plenum and the space meets listed conditions (e.g., fan runs continuously, space not mechanically heated/cooled, or space is maintained at positive pressure) — § 504.1.1.
- Domestic kitchen downdraft ducts under concrete slabs: Schedule 40 PVC allowed if the trench is backfilled with sand/gravel, joints solvent-cemented, duct terminates above grade with backdraft damper, and limited PVC coupling protrusion — § 504.3 exception.
- Clothes dryer transition ducts may be used up to 6 ft but must not be concealed and must be listed per UL 2158A or installed to the dryer manufacturer’s instructions — § 504.4.2.2.
- Where a dryer exhaust power ventilator (UL 705 listed) is used, the dryer manufacturer’s instructions and the power-vent manufacturer instructions control allowable duct length — § 504.4.2.3.
- Termination distances from property lines, openings, public ways, and air intakes are defined in § 502.2.1 (environmental air) and must be observed when siting terminations.
If you need the full text of a specific exception or a subsection not quoted above (for example § 504.4.4), I do not have that subsection present in the retrieved excerpts — tell me and I will locate and cite it directly.
Common mistakes
- Installing a screen at a dryer termination (screens are prohibited for dryer outlets) — violates § 504.4.
- Using screws, rivets, or fasteners that project into the dryer duct and catch lint — prohibited by § 504.4.
- Using nonmetallic duct for kitchen range hood exhaust (except the narrow under-slab PVC exception) — kitchen exhaust ducts must be metal, smooth interior, sealed with UL 181A tape/mastic per § 504.3.
- Exceeding the dryer duct 14 ft limitation (without approved power ventilator or manufacturer allowance) or failing to deduct for extra elbows — see § 504.4.2.1.
- Combining environmental exhaust with other systems or failing to maintain negative pressure when combined — prohibited unless done per § 504.2.
- Concealing a listed transition duct (transition ducts ≤ 6 ft must not be concealed) — § 504.4.2.2.
Worked example — dryer duct math and checks
Scenario: A homeowner plans to install a Type 1 domestic dryer. The proposed run is 10 ft horizontal from the dryer to an exterior wall, plus 3 ft vertical rise to the termination, and the run includes three 90° elbows.
Step-by-step code check:
- Combined horizontal + vertical length = 10 ft + 3 ft = 13 ft. Compare to maximum allowed 14 ft per § 504.4.2.1.
- There are 3 elbows — that is 1 elbow in excess of two. Per § 504.4.2.1 subtract 2 ft for each elbow in excess of two: allowed maximum becomes 14 ft − 2 ft = 12 ft.
- The proposed combined length (13 ft) exceeds the adjusted allowed length (12 ft). Therefore the run is NOT code-compliant unless:
- the installer reduces duct length/elbows, or
- the installer uses an approved exhaust duct power ventilator (UL 705) and follows that manufacturer’s allowed duct length, or
- follows a dryer manufacturer's stated permitted installation (and is approved by the AHJ) — see § 504.4.2.3.
Other required checks for the example:
- Use rigid metal, smooth interior duct with thickness ≥ 0.016 in and diameter ≥ 4 in — § 504.4.2.
- Provide makeup air if the dryer is in a closet: 100 in² or equivalent per § 504.4.1(1).
- Terminate outdoors per § 502.2.1 with required clearances and include a backdraft damper at termination (no screen).
Related provisions
- Termination and exhaust opening protection — § 502.1 and § 502.2.1 (clearances, where exhaust may terminate)
- Duct construction and materials (Chapter 6) — see § 602.4 (metallic) and § 602.5 (nonmetallic) for material listings and UL requirements.
- Duct leakage testing and cross-contamination rules — § 603.9.2 (leakage testing limits) and § 603.10 (cross contamination / pressurization)
- Fan and motor requirements for exhaust systems (fire/explosion/corrosion considerations) — § 503.1 and § 503.2.
- Commercial kitchen exhaust — Part II (Type I / Type II) and specific termination rules: § 510.9 and § 519.5 for commercial systems referenced from Part I.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 504.0 High relevance — show source text
Equipment used to exhaust explosive or flammable vapors, fumes, or dusts shall bear an identification plate stating the ventilation rate for which the system was designed.
Fans located in systems conveying corrosives shall be of materials that are resistant to the corrosive or shall be coated
with corrosion-resistant materials.
504.0 Environmental Air Ducts.
504.1 General. Where not specified in this chapter, exhaust ducts shall be constructed and installed in accordance with
Chapter 6 and shall be airtight as approved by the Authority Having Jurisdiction. Environmental air ducts that have an alternate function as a part of an approved smoke control sys
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501.0 General.
501.1 Applicability. This chapter includes requirements for environmental air ducts, product-conveying systems, and commercial hoods and kitchen ventilation. Part I addresses environmental air ducts and product conveying systems. Part II addresses commercial hoods and kitchen ventilation.
502.0 Termination.
502.1 Exhaust Opening Protection. Exhaust openings terminating to the outdoors shall be covered with a corrosionresistant screen having not less than [1] ⁄ 4 of an inch (6.4 mm) openings, and shall have not more than [1] ⁄ 2 of an inch (12.7 mm) openings.
Exception: Clothes dryers.
502.2 Termination of Exhaust Ducts. Exhaust ducts shall terminate in accordance with Section 502.2.1 through Section 502.2.4. Classes of air shall be as defined in Section
203.0 and classified in Section 403.9.
502.2.1 Environmental, Class 1, and Class 2 Air Ducts. Environmental, Class 1, and Class 2 air duct exhaust shall terminate not less than 3 feet (914 mm) from a property line, 10 feet (3048 mm) above a public way, 3 feet (914 mm) from openings into the building and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1. The discharge of dryer exhaust ducts shall not terminate over a public way or over an area where condensate or vapor could create a nuisance or hazard.
502.2.2 Class 3 Air Ducts. Class 3 air duct exhaust shall terminate not less than 10 feet (3048 mm) from a property line, 3 feet (914 mm) from exterior walls or roofs that are in the direction of the exhaust discharge, 10 feet (3048 mm) from openings into the building, 10 feet (3048 mm) above adjoining grade, and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1.
CMC § 2025 High relevance — show source text
The building’s structural stability is protected by the regulations for cutting and notching of structural members.
Chapter 4 Ventilation Air. Chapter 4 regulates the minimum requirements for ventilation air supply, exhaust, and makeup air for occupiable spaces within a building. Building ventilation is one important factor affecting the relationship between airborne transmission of respiratory infections and the health and productivity of workers. Ventilation air may be composed of mechanical or natural ventilation, infiltration, recirculated air, transfer air, or a suitable combination of these. Providing a comfortable and healthy indoor environment for building occupants is of primary concern. When determining ventilation rates, typical and unusual significant sources of indoor pollution need to be controlled. Areas such as kitchens, bathrooms, and laundries are all built to allow specific functions. These spaces produce pollutants such as moisture, odors, volatile organic compounds, particles, or combustion byproducts. The purpose of local exhaust is to control concentrates of these pollutants in the room into which they were emitted and to reduce the spread of pollutants into other parts of the occupancy. Local exhaust ventilation is the source control for pollution that is expected in certain rooms. Using local exhaust to extract contaminants before they mix with the indoor environment is essential.
Chapter 5 Exhaust Systems. Chapter 5 regulates the minimum requirements for exhaust systems. Chapter 5 contains two parts: Part I provides exhaust requirements for environmental air ducts and product conveying systems; Part II provides exhaust requirements for commercial hoods and kitchen ventilation. Environmental air ducts include exhaust ducts used for transporting the air from domestic kitchens, bathrooms, shower rooms, locker rooms, janitor’s closets, and clothes dryers. Systems that carry nonabrasive exhaust, such as smoke, moderate abrasives such as sawdust, and high abrasives such as manganese or acid vapors use product-conveying ducts. Part II provides the minimum fire safety requirements related to the design, installation, inspection, and maintenance of grease-type operations, such as cooking, for both fuel-gas and solid fuel. Cooking produces a significant amount of smoke, fumes, vapors, heat, and other pollutants. Therefore, acceptable kitchen ventilation is necessary to prevent fires from the build-up of grease and to provide the occupants protection from smoke, unpleasant odors, pollutants, and dangerous gases. There are two types of exhaust hoods (Type I and Type II) used in commercial kitchen applications.
xiv 2025 CALIFORNIA MECHANICAL CODE
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FORMAT OF THE UNIFORM MECHANICAL CODE
Type I hoods are intended to be installed above equipment or appliances that generate grease or smoke. Type II hoods are intended to be installed above equipment or appliances that generate steam, vapor, heat, or odors and do not generate grease or smoke.
Chapter 6 Duct Systems. Chapter 6 regulates requirements for ducts and plenums that are portions of a heating, cooling, ventilation, or exhaust system. This chapter contains material and installation requirements for metal, gypsum, factory-made, flexible, and plastic ducts. It also contains provisions for duct insulation, dampers, fire protection, and automatic shutoff for the building’s air distribution system. It is worth noting that exhaust ducts are regulated by Chapter 5, and combustion air ducts are regulated by Chapter 7.
CMC § 502.2 High relevance — show source text
Exception: Clothes dryers.
502.2 Termination of Exhaust Ducts. Exhaust ducts shall terminate in accordance with Section 502.2.1 through Section 502.2.4. Classes of air shall be as defined in Section
203.0 and classified in Section 403.9.
502.2.1 Environmental, Class 1, and Class 2 Air Ducts. Environmental, Class 1, and Class 2 air duct exhaust shall terminate not less than 3 feet (914 mm) from a property line, 10 feet (3048 mm) above a public way, 3 feet (914 mm) from openings into the building and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1. The discharge of dryer exhaust ducts shall not terminate over a public way or over an area where condensate or vapor could create a nuisance or hazard.
502.2.2 Class 3 Air Ducts. Class 3 air duct exhaust shall terminate not less than 10 feet (3048 mm) from a property line, 3 feet (914 mm) from exterior walls or roofs that are in the direction of the exhaust discharge, 10 feet (3048 mm) from openings into the building, 10 feet (3048 mm) above adjoining grade, and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1.
502.2.3 Product Conveying, Flammable, and Class 4 Air Ducts. Ducts conveying Class 4 air or explosive or flammable vapors, fumes, or dusts shall terminate not less than 30 feet (9144 mm) from a property line, 10 feet (3048 mm) from openings into the building, 6 feet (1829 mm) from exterior walls or roofs that are in the direction of the exhaust discharge, 30 feet (9144 mm) from combustible walls or openings into the building that are in the direction of the exhaust discharge, 10 feet (3048 mm) above adjoining grade, and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1.
Exception: Type I Hood exhaust termination shall be in accordance with Section 510.9.
2025 CALIFORNIA MECHANICAL CODE 99
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EXHAUST SYSTEMS
tem in accordance with Section 505.7.3 do not require design as Class 1 product-conveying ducts.
Exceptions:
(1) Ductless range hoods where installed in accordance with the manufacturer’s installation instructions.
(2) Condensing clothes dryers where installed in accordance with the manufacturer’s installation instructions.
504.1.1 Backdraft Protection. Exhaust ducts shall terminate outside the building and shall be equipped with backdraft dampers or with motorized dampers that automatically shut where the systems or spaces served are not in use. [OSHPD 1, 1R, 2, 4 & 5] Exception: Back- draft dampers are not required when the exhaust fan must operate continuously.
Exception: Where the exhaust duct does not discharge into a common exhaust plenum and one of the following:
(1) The exhaust fan runs continuously.
CMC § 510.9. High relevance — show source text
Exception: Type I Hood exhaust termination shall be in accordance with Section 510.9.
2025 CALIFORNIA MECHANICAL CODE 99
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EXHAUST SYSTEMS
tem in accordance with Section 505.7.3 do not require design as Class 1 product-conveying ducts.
Exceptions:
(1) Ductless range hoods where installed in accordance with the manufacturer’s installation instructions.
(2) Condensing clothes dryers where installed in accordance with the manufacturer’s installation instructions.
504.1.1 Backdraft Protection. Exhaust ducts shall terminate outside the building and shall be equipped with backdraft dampers or with motorized dampers that automatically shut where the systems or spaces served are not in use. [OSHPD 1, 1R, 2, 4 & 5] Exception: Back- draft dampers are not required when the exhaust fan must operate continuously.
Exception: Where the exhaust duct does not discharge into a common exhaust plenum and one of the following:
(1) The exhaust fan runs continuously.
(2) The exhaust duct serves space(s) that are not mechanically heated or cooled.
(3) The space served is maintained at positive pressure. 504.2 Independent Exhaust Systems. Single or combined mechanical exhaust systems for environmental air shall be independent of other exhaust systems. Combined exhaust systems shall operate at negative pressure and shall terminate in accordance with Section 502.2.1. Clothes dryer exhaust systems shall be independent of all other exhaust systems except where permitted in Section 504.4.4. 504.3 Domestic Cooking Exhaust Equipment. Domestic cooking exhaust equipment shall comply with the following requirements, as applicable:
(1) The fan for overhead range hoods and downdraft exhaust equipment not integral with the cooking appliance shall comply with UL 507.
(2) Overhead range hoods and downdraft exhaust equipment with integral fans shall comply with UL 507.
(3) Domestic cooking appliances with integral downdraft exhaust equipment shall comply with UL 858 or CSA/ANSI Z21.1/CSA 1.1.
(4) Microwave ovens with integral exhaust for installation over the cooking surface shall comply with UL 923.
(5) All domestic kitchen exhaust ducts used in conjunction with domestic range or cooktop hoods shall be constructed of metal and shall have smooth interior surfaces, fastened and sealed with duct mastic or metal tapes that meet the requirements of UL 181A.
Exception: Ducts for domestic kitchen downdraft grillrange ventilation installed under a concrete slab floor shall be permitted to be of approved Schedule 40 PVC provided the following conditions are met:
(1) The under-floor trench in which the duct is installed shall be completely backfilled with sand or gravel.
(2) Not more than 1 inch (25.4 mm) of 6 inch diameter (150 mm) PVC coupling shall be permitted to protrude above the concrete floor surface.
(3) PVC pipe joints shall be solvent cemented to provide an air and greasetight duct.
(4) The duct shall terminate above grade outside the building and shall be equipped with a backdraft damper.
(6) Range hoods shall discharge to the outdoors through a single wall duct and shall not terminate in an attic or crawl space.
CMC § 0.50 Medium relevance — show source text
(8) Materials in a hazardous fabrication area including the areas above and below the fabrication area sharing a common air recirculation path with the fabrication area.
(9) Plastic water distribution piping listed and labeled for use in plenums in accordance with UL 2846 as having a peak optical density not greater than 0.50, an average optical density not greater than 0.15, and a flame spread distance not greater than 5 feet (1524 mm), and installed in accordance with its listing, shall be permitted.
602.2.1 Electrical. Electrical wiring in plenums shall comply with the California Electrical Code . Electrical wires and cables and optical fiber cables shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15, and a peak optical density not exceeding 0.5, where tested in accordance with NFPA 262.
602.2.2 Fire Sprinkler Piping. Nonmetallic fire sprinkler piping in plenums shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15 and, a peak optical density not exceeding 0.5, where tested in accordance with UL 1887.
602.2.3 Pneumatic Tubing. Nonmetallic pneumatic tubing in plenums shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15, and a peak optical density not exceeding 0.5, where tested in accordance with UL 1820.
602.2.4 Discrete Products in Plenums. Discrete plumbing, mechanical, and electrical products that are located in a plenum and have exposed combustible material shall be listed and labeled in accordance with UL
602.3 Tall Wood (Mass Timber) Buildings. Duct systems installed in tall wood (mass timber) buildings shall comply with the following:
(1) Be designed by a registered design professional in accordance with this code and the building code.
(2) Be designed to accommodate expansion, contraction, and differential movement between parts of a tall wood (mass timber) building in accordance with Section 316.2.
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DUCT SYSTEMS
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602.4 Metallic. Ducts, plenums, or fittings of metal shall comply with SMACNA HVAC Duct Construction Standards – Metal and Flexible. Flexible metallic ducts shall comply with UL 181.
602.5 Nonmetallic Ducts. Nonmetallic ducts shall comply with Section 602.5.1, Section 602.5.2, Section 602.5.3, Section 602.5.4, or Section 602.5.5.
602.5.1 Phenolic. Phenolic duct, plenum, or fitting material shall comply with UL 181. Ducts, plenums, or fittings of phenolic shall be constructed in accordance with SMACNA Phenolic Duct Construction Standards or the conditions of its listing.
CMC § 402.1 Medium relevance — show source text
Table 4-B Filter Efficiencies for Central
Ventilation and Air-Conditioning Systems In General Acute Care Hospitals, Outpatient Facilities,
Licensed Clinics and Acute
Psychiatric Hospitals . . . . . . . . . . . 91
Table 4-C Filter Efficiencies for Central
Ventilation and Air-Conditioning Systems In Skilled Nursing Facilities, Intermediate Care
Facilities and Correctional
Treatment Centers . . . . . . . . . . . . . 91
Table 402.1 Minimum Ventilation Rates in
Breathing Zone . . . . . . . . . . . . . . . . 92
Table 403.2.2 Zone Air Distribution Effectiveness. . 95
Table 403.7 Minimum Exhaust Rates. . . . . . . . . 96
CHAPTER 5 EXHAUST SYSTEMS . . . . . . . . . . 97
501.0 General. . . . . . . . . . . . . . . . . . . . . . 99
501.1 Applicability. . . . . . . . . . . . . . . . . . . 99
502.0 Termination. . . . . . . . . . . . . . . . . . . 99
502.1 Exhaust Opening Protection. . . . . . 99
502.2 Termination of Exhaust Ducts . . . . 99
2025 CALIFORNIA MECHANICAL CODE
Part I Environmental Air Ducts and
Product-Conveying Systems . . . . 99
503.0 Motors, Fans, and Filters . . . . . . . . 99
503.1 General. . . . . . . . . . . . . . . . . . . . . . 99
503.2 Fans . . . . . . . . . . . . . . . . . . . . . . . . 99
504.0 Environmental Air Ducts . . . . . . . . . 99
504.1 General. . . . . . . . . . . . . . . . . . . . . . 99
504.2 Independent Exhaust Systems. . . 100
504.3 Domestic Cooking Exhaust Equipment . . . . . . . . . . . . . . . . . . 100
504.4 Clothes Dryers . . . . . . . . . . . . . . . 100
504.5 Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) Systems . . . . . . 101
505.0 Product-Conveying Systems . . . . 101
505.1 General. . . . . . . . . . . . . . . . . . . . . 101
505.2 Incompatible Materials . . . . . . . . . 102
505.3 Flammability Limit. . . . . . . . . . . . . 102
505.4 Air-Moving Devices. . . . . . . . . . . . 102
505.5 Generating Flames, Sparks, or Hot Materials . . . . . . . . . . . . . . 102
505.6 Fire Dampers . . . . . . . . . . . . . . . . 102
505.7 Fire Detection, Smoke Control and Alarm Systems . . . . . . . . . . . . 102
CMC § 503.1 Medium relevance — show source text
503.1 General. . . . . . . . . . . . . . . . . . . . . . 99
503.2 Fans . . . . . . . . . . . . . . . . . . . . . . . . 99
504.0 Environmental Air Ducts . . . . . . . . . 99
504.1 General. . . . . . . . . . . . . . . . . . . . . . 99
504.2 Independent Exhaust Systems. . . 100
504.3 Domestic Cooking Exhaust Equipment . . . . . . . . . . . . . . . . . . 100
504.4 Clothes Dryers . . . . . . . . . . . . . . . 100
504.5 Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) Systems . . . . . . 101
505.0 Product-Conveying Systems . . . . 101
505.1 General. . . . . . . . . . . . . . . . . . . . . 101
505.2 Incompatible Materials . . . . . . . . . 102
505.3 Flammability Limit. . . . . . . . . . . . . 102
505.4 Air-Moving Devices. . . . . . . . . . . . 102
505.5 Generating Flames, Sparks, or Hot Materials . . . . . . . . . . . . . . 102
505.6 Fire Dampers . . . . . . . . . . . . . . . . 102
505.7 Fire Detection, Smoke Control and Alarm Systems . . . . . . . . . . . . 102
505.8 Product-Conveying Duct Classifications. . . . . . . . . . . . . . . . 102
505.9 Minimum Velocities and
Circulation. . . . . . . . . . . . . . . . . . . 102
505.10 Makeup Air . . . . . . . . . . . . . . . . . . 102
505.11 Hoods and Enclosures . . . . . . . . . 103
505.12 Pharmacies – Compounding Area of Parenteral Solutions . . . . 103
506.0 Product-Conveying Ducts. . . . . . . 103
506.1 Materials . . . . . . . . . . . . . . . . . . . . 103
Table 505.9 Range of Minimum Duct Design Velocities. . . . . . . . . . . . . . . . . . . . 103
Table 506.2(1) Minimum Sheet Metal Thickness for Round Ducts . . . . . . . . . . . . . . 104
506.2 Construction . . . . . . . . . . . . . . . . . 105
Table 506.2(2) Minimum Sheet Metal Thickness for Rectangular Ducts. . . . . . . . . . 106
506.3 Penetrations . . . . . . . . . . . . . . . . . 106
506.4 Condensate . . . . . . . . . . . . . . . . . 107
506.5 Fittings . . . . . . . . . . . . . . . . . . . . . 107
California Mechanical Code Medium relevance — show source text
A mechanical code, as with any other code, is intended to be adopted as a legally enforceable document to safeguard health, safety, property, and public welfare. The code cannot be effective without satisfactory provisions for its administration and enforcement. The Authority Having Jurisdiction is to review the proposed and completed work and decide whether a mechanical system conforms to the code requirements. As a public servant, the Authority Having Jurisdiction enforces the code in an unbiased proper manner. The design professional is responsible for the design of a safe mechanical system. The contractor is responsible for installing the system in accordance with the plans.
Chapter 2 Definitions. To maintain consistency and encourage the use of common terminology, Chapter 2 establishes definitions to provide clarity of terms and promote the use of a common language throughout the code. Understanding definitions within the context of their application enables greater collaboration, efficiency, standardization, and interpretation in applying and enforcing terms used throughout the code. Codes are technical documents, and every term can impact the meaning of the code text. Terms not defined have a normally accepted meaning.
Chapter 3 General Regulations. Chapter 3 regulates the general requirements, not specific to other chapters, for installing mechanical systems. Many regulations are not specific mechanical requirements but relate to the overall mechanical system. This chapter contains safety requirements for appliance location and installation, appliance and system access, condensate disposal, clearances to combustibles, and return or outside air used in mechanical systems. Listing method of approval, based on applicable nationally recognized standards for the safe and proper installation of mechanical systems, is essential to ensure protection of public health, safety, and welfare. These safety requirements provide protection for piping, tubing, materials, and structures, including provisions for installation practices addressing stress and strain, sleeving, and hanger support. The building’s structural stability is protected by the regulations for cutting and notching of structural members.
Chapter 4 Ventilation Air. Chapter 4 regulates the minimum requirements for ventilation air supply, exhaust, and makeup air for occupiable spaces within a building. Building ventilation is one important factor affecting the relationship between airborne transmission of respiratory infections and the health and productivity of workers. Ventilation air may be composed of mechanical or natural ventilation, infiltration, recirculated air, transfer air, or a suitable combination of these. Providing a comfortable and healthy indoor environment for building occupants is of primary concern. When determining ventilation rates, typical and unusual significant sources of indoor pollution need to be controlled. Areas such as kitchens, bathrooms, and laundries are all built to allow specific functions. These spaces produce pollutants such as moisture, odors, volatile organic compounds, particles, or combustion byproducts. The purpose of local exhaust is to control concentrates of these pollutants in the room into which they were emitted and to reduce the spread of pollutants into other parts of the occupancy. Local exhaust ventilation is the source control for pollution that is expected in certain rooms. Using local exhaust to extract contaminants before they mix with the indoor environment is essential.
Chapter 5 Exhaust Systems. Chapter 5 regulates the minimum requirements for exhaust systems. Chapter 5 contains two parts: Part I provides exhaust requirements for environmental air ducts and product conveying systems; Part II provides exhaust requirements for commercial hoods and kitchen ventilation. Environmental air ducts include exhaust ducts used for transporting the air from domestic kitchens, bathrooms, shower rooms, lo
CMC § 1208.2 Medium relevance — show source text
1208.2 Health Care (see Institutional I-1 and Institutional I-2) Ambulatory care facilities 422 Clinic, outpatient 304.1 Hospitals 308.3 Health-hazard Materials 307.2, Table 414.2.5.1, 415.2
Heat Vents 910 Heating (see Mechanical) 101.4.2 Aircraft hangars 412.3.4 Fire pump rooms 913.3 Fireplace 2111 Masonry heaters 2112 Parking garages 406.2.9 Repair garages 406.2.9 Height, Building 503, 504, 505, 508, 510 Limitations 503 Mixed construction types 510 Modifications 504 Roof structures 504.3, 1511 Helical Pile 1810.3.1.5, Table 1810.3.2.6, 1810.3.5.3.5, 1810.4.11 Heliport Live loads 1607.6 Higher Education Laboratories 428 High-Piled Combustible Storage 413, 907.2.16, 910.2.2 High-Pressure Decorative Exterior-Grade Compact Laminates 1408 High-Rise Buildings 403 Alarms and detection 403.4.1, 403.4.2,
907.2.14
Application 403.1 Construction 403.2
Elevators 403.6, 1009.2.1, 3007, 3008 Emergency power 403.4.8, 2702.2 Emergency systems 403.4 Fire command station 403.4.6 Fire department communication 403.4.4, 403.4.5 Fire service elevators 403.6.1, 3007 Occupant evacuation elevators 403.6.2, 3008 Smoke removal 403.4.7 Smokeproof enclosure 403.5.4, 1023.12 Sprayed fire-resistant materials (SFRM) 403.2.3
Sprinkler protection 403.3, 903.2.11.3 Stairways 403.5, 1023, 1025 Standby power 403.4.8, 2702.2 Structural integrity 403.2.2, 1616 Super high-rise (over 420 feet) 403.2.1, 403.2.2, 403.2.3, 403.3.1, 403.5.2 Voice alarm 403.4.4, 907.2.14 Zones 907.6.3, 907.6.4 Historic Buildings 101.4.7 Flood provisions G106.3
Horizontal Assembly 711 Continuity 509.4.1, 711.2.2, 711.2.3, 713.11, 713.12 Fire-resistance rating 603.1, 703, 704.3.2, 707.3.10, 711.2.4 Glazing, rated 716.3.4 Group I-1 420.3 Group R 420.3 Incidental 509.4
CMC § 502.2.4 Medium relevance — show source text
98 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
CHAPTER 5
EXHAUST SYSTEMS
502.2.4 Commercial Kitchen Ducts. Commercial
kitchens exhaust ducts shall terminate in accordance with
Section 510.9 for Type I exhaust systems or Section 519.5 for Type II exhaust systems.
Part I – Environmental Air Ducts and Product-Conveying Systems.
503.0 Motors, Fans, and Filters.
503.1 General. Motors and fans shall be sized to provide the required air movement. Motors in areas that contain flammable vapors or dusts shall be of a type approved for such environments. A manually operated remote control installed at an approved location shall be provided to shut off fans or blowers in flammable vapor or dust systems. Equipment used in operations that generate explosive or flammable vapors, fumes, or dusts shall be interlocked with the ventilation system so that the equipment cannot be operated unless the ventilation fans are in operation. Motors for fans used to convey flammable vapors or dusts shall be located outside the duct or shall be protected with approved shields and dustproofing. Where belts are used, they shall not enter the duct unless the belt and pulley within the duct are enclosed. Motors and fans shall be accessible for servicing and maintenance.
503.2 Fans. Parts of fans in contact with explosive or flammable vapors, fumes, or dusts shall be of nonferrous or nonsparking materials, or their casing shall be lined or constructed of such material. Where the size and hardness of
materials passing through a fan are capable of producing a spark, both the fan, and the casing shall be of nonsparking materials. Where fans are required to be spark-resistant, their bearings shall not be within the airstream, and parts of the fan shall be grounded. Fans in systems handling materials that are likely to clog the blades, and fans in buffing or woodworking exhaust systems, shall be of the radial-blade or tube-axial type.
Equipment used to exhaust explosive or flammable vapors, fumes, or dusts shall bear an identification plate stating the ventilation rate for which the system was designed.
Fans located in systems conveying corrosives shall be of materials that are resistant to the corrosive or shall be coated
with corrosion-resistant materials.
504.0 Environmental Air Ducts.
504.1 General. Where not specified in this chapter, exhaust ducts shall be constructed and installed in accordance with
Chapter 6 and shall be airtight as approved by the Authority Having Jurisdiction. Environmental air ducts that have an alternate function as a part of an approved smoke control sys
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501.0 General.
501.1 Applicability. This chapter includes requirements for environmental air ducts, product-conveying systems, and commercial hoods and kitchen ventilation. Part I addresses environmental air ducts and product conveying systems. Part II addresses commercial hoods and kitchen ventilation.
502.0 Termination.
502.1 Exhaust Opening Protection. Exhaust openings terminating to the outdoors shall be covered with a corrosionresistant screen having not less than [1] ⁄ 4 of an inch (6.4 mm) openings, and shall have not more than [1] ⁄ 2 of an inch (12.7 mm) openings.
CMC § 709.4.2 Medium relevance — show source text
Requirements 709.4.2, 1009.6 Signage 1009.10, 1009.11, 1009.9, 1013.4, 1143A, 11B-216, 11B-703 Two-way communication 1009.6.5, 1009.8, 1026.6 Where required 1009.2, 1009.3.3, 1009.4.2 Assembly Occupancy (Group A) 303, 1030 Accessibility Chapters 11A and 11B Alarms and detection 907.2.1
Area 503, 506, 507, 508 Bleachers (see Bleachers) Folding and telescopic seating (see Bleachers) General 303.1 Grandstands (see Grandstands) Group-specific provisions A-1 303.2
A-2 303.3
A-3 303.4
A-4 303.5
Motion picture theater 409, 507.12 Special amusement areas 411 Stages and platforms 410 Height 503, 504, 505, 508, 510 Incidental uses 509 Interior finishes Table 803.13, 804 Live load Table 1607.1 Means of egress Aisles 1018.2, 1030.9, 1030.10,
1030.11
INDEX-2 2025 CALIFORNIA BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
INDEX
Limitations on authority 113.2 Membership of board B101.3 Notice of meeting B101.5 Qualifications 113.3, B101.3.1 Boiler Room
Exits 1006.2.2.1
Bolts
Anchor rods 1901.3 Bonding, Masonry 1805.2.2, 2103.2.2, 2113.3.1, 2510.7 Bottle-Filling Stations 202 Braced Wall Line 202
Bracing 2308.10 Seismic requirements 2308.10.10.2, 2308.10.6.2, 2308.10.8 Sill anchorage 2308.10.7.3 Spacing 2308.10.1 Support 2308.10.8 Temporary 3103.5 Braced Wall Panel 202 Alternative bracing 2308.10.5.1, 2308.10.5.2
Connections 2308.10.7 Length 2308.10.4 Location 2308.10.2
Method 2308.10.3 Brick (see Masonry) Building Area (see Area, Building) 502.1, 503, 505, 506, 507, 508, 510 Demolition 3303
Existing 101.4.7 Fire walls 706.1 Height (see Height, Building) 502.1, 503, 504, 505, 508, 510 Occupancy classification Chapter 3 Party walls 706.1.1 Relocatable 3113
Replicable Appendix N Building, Existing 202 Building Department 103 Building Official Duties and powers 103, 104 Qualifications Appendix A Records 104.7
CMC § 0.65 Medium relevance — show source text
Duct leakage tests 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. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:
Lmax = CLP [0.65 ] (Equation 603.9.2)
Where:
Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface
area.
CL = six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).
P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).
Exception: Transfer air duct operating at less than 1 inch of water column (0.25 kPa). 603.9.2.1 Duct Leakage Tests for Buildings that Meet Air Distribution System Duct Leak- age Sealing Criteria in Title 24, Part 6. For duct leakage testing, see California Energy Code Sec- tions 150.0(m)(11) for single family buildings, Sec- tion 160.3(b) for multifamily buildings, and Section 120.4(g) for nonresidential and Hotel Motel build- ings. 603.10 Cross Contamination. Exhaust ducts that convey Class 4 air shall be negatively pressurized relative to ducts, plenums, or occupiable spaces through which the ducts pass. Exhaust ducts under positive pressure that convey Class 2 or Class 3 air shall not extend into or pass through ducts, plenums, or occupiable spaces other than the space from which the exhaust air is drawn.
603.11 Underground Installation. Ducts installed underground shall be approved for the installation and shall have a slope of not less than [1] ⁄ 8 inch per foot (10.4 mm/m) back to the main riser. Ducts, plenums, and fittings shall be permitted to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab, provided the joints are sealed and duct is secured in accordance with SMACNA
HVAC Duct Construction Standards – Metal and Flexible.
Metal ducts where installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured in accordance with SMACNA HVAC Duct Con struction Standards – Metal and Flexible.
Frequently asked questions
Can I use flexible foil duct for my dryer?
Flexible listed transition ducts up to 6 ft are permitted only as the connection between the dryer and the exhaust duct and must not be concealed; the main dryer exhaust must be rigid metal per § 504.4.2.2 and § 504.4.2.
Are backdraft dampers always required at terminations?
Yes — exhaust ducts shall terminate outside and be equipped with backdraft dampers unless an exception applies (for example, where the fan must operate continuously or other listed conditions) — § 504.1.1.
Can I route a kitchen range-hood duct into the attic?
No — domestic range hoods must discharge to the outdoors through a single-wall duct and shall not terminate in an attic or crawl space — § 504.3.
What if my dryer run is longer than 14 ft?
You must shorten the run or reduce elbows, or use an approved dryer exhaust duct power ventilator (UL 705 listed) with the manufacturer’s allowed duct length, or follow dryer manufacturer instructions approved by the AHJ — § 504.4.2.1 and § 504.4.2.3.
Are screens allowed at dryer terminations?
No — screens are not permitted at clothes dryer terminations; the code specifically prohibits screens at dryer terminations and requires a backdraft damper and outdoor termination per § 504.4 and § 502.2.1.
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