CMC · California Mechanical Code
What materials and construction are required for combustion air ducts?
Combustion air ducts must be built of galvanized steel (or an equivalent material) and installed so the duct serves only one space, ends in an unobstructed opening, is not screened in attics, does not slope downward for horizontal upper ducts, and has exterior intakes at least 12 inches above adjacent finished ground. These requirements are stated in California Mechanical Code **§ 701.11**.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
Combustion air ducts must be made of galvanized steel or a material with equivalent corrosion resistance, strength, and rigidity, and they must terminate and be routed so they provide free, unobstructed air to the appliance. These installation and siting requirements are stated in § 701.11 . Other construction restrictions (single-space service, attic termination rules, slope rules, etc.) are also listed in § 701.11 .
The single most important rule: combustion air ducts must be constructed of galvanized steel (or an equivalent material) and installed so the termination is unobstructed and serves a single space in accordance with § 701.11.
Requirements in detail
Material requirement
- Material: Galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity. This is mandatory for combustion air ducts. § 701.11(1) .
Routing and termination
- Terminate in an unobstructed space that allows free movement of combustion air to the appliance. § 701.11(2) .
- Ducts must serve a single space (do not use one combustion-air duct to serve multiple rooms). § 701.11(3) .
- Ducts shall not serve both upper and lower combustion-air openings where both openings are used; separation between ducts serving upper and lower openings must be maintained to the source. § 701.11(4) .
- When a combustion-air duct terminates in an attic, it must not be screened at the termination. § 701.11(5) .
- Horizontal upper combustion-air ducts shall not slope downward toward the source of combustion air (i.e., avoid downhill slope toward appliance). § 701.11(6) .
- The space remaining around a chimney liner, gas vent, special gas vent, or plastic piping inside a chimney shall not be used to supply combustion air. § 701.11(7) .
- Exterior combustion-air intake openings: the lowest side of the intake opening must be at least 12 inches (305 mm) vertically above the adjoining finished ground level. § 701.11(8) .
Decision-relevant dimensions / values (quick reference table)
| Decision factor | Requirement / value | Code Reference |
|---|---|---|
| Duct material | Galvanized steel or equivalent corrosion resistance/strength/rigidity | § 701.11(1) |
| Termination | Must terminate in an unobstructed space for free air movement | § 701.11(2) |
| Serving spaces | Single space only (one duct → one served space) | § 701.11(3) |
| Upper vs lower openings | Ducts must not serve both upper and lower openings where used | § 701.11(4) |
| Attic terminations | No screening where duct terminates in attic space | § 701.11(5) |
| Slope for horizontal upper ducts | Must not slope downward toward source | § 701.11(6) |
| Chimney voids | Remaining space around chimney liners not permitted as combustion-air supply | § 701.11(7) |
| Intake opening clearance | Lowest side ≥ 12 in (305 mm) above adjoining finished ground | § 701.11(8) |
| Dwelling-unit stud/joist spaces (exception) | Allowed to convey combustion air if no more than one fireblock is removed | Exception to § 701.11(1) |
Construction quality (where to look)
- The material choice and equivalent performance are declared in § 701.11(1), but construction practices (joints, sealing, flexible duct rules, fire characteristics, etc.) are covered elsewhere in the CMC (see Related provisions below). For example, duct joint/seal requirements and acceptable closure materials are addressed in Chapter 6 (e.g., § 603.9); metallic duct construction standards appear in § 602.4. See Related provisions for exact sections and references.
Exceptions & special cases
- Dwelling-unit framing: unobstructed stud and joist spaces within dwelling units are permitted to convey combustion air provided not more than one fireblock is removed (Exception to § 701.11(1)).
- The rule prohibiting use of chimney liner voids has an exception for direct-vent appliances designed for installation in a solid fuel-burning fireplace, but only when installed per the manufacturer’s instructions (Exception to § 701.11(7)).
- Note: § 701.11 sets the material and siting requirements but does not re-state general duct construction standards (sealing, pressure class, UL listings). Those provisions remain in Chapters 6 and 3 and must be followed where applicable (see Related provisions).
Common mistakes
- Using unprotected or non‑equivalent thin metal (not meeting the “equivalent corrosion resistance, strength, and rigidity” requirement) instead of galvanized steel. Refer to § 701.11(1).
- Routing a combustion-air duct to serve multiple rooms or both upper and lower openings (violates § 701.11(3) and § 701.11(4)).
- Terminating an attic duct with a screen — attic terminations must not be screened per § 701.11(5).
- Installing a horizontal upper duct that slopes down toward the appliance (violates § 701.11(6)).
- Assuming the space around a chimney liner is available for combustion air (prohibited by § 701.11(7)).
- Forgetting the 12 in (305 mm) minimum vertical clearance from finished ground for exterior intake openings (required by § 701.11(8)).
- Overlooking other CMC requirements that affect duct construction quality (e.g., sealing, closure systems, UL listings). See Related provisions for those requirements.
Worked example — concrete scenario
Scenario: You must supply outdoor combustion air to a single gas-fired water heater located in a garage. You choose to run a single horizontal duct from an exterior wall into the garage.
Steps and how § 701.11 applies:
- Material: Choose galvanized steel duct (meets § 701.11(1)).
- Termination: Place the duct termination in an unobstructed area inside the garage so air can move freely to the water heater; do not enclose the termination behind shelving or a grille that would block flow (§ 701.11(2)).
- Single-space service: Ensure the duct serves only the garage (one space) — do not branch it to serve another room (§ 701.11(3)).
- Upper/lower openings: Because only a single (lower) opening is used here, no conflict with the “not serve both upper and lower openings” rule exists; if you were using both upper and lower openings you must keep their ducts separated to the source (§ 701.11(4)).
- Slope: If this is a horizontal run to the garage and it is serving an upper opening, ensure it does not slope downward toward the source (applies to horizontal upper ducts) (§ 701.11(6)). If it’s a lower opening, slope rule for horizontal upper ducts does not apply in the same way, but good practice is to avoid trapping water or debris.
- Exterior intake elevation: Place the exterior intake so the lowest side is ≥ 12 in (305 mm) above the adjoining finished ground (§ 701.11(8)). Measure from the adjacent finished grade to the lowest side of the intake opening.
Result: A galvanized-steel duct, terminating unobstructed inside the garage, serving only that space, and with the exterior intake placed at least 12 in above finished ground, will satisfy the construction and siting rules in § 701.11 for this simple application.
Related provisions
- § 701.10 — Louvers, grilles, and screens (net free area; screen mesh sizes) — relevant when you use louvers or screens at intakes.
- § 701.9 — Mechanical combustion-air supply (rates, interlocks) — use when combustion air is provided mechanically rather than by passive ducts.
- § 603.9 — Joints and seams; closure systems and Seal Class A requirements — affects how ducts are joined and sealed.
- § 602.4 — Metallic duct construction standards (SMACNA/UL requirements) — applies to acceptable metal duct construction and referenced standards.
- § 603.1.3 — Protection/separation from earth for ducts (installation considerations).
(These related sections must be followed in addition to the specific combustion-air duct rules of § 701.11 where applicable.)
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 9.3.7.1 High relevance — show source text
Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
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COMBUSTION AIR
701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
(7) The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
CMC § 9.3.4 High relevance — show source text
[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
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COMBUSTION AIR
701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
CMC § 505.3. High relevance — show source text
The design of the hood or enclosure shall be such that air currents created by the exhaust systems will capture the contaminants and transport them directly to the exhaust duct. The volume of air shall be sufficient to dilute explosive or flammable vapors, fumes, or dusts in accordance with Section 505.3. Hoods of steel shall have a base metal thickness not less than 0.027 of an inch (0.686 mm) (No. 22 gauge) for Class 1 and Class 5 metal duct systems; 0.033 of an inch (0.838 mm) (No. 20 gauge) for hoods serving a Class 2 duct system; 0.044 of an inch (1.118 mm) (No. 18 gauge) for hoods serving a Class 3 duct system; and 0.068 of an inch (1.727 mm) (No. 14 gauge) for hoods serving a Class 4 duct system.
Approved nonmetallic hoods and duct systems shall be permitted to be used for Class 5 corrosive systems where the corrosive mixture is nonflammable. Metal hoods used with Class 5 duct systems shall be protected with an approved corrosion-resistant material. Edges of hoods shall be rounded. The minimum clearance between hoods and combustible construction shall be the clearance required by the duct system.
505.12 Pharmacies – Compounding Area of Par- enteral Solutions. [CA – Board of Pharmacy] The pharmacy shall have a designated area for the preparation of sterile products for dispensing which shall be ventilated in a manner not interfering with laminar air flow.
Note: For additional pharmacy building standard require- ments, see Chapter 12, California Building Code.
505.12.1 P harmacies – Laminar Flow Biological Safety Cabinet. [CA – Board of Pharmacy] In all pharmacies preparing parenteral cytotoxic agents, all com- pounding shall be conducted within a certified Class II Type A or Class II Type B vertical laminar airflow hood with bag in – bag out design. The pharmacy must ensure that con-
taminated air plenums that are under positive air pressure are leak tight. Note: For additional pharmacy building standard requirements, see Chapter 12, California Building Code.
506.0 Product-Conveying Ducts.
506.1 Materials. Materials used in product-conveying duct systems shall be suitable for the intended use and shall be of rigid sheet metal.
Exceptions:
(1) Asbestos-cement, concrete, clay, or ceramic materials shall be permitted to be used where it is shown that these materials will be equivalent to metal ducts installed in accordance with this chapter.
(2) Ducts serving a Class 5 system shall be permitted to be constructed of approved nonmetallic material where the corrosive characteristics of the material being conveyed make a metal system unsuitable and where the mixture being conveyed is nonflammable.
Approved nonmetallic material shall be either a listed product having a flame-spread index not exceeding 25 and a smoke-developed rating of 50 or less on both inside and outside surfaces without evidence of continued progressive combustion, or shall have a flame-spread index not exceeding 25 and shall be installed with an automatic fire-sprinkler protection system inside the duct.
CMC § 601.0 High relevance — show source text
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CHAPTER 6
DUCT SYSTEMS
601.0 General.
601.1 Applicability. Ducts and plenums that are portions of a heating, cooling, ventilation, or exhaust system shall comply with the requirements of this chapter, Chapter 5 for exhaust ducts, and Chapter 7 for combustion air ducts. 601.2 Sizing Requirements. Duct systems shall be sized in accordance with applicable standards in Chapter 18 or by other approved methods.
Exception: Residential duct systems shall be sized in accordance with ACCA Manual D, ACCA Manual Zr, as applicable, or by other approved methods.
601.2.1 California Energy Code Residential Return Duct Sizing Requirements. California Energy Code Tables 150.0-B and 150.0-C specify return duct sizing requirements for single return and multiple return duct systems, respectively, that are applicable as an alternative to confirming system airflow via field verification and diagnostic testing. See California Energy Code Section 150.0(m)13 for provisions applicable to newly con- structed buildings, and Section 150.2(b)(1)(F)(ii)(a) for alterations.
602.0 Material.
602.1 General. Materials used for duct systems shall comply with Section 602.2 through Section 602.7 as applicable. Exception: [OSHPD 1, 1R, 2, 3, 4 & 5] See Section 407.4.2.
[Not permitted for OSHPD 1, 1R, 2, 3, 4 & 5] Concealed building spaces or independent construction within buildings shall be permitted to be used as ducts or plenums. Gypsum board shall not be used for positive pressure ducts.
Exception: In healthcare facilities, concealed spaces shall not be permitted to be used as ducts or plenums.
602.2 Combustibles Within Ducts or Plenums. Materials exposed within ducts or plenums shall be noncombustible or shall have a flame spread index not to exceed 25 and a smoke-developed index not to exceed 50, where tested as a composite product in accordance with ASTM E84 or UL 723. Plastic piping installed in plenums shall be tested in accordance with all requirements of ASTM E84 or UL 723. Mounting methods, supports and sample sizes of materials for testing that are not specified in ASTM E84 or UL 723 shall be prohibited.
Exceptions:
(1) Return-air and outside-air ducts, plenums, or concealed spaces that serve a dwelling unit.
(2) Air filters in accordance with the requirements of Section 311.2.
(3) Water evaporation media in an evaporative cooler.
(4) Charcoal filters where protected with an approved fire suppression system.
(5) Products listed and labeled for installation within plenums in accordance with Section 602.2.1 through Section 602.2.4.
(6) Smoke detectors in accordance with the requirements of Section 609.0.
(7) Duct insulation, coverings, and linings and other supplementary materials installed in accordance with Section 605.0.
CMC § 2025 High relevance — show source text
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.
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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.
Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.
Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.
Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.
Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.
CMC § 602.2.4 High relevance — show source text
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.
602.5.2 Gypsum. Where gypsum products are exposed in ducts or plenums, the air temperature shall be restricted to a range from 50°F (10°C) to 125°F (52°C), and moisture content shall be controlled so that the material is not adversely affected. All gypsum products shall have a mold or mildew resistant surface. For the purpose of this section, gypsum products shall not be exposed in supply ducts.
602.5.3 Air Dispersion Systems. Air dispersion systems shall be listed and labeled in accordance with UL
602.5.4 Fibrous Glass Duct. Fibrous glass ducts, plenums, or fittings shall be constructed in accordance with SMACNA Fibrous Glass Duct Construction Stan dards or NAIMA Fibrous Glass Duct Construction Stan dards.
602.5.5 Other Materials. Flexible and rigid ducts, plenums, or fittings for use in heating, ventilation, and air conditioning systems of other nonmetallic materials listed and labeled to UL 181 shall be permitted.
Exception: Plastic ducts shall comply with Section 603.5.
602.6 Vibration Isolators. Vibration isolation connectors installed between mechanical equipment and metal ducts (or casings) shall be made of an approved material and shall not exceed 10 inches (254 mm) in length.
602.7 Corridors. Corridors shall not be used to convey air to or from rooms where the corridor is required to be of fire-resistive construction in accordance with the building code except where permitted by the building code. Corridors shall not serve as supply, return, exhaust, relief, or ventilation air ducts.
603.0 Installation of Ducts.
CMC § 602.5.2 High relevance — show source text
602.5.2 Gypsum. Where gypsum products are exposed in ducts or plenums, the air temperature shall be restricted to a range from 50°F (10°C) to 125°F (52°C), and moisture content shall be controlled so that the material is not adversely affected. All gypsum products shall have a mold or mildew resistant surface. For the purpose of this section, gypsum products shall not be exposed in supply ducts.
602.5.3 Air Dispersion Systems. Air dispersion systems shall be listed and labeled in accordance with UL
602.5.4 Fibrous Glass Duct. Fibrous glass ducts, plenums, or fittings shall be constructed in accordance with SMACNA Fibrous Glass Duct Construction Stan dards or NAIMA Fibrous Glass Duct Construction Stan dards.
602.5.5 Other Materials. Flexible and rigid ducts, plenums, or fittings for use in heating, ventilation, and air conditioning systems of other nonmetallic materials listed and labeled to UL 181 shall be permitted.
Exception: Plastic ducts shall comply with Section 603.5.
602.6 Vibration Isolators. Vibration isolation connectors installed between mechanical equipment and metal ducts (or casings) shall be made of an approved material and shall not exceed 10 inches (254 mm) in length.
602.7 Corridors. Corridors shall not be used to convey air to or from rooms where the corridor is required to be of fire-resistive construction in accordance with the building code except where permitted by the building code. Corridors shall not serve as supply, return, exhaust, relief, or ventilation air ducts.
603.0 Installation of Ducts.
603.1 General. Air ducts shall be installed in accordance with this chapter and the installation instructions.
603.1.1 Pressure Classification. The pressure classification of ducts shall be not less than the design operating pressure of the air distribution in which the duct is utilized. All ducts regardless of pressure classification(s) shall be sealed to Seal Class A.
603.1.2 Air Temperature. The temperature of the air to be conveyed in a duct shall not exceed 250°F (121°C).
603.1.3 Protection. Air ducts, other than plastic ducts, shall be installed with not less than 4 inches (102 mm) of
separation from earth, except where installed as a liner inside of concrete, tile, or metal pipe and shall be protected from physical damage.
603.1.4 Vertical Risers. Ducts listed and labeled to UL 181 shall not be used for vertical risers in air-duct systems serving more than two stories.
603.1.5 Penetrations. Ducts listed and labeled to UL 181 shall not penetrate a fire-resistance-rated assembly or construction.
603.1.6 Earthquake Loads. Ducts located in structures that are installed in areas classified as seismic design categories shall be restrained to resist displacement due to earthquake motion.
603.2 Under-Floor or Crawl Space. Air ducts installed under a floor in a crawl space shall be installed in accordance with the following:
(1) Shall not prevent access to an area of the crawl space.
(2) Where it is required to move under ducts for access to areas of the crawl space, a vertical clearance of not less than 18 inches (457 mm) shall be provided.
CMC § 701.6.1 High relevance — show source text
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COMBUSTION AIR
FIGURE 701.6.1(3) ALL COMBUSTION AIR FROM OUTDOORS
THROUGH HORIZONTAL DUCTS
[NFPA 54: FIGURE A.9.3.3.1(2)]
FIGURE 701.6.2
ALL COMBUSTION AIR FROM OUTDOORS THROUGH
SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
701.7.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 701.5. [NFPA 54:9.3.4(1)]
701.7.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 701.6. [NFPA 54:9.3.4(2)]
701.7.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 701.6, multiplied by the reduction factor. The minimum dimension of air openings shall not be less than 3 inches (76 mm).
[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening.
CMC § 1108.0 High relevance — show source text
Equipment and controls in. . . . . . . . . . . . . . . . . . 1108.0
Ventilation of . . . . . . . . . . . . . . . . . . . 1106.2.7, 1106.2.8
Where required . . . . . . . . . . . . . . . . . . . . . . . . . . 1106.0
MAKEUP AIR . . . . . . . . . . . . . . . . . . . . . . (see Air, Makeup)
MARKING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.1.1
MASONRY, CHIMNEYS . . . . . . . . . (see Chimney, masonry)
MATERIALS
Alternate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.2
For construction of
hoods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508.3
For piping, containers, and valves of
refrigeration systems . . . . . . . . . . . . . . . . . . . . 1109.1
For ducts . . . . . . . . . . . . . . . . . . . 506.1, 510.5.1, 602.0
For fuel piping. . . . . . . . . . . . . . . . . . . . 1308.0, D 109.0
For piping, tubing
and fittings hydronics . . . . . . . . . 1210.0, Table 1210.1
For vent connectors. . . . . . . . . . . 802.10.1.1–802.10.1.4
MECHANICAL
Combustion air supply. . . . . . . . . . . . . . . 701.9, E 502.6
Equipment Schedules [OSHPD] . . . . . . . . . . . . . 323.0
Exhaust system. . . . . . . . . . . . . . . . . . . . . . . . . . . 505.1
Ventilating system . . . . . . . . . . . 402.3, 1106.7, E 502.6,
E 605.1.2, Table E 502.6
MECHANICAL EXHAUSTING FOR
Bathrooms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E 605.2
Low-rise residential dwelling . . . . . . . . . . . . . . E 605.1.3
Product conveying. . . . . . . . . . . . . . . . . . . . . . . . . 505.0
Refrigeration . . . . . . . . . . . . . . . . . . . . 1105.5.2, 1106.0,
MECHANICAL JOINTS . . . . . . . . . . . . . . . . . . . . (see Joints
and connections)
CMC § 120.4 High relevance — show source text
Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).
Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.
(a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.
Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:
Outdoors; or
In a space between the roof and an insulated ceiling; or
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
2025 CALIFORNIA ENERGY CODE 77
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS
Portions of supply-air ducts that are not in one of these spaces, including ducts buried in concrete slab, shall be insulated to a minimum installed level of R-4.2 or be enclosed in directly conditioned space.
(b) Duct and plenum materials.
- Factory-fabricated duct systems. A. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. B. All pressure-sensitive tapes, heat-activated tapes, and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. C. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. D. All ductwork and plenums with pressure class ratings shall be constructed to Seal Class A. Joints and seams of duct systems and their components shall not be sealed with cloth-back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands.
Exception to Section 120.4(b)1D: Ductwork located in occupied space and exposed to view is not required to meet Seal Class A.
CMC § 703.1 High relevance — show source text
ii. At all locations where noninsulated portions of the duct system penetrate into unconditioned space, the penetration shall be draft stopped compliant with CFC Sections 703.1 and 704.1 and air-sealed to the construction materials that are penetrated, using materials compliant with CMC Section E502.4.2 to prevent air infiltration into the cavity. All connections in unconditioned space are insulated to a minimum of R-6.0 as confirmed by visual inspection. b. Ducts do not require insulation when the duct system is located entirely in conditioned space. For buildings with three or fewer habitable stories, duct systems located entirely in conditioned space shall be confirmed through field verification and diagnostic testing in accordance with the requirements of Reference Residential Appendix RA3.1.4.3.8. iii. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. iv. Openings shall be sealed with mastic, tape or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B or aerosol sealant that meets the requirements of UL 723. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used. v. Building cavities, support platforms for air handlers, and plenums designed or constructed with materials other than sealed sheet metal, duct board or flexible duct shall not be used for conveying conditioned air. Building cavities and support platforms may contain ducts. Ducts installed in cavities and support platforms shall not be compressed to cause reductions in the cross-sectional area of the ducts. Exception to Section 160.3(b)5A: Ducts and fans integral to a wood heater or fireplace. B. Factory-fabricated duct systems . i. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. ii. All pressure-sensitive tapes, heat-activated tapes and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. iii. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. iv. Joints and seams of duct systems and their components shall not be sealed with cloth back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands. C. Field-fabricated duct systems. i. Factory-made rigid fiberglass and flexible ducts for field-fabricated duct systems shall comply with UL 181. All pressure-sensitive tapes, mastics, aerosol sealants or other closure systems used for installing field-fabricated duct systems shall meet the applicable requirements of UL 181, UL 181A and UL 181B.
ii. Mastic sealants and mesh.
a. Sealants shall comply with the applicable requirements of UL 181, UL 181A and UL 181B, and be nontoxic and water resistant.
b. Sealants for interior applications shall be tested in accordance with ASTM C731 and D2202, incorporated herein by reference.
2025 CALIFORNIA ENERGY CODE 215
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
CMC § 8.5 High relevance — show source text
21_–0.27|75|0.75 inch|0.75 inch|1.0 inch|1.0 inch|1.0 inch| |Residential
40–60|0.21–0.27|75|R-6|R-5|R-7|R-6|R-5| |Nonresidential
40–60|0.21–0.27|75|0.5 inch|0.5 inch|1.0 inch|1.0 inch|1.0 inch| |Nonresidential
40–60|0.21–0.27|75|R-3|R-3|R-7|R-6|R-5| |Below 40|0.20–0.26|50|1.0 inch|1.5 inches|1.5 inches|1.5 inches|1.5 inches| |Below 40|0.20–0.26|50|R 8.5|R 14|R 12|R 10|R 9|Footnote to Table 120.3-A-1 and Table 120.3-A-2: These thicknesses are based on energy efficiency considerations only. Issues such as water vapor permeability or surface condensation sometimes require vapor retarders or additional insulation.
SECTION 120.4—REQUIREMENTS FOR AIR DISTRIBUTION SYSTEM DUCTS AND PLENUMS
Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).
Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.
(a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.
Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:
Outdoors; or
In a space between the roof and an insulated ceiling; or
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
2025 CALIFORNIA ENERGY CODE 77
Frequently asked questions
Can I use plastic or flexible duct for combustion air?
No — § 701.11(1) requires galvanized steel or a material of equivalent corrosion resistance, strength, and rigidity. If you propose an alternate material you must show equivalence.
May I place a screen on the exterior intake to keep out pests?
Screens are allowed generally (see § 701.10 for mesh size), but draping/screening is specifically prohibited where a combustion-air duct terminates in an attic per § 701.11(5). For exterior intakes, check § 701.10 for minimum screen mesh (1/4 in (6.4 mm)) and free-area calculations.
Can I use the space around a chimney liner to bring combustion air to an appliance?
No — § 701.11(7) prohibits using the remaining space surrounding a chimney liner or vent as a combustion-air supply, with limited exception for certain direct‑vent appliances installed in solid fuel fireplaces per the manufacturer’s instructions.
If my combustion air duct runs through the attic, does it need to be insulated or sealed a certain way?
§ 701.11 covers material and siting; insulation/sealing and closure systems are addressed elsewhere (Chapter 6). Refer to § 603.9 for sealing/closure requirements and to applicable energy-code/duct-insulation rules.
Is it acceptable to remove multiple fireblocks to run combustion air through stud/joist spaces?
No — the exception in § 701.11(1) permits using unobstructed stud/joist spaces only if not more than one fireblock is removed. Exceeding that would violate the exception conditions.
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