CMC · California Mechanical Code
What products are allowed inside plenums and what listing is required?
If something with plastic or other combustible parts will sit exposed inside an HVAC plenum, California Mechanical Code requires it be specifically tested and listed for plenum use (most discrete devices must be UL 2043‑listed). General materials in plenums must also meet flame and smoke limits or be noncombustible; special tests and listings apply to cables, sprinkler pipe, pneumatic tubing and some plastic piping. Always check the product’s listing and the specific Code section that applies.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
In plain English: products with exposed combustible parts are generally not allowed in plenums unless specifically tested and listed for plenum use. The Code requires that discrete plumbing, mechanical and electrical products located in a plenum with exposed combustible material be listed and labeled in accordance with UL 2043 (see § 602.2.4). More broadly, materials exposed within ducts or plenums must be noncombustible or meet the flame- and smoke‑performance limits in § 602.2.
The single most important rule: if a part of your equipment will be exposed inside an air plenum and it’s combustible, it must be a product specifically tested and listed for plenums (UL 2043) or otherwise meet the specific plenum-test limits in the Code.
Requirements in detail
Key definitions (first mention)
- Plenum — a space used to convey conditioned air as part of an HVAC system (see Chapter 6 applicability).
- Listed and labeled for use in plenums — product evaluation by a recognized laboratory with a listing specifically for plenum installation (see § 602.2.4).
Decision table — what to use in a plenum
| Product category | Allowed inside plenum? | Listing / test required | Key numeric thresholds / limits | Code Reference |
|---|---|---|---|---|
| General exposed building materials (walls, ceilings, finishes exposed to plenum air) | Only if noncombustible or meet flame/smoke limits | ASTM E84 or UL 723 composite test | Flame spread index ≤ 25, Smoke-developed index ≤ 50 | § 602.2 |
| Discrete plumbing, mechanical, electrical products with exposed combustible material (examples: junction boxes, small devices with plastic housings) | Permitted only if listed for use in plenums | UL 2043 listing and label | Listing (UL 2043) — listing required (no numeric values in § 602.2.4 itself) | § 602.2.4 |
| Electrical wires/cables and optical fiber cables | Permitted only if listed and labeled for plenum use | NFPA 262 test (plenum) | Flame spread distance ≤ 5 ft (1524 mm); average optical density ≤ 0.15; peak optical density ≤ 0.5 | § 602.2.1 |
| Nonmetallic fire‑sprinkler piping | Permitted when listed for plenum use | UL 1887 | Flame/optical limits per UL 1887 (see § 602.2.2) | § 602.2.2 |
| Nonmetallic pneumatic tubing | Permitted when listed for plenum use | UL 1820 | Flame/optical limits per UL 1820 (see § 602.2.3) | § 602.2.3 |
| Plastic water‑distribution piping (special exception) | Permitted if listed for plenums | UL 2846 | Peak optical density ≤ 0.50, average optical density ≤ 0.15, flame spread distance ≤ 5 ft; installed per listing | Exception in § 602.2 |
| Factory‑fabricated ducts / duct closure systems | Allowed when listed | UL 181 (ducts and closure systems) | See UL 181 requirements | Chapter 6 references / energy code adoption (UL 181) |
| Air dispersion systems | Allowed only as listed | UL 2518 | Listed and labeled for use as installed | § 602.5.3 |
Notes: The table lists the Code-required listing/test for each product category. Where the Code specifies numeric performance limits, those are shown; where the Code requires a listing standard (e.g., UL 2043), the listing is the compliance requirement.
What § 602.2.4 actually says
- § 602.2.4 requires that discrete plumbing, mechanical, and electrical products located in a plenum that have exposed combustible material shall be listed and labeled in accordance with UL 2043. This is the controlling sentence for “discrete products.”
What § 602.3 covers (when it controls)
- § 602.3 addresses duct systems in tall wood (mass timber) buildings. It does not change the UL‑listing requirements for plenums; instead it requires that ducts in tall wood buildings be designed by a registered design professional and accommodate building movement (see § 602.3 (1)–(2)). Use § 602.3 when the project is a tall wood building.
Exceptions & special cases
- Return‑air and outside‑air ducts, plenums, or concealed spaces that serve a dwelling unit are excepted from some requirements in § 602.2. Check the exception text before assuming full plenum restrictions apply.
- Air filters, evaporative cooler media, charcoal filters (with approved suppression), duct insulation installed per Chapter 6/Section 605.0, smoke detectors, and products listed per § 602.2.1–602.2.4 are explicitly excepted or regulated separately by § 602.2 exceptions. Review the exception list in § 602.2.
- A special allowance (Exception 9 under § 602.2) permits certain plastic water‑distribution piping if it is listed to UL 2846 and meets the stated optical/flame criteria and installation requirements.
- In tall wood (mass timber) buildings, use § 602.3 to determine additional design obligations; it does not relieve listing requirements in § 602.2.4.
Common mistakes
- Assuming “plenum‑rated” is optional: if a product will be exposed inside the plenum and is combustible, it must be listed for plenum use (UL 2043 or other specific standard) — not merely labeled “low smoke” or “low flame” without the proper plenum listing. See § 602.2.4.
- Installing ordinary nonmetallic cables in plenums without verifying NFPA 262 plenum rating (flame/optical limits). The Code requires plenum‑listed cables (see § 602.2.1).
- Using gypsum or other building materials exposed in supply plenums without observing temperature/moisture limits — gypsum is restricted for supply ducts and has temperature/moisture conditions in § 602.5.2.
- Confusing duct material listings (UL 181) with discrete product listings (UL 2043). Ducts/closures and discrete devices have different listing standards; follow the specific § references.
Worked example — concrete scenario with numbers
Scenario: You are specifying installation of an electrical junction box assembly that will be mounted and exposed inside an office building return plenum.
Step 1 — Determine classification: the assembly has a plastic housing (combustible) and will be exposed to plenum air, so it is a discrete product with exposed combustible material. The Code point: § 602.2.4 requires listing to UL 2043.
Step 2 — Verify listing: obtain the manufacturer’s label or data sheet showing the product is listed and labeled for use in plenums to UL 2043. Without the UL 2043 listing the product is not permitted in the plenum. Outcome: If UL 2043 listing present → permitted (assuming installation per listing). If not → remove or relocate product outside plenum.
Alternate numeric example (cable run): You need to run nonmetallic power/data cable through the same plenum. Per § 602.2.1, the cable must be listed for plenum use and meet the NFPA 262 test criteria: flame spread distance ≤ 5 ft (1524 mm), average optical density ≤ 0.15, peak optical density ≤ 0.5. If the cable’s listing shows compliance with NFPA 262/“plenum-rated cable,” it may be installed; if it’s riser-rated only, it is not permitted in the plenum.
Related provisions
- § 602.2 — Combustibles within ducts or plenums (general flame/smoke limits and exceptions).
- § 602.2.1 — Electrical wiring and cables in plenums; NFPA 262 testing and numeric optical/flame limits.
- § 602.2.2 — Nonmetallic fire sprinkler piping listing (UL 1887) and limits.
- § 602.2.3 — Pneumatic tubing listing (UL 1820) and limits.
- § 602.2.4 — Discrete products in plenums; UL 2043 listing requirement.
- § 602.3 — Duct systems for tall wood (mass timber) buildings — design and movement accommodation.
- § 602.5.3 — Air dispersion systems listed to UL 2518.
- UL/industry standards referenced by Chapter 6 (example: UL 181, UL 2043, NFPA 262) — see the relevant sections above and the California Energy Code adoption notes for additional material/labeling expectations.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 602.2 High relevance — show source text
[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.
(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.
CMC § 0.50 High 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.
2025 CALIFORNIA MECHANICAL CODE 137
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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 § 1305.2.6 High relevance — show source text
TABLE 1305.2.6(2)—CONSTRUCTION-TYPE FACTOR Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 FACTOR TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION TYPE OF CONSTRUCTION FACTOR IA IB IIA IIB IIIA IIIB IV VA VB 1.2 1.5 2.2 3.5 2.5 3.5 2.3 3.3 7 1305.2.6.1 Vertical opening formula. The following formula shall be used in computing vertical opening value.
Equation 13-6 VO = PV × CF
where:
VO = Vertical opening value. The calculated value shall not be greater than positive 2.0.
PV = Protection value from Table 1305.2.6(1).
CF = Construction-type factor from Table 1305.2.6(2).
1305.2.7 HVAC systems. Evaluate the ability of the HVAC system to resist the movement of smoke and fire beyond the point of origin. Under the categories in Section 1305.2.7.1, determine the appropriate value and enter that value into Table 1306.1 under Safety Parameter 1305.2.7, HVAC Systems, for fire safety, means of egress and general safety. Facilities in Group I-2 occupancies meeting Category a, b or c shall be considered to fail the evaluation.
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1305.2.7.1 Categories. The categories for HVAC systems are:
Category a—Plenums not in accordance with Section 602 of the International Mechanical Code . -10 points.
Category b—Air movement in egress elements not in accordance with Section 1020.6 of the International Building Code . -5 points.
Category c—Both Categories a and b are applicable. -15 points.
Category d—Compliance of the HVAC system with Section 1020.6 of the International Building Code and Section 602 of the International Mechanical Code . 0 points.
Category e—Systems serving one story; or a central boiler/chiller system without ductwork connecting two or more stories or where systems have no ductwork. +5 points.
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.
2025 CALIFORNIA MECHANICAL CODE 137
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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 § 13-7 High relevance — show source text
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1305.2.7.1 Categories. The categories for HVAC systems are:
Category a—Plenums not in accordance with Section 602 of the International Mechanical Code . -10 points.
Category b—Air movement in egress elements not in accordance with Section 1020.6 of the International Building Code . -5 points.
Category c—Both Categories a and b are applicable. -15 points.
Category d—Compliance of the HVAC system with Section 1020.6 of the International Building Code and Section 602 of the International Mechanical Code . 0 points.
Category e—Systems serving one story; or a central boiler/chiller system without ductwork connecting two or more stories or where systems have no ductwork. +5 points.
1305.2.8 Automatic fire detection. Evaluate the smoke detection capability based on the location and operation of automatic fire detectors in accordance with the International Mechanical Code and Section 907 of the International Building Code. Under the categories and occupancies in Table 1305.2.8, determine the appropriate value and enter that value into Table 1306.1 under Safety Parameter 1305.2.8, Automatic Fire Detection, for fire safety, means of egress and general safety. Facilities in Group I-2 occupancies meeting Category a, b or c shall be considered to fail the evaluation.
TABLE 1305.2.8—AUTOMATIC FIRE DETECTION VALUES Col2 Col3 Col4 Col5 Col6 Col7 OCCUPANCY CATEGORIES CATEGORIES CATEGORIES CATEGORIES CATEGORIES CATEGORIES OCCUPANCY a b c d e f A-1, A-3, F, M, R, S-1 -10 -5 0 2 6 NA A-2 -25 -5 0 5 9 NA A-4, B, E, S-2 -4 -2 0 4 8 NA I-2 NP NP NP 4 5 2 NA = Not Applicable.
NP = Not Permitted.NA = Not Applicable.
NP = Not Permitted.NA = Not Applicable.
NP = Not Permitted.NA = Not Applicable.
NP = Not Permitted.NA = Not Applicable.
NP = Not Permitted.NA = Not Applicable.
NP = Not Permitted.NA = Not Applicable.
NP = Not Permitted.1305.2.8.1 Categories. The categories for automatic fire detection are:
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 § 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.
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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 § 1.0 High relevance — show source text
D. The outdoor design temperatures for cooling shall be no greater than the 1.0 percent Cooling Dry Bulb and Mean Coincident Wet Bulb values.
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MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
- Outdoor condensing units . A. Clearances. Installed air conditioner and heat pump outdoor condensing units shall have a clearance of at least 5 feet (1.5 meters) from the outlet of any dryer vent. B. Liquid line drier. Installed air conditioner and heat pump systems shall be equipped with liquid line filter driers if required, as specified by manufacturer’s instructions.
- Central forced-air heating furnaces . A. Temperature rise . Central forced-air heating furnace installations shall be configured to operate in conformance with the furnace manufacturer's maximum inlet-to-outlet temperature rise specifications.
- Air-distribution and ventilation system ducts, plenums and fans. A. CMC compliance.
i. All air-distribution system ducts and plenums, including, but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0 and ANSI/SMACNA006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. ii. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection a or b below:
a. Ducts shall have a minimum installed level of R-6.0, or
Exception to Section 160.3(b)5Aiia : Portions of the duct system located in conditioned space below the ceiling separating the occupiable space from the attic are not required to be insulated if all of the following conditions are met: i. The noninsulated portion of the duct system is located entirely inside the building’s thermal envelope as confirmed by visual inspection. 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.
CMC § 1305.2.6 High relevance — show source text
PV = Protection value from Table 1305.2.6(1).
CF = Construction-type factor from Table 1305.2.6(2).
1305.2.7 HVAC systems. Evaluate the ability of the HVAC system to resist the movement of smoke and fire beyond the point of origin. Under the categories in Section 1305.2.7.1, determine the appropriate value and enter that value into Table 1306.1 under Safety Parameter 1305.2.7, HVAC Systems, for fire safety, means of egress and general safety. Facilities in Group I-2 occupancies meeting Category a, b or c shall be considered to fail the evaluation.
2025 CALIFORNIA EXISTING BUILDING CODE 13-7
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
PERFORMANCE COMPLIANCE METHODS
1305.2.7.1 Categories. The categories for HVAC systems are:
Category a—Plenums not in accordance with Section 602 of the International Mechanical Code . -10 points.
Category b—Air movement in egress elements not in accordance with Section 1020.6 of the International Building Code . -5 points.
Category c—Both Categories a and b are applicable. -15 points.
Category d—Compliance of the HVAC system with Section 1020.6 of the International Building Code and Section 602 of the International Mechanical Code . 0 points.
Category e—Systems serving one story; or a central boiler/chiller system without ductwork connecting two or more stories or where systems have no ductwork. +5 points.
1305.2.8 Automatic fire detection. Evaluate the smoke detection capability based on the location and operation of automatic fire detectors in accordance with the International Mechanical Code and Section 907 of the International Building Code. Under the categories and occupancies in Table 1305.2.8, determine the appropriate value and enter that value into Table 1306.1 under Safety Parameter 1305.2.8, Automatic Fire Detection, for fire safety, means of egress and general safety. Facilities in Group I-2 occupancies meeting Category a, b or c shall be considered to fail the evaluation.
CMC § 160.3 Medium relevance — show source text
Exception 2 to Section 160.3(c)1: Piping that conveys fluids with a design operating temperature range between 60°F and 105°F.
Exception 3 to Section 160.3(c)1: Where the heat gain or heat loss to or from piping without insulation will not increase building source energy use. Exception 4 to Section 160.3(c)1: Piping that penetrates framing members shall not be required to have pipe insulation for the distance of the framing penetration. Metal piping that penetrates metal framing shall use grommets, plugs, wrapping or other insulating material to ensure that no contact is made with the metal framing. 2. Requirements for air distribution system, ducts and plenum. Multifamily common areas shall comply with the applicable requirements of Sections 160.3(c)2A through 160.3(c)2F. 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 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. B. 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:
i. Outdoors; or ii. In a space between the roof and an insulated ceiling; or iii. In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or iv. In an unconditioned crawl space; or v. In other unconditioned spaces. 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. C. Duct and plenum materials. i. 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. 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.
CMC § 1.11.0. Medium relevance — show source text
Adopting Agency BSC BSC-
CGSFM HCD Col6 Col7 DSA Col9 Col10 OSHPD Col12 Col13 Col14 Col15 Col16 Col17 BSCC DPH AGR DWR CEC CA SL SLC Adopting Agency BSC BSC-
CGSFM 1 2 1-AC AC ** SS** ** SS/CC** 1 1R 2 3 4 5 6 6 6 6 6 6 6 6 6 Adopt Entire Chapter X X X X X X X X X X X X Adopt Entire Chapter as
amended (amended sections
listed below)Adopt only those sections
that are listed belowChapter/Section This state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.0.
2025 CALIFORNIA MECHANICAL CODE 151
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
152 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 8
CHIMNEYS AND VENTS
801.0 General.
801.1 Applicability. The requirements of this chapter shall govern the venting of fuel-burning appliances.
801.2 Venting of Gas Appliances. Low-heat and medium-heat gas appliances shall be vented in accordance with this chapter. Other gas appliances shall be vented in accordance with NFPA 211 or other applicable standards.
801.3 Appliances Fueled by Other Fuels. Appliances fueled by fuels other than gas shall be vented in accordance with NFPA 211 and the appliance manufacturer’s instructions.
802.0 Venting of Appliances.
802.1 Listing. Type B and Type B-W gas vents shall comply with UL 441, Type L gas vents shall comply with UL 641.
802.1.1 Installation. Listed chimneys and vents shall be installed in accordance with this chapter and the manufacturer’s installation instructions. [NFPA 54:12.2.1]
CMC § 104.1 Medium relevance — show source text
[A] 104.1 General. The building official is hereby authorized and directed to enforce the provisions of this code.
[A] 104.2 Determination of compliance. The building official shall have the authority to determine compliance with this code, to render interpretations of this code and to adopt policies and procedures in order to clarify the application of its provisions. Such interpretations, policies and procedures:
- Shall be in compliance with the intent and purpose of this code.
- Shall not have the effect of waiving requirements specifically provided for in this code.
[A] 104.2.1 Listed compliance. Where this code or a referenced standard requires equipment, materials, products or services to be listed and a listing standard is specified, the listing shall be based on the specified standard. Where a listing standard is not specified, the listing shall be based on an approved listing criteria. Listings shall be germane to the provision requiring the listing. Installation shall be in accordance with the listing and the manufacturer’s instructions, and where required to verify compliance, the listing standard and manufacturer’s instructions shall be made available to the building official.
[A] 104.2.2 Technical assistance. To determine compliance with this code, the building official is authorized to determine compliance with this code, to require the owner or owner’s authorized agent to provide a technical opinion and report.
[A] 104.2.2.1 Cost. A technical opinion and report shall be provided without charge to the jurisdiction.
[A] 104.2.2.2 Preparer qualifications. The technical opinion and report shall be prepared by a qualified engineer, specialist, laboratory or specialty organization acceptable to the building official. The building official is authorized to require design submittals to be prepared by, and bear the stamp of, a registered design professional.
[A] 104.2.2.3 Content. The technical opinion and report shall analyze the properties of the design, operation or use of the building or premises and the facilities and appurtenances situated thereon to identify and propose necessary recommendations.
[A] 104.2.2.4 Tests. Where there is insufficient evidence of compliance with the provisions of this code, the building official shall have the authority to require tests as evidence of compliance. Test methods shall be as specified in this code or by other recognized test standards. In the absence of recognized test standards, the building official shall approve the testing procedures. Such tests shall be performed by a party acceptable to the building official.
[A] 104.2.3 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved.
Exception: Performance-based alternative materials, designs or methods of construction and equipment complying with the International Code Council Performance Code . This exception shall not apply to alternative structural materials or to alternative structural designs. [DSA-SS, DSA-SS/CC] The International Code Council Performance Code is not permitted by DSA.
[DSA-SS, DSA-SS/CC & OSHPD 1, 1R, 2, 4 & 5] Alternative system shall satisfy ASCE 7 Section 1.3, unless more restrictive require- ments are established by this code for an equivalent system.
[DSA-SS, DSA-SS/CC] Alternative systems shall also satisfy the California Administrative Code, Section 4-304.
Frequently asked questions
What does “listed and labeled for use in plenums” mean?
It means a recognized testing laboratory has evaluated the product for plenum use and the manufacturer’s documentation and label indicate it is listed for that use (for many discrete products that listing is UL 2043).
If a product is UL 2043 listed, can I install it anywhere in the plenum?
Generally yes — a UL 2043 listing permits installation in plenums when installed per the product listing and manufacturer instructions, but always check other Code exceptions/limits and job‑specific conditions. § 602.2.4 is the controlling requirement.
Are there numeric flame/smoke limits I should watch for?
Yes — the general plenum material limits in § 602.2 are flame spread index ≤ 25 and smoke-developed index ≤ 50 (ASTM E84 or UL 723). For cables and some nonmetallic piping the Code uses optical density/flame spread distance criteria (see § 602.2.1, § 602.2.2, Exception 9).
Does § 602.3 change product listing requirements for mass timber buildings?
No. § 602.3 adds design and movement requirements for ducts in tall wood (mass timber) buildings; it does not waive or replace the UL‑listing requirements in § 602.2 and § 602.2.4.
What if I can’t find a UL 2043 listed version of the device I need?
You must either use a listed alternative, relocate the device out of the plenum, or pursue an approved alternative (e.g., a tested and accepted equivalency or a design submittal prepared by a registered design professional and accepted by the authority having jurisdiction). See building official authority on listed compliance.
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