CMC · California Mechanical Code

Exhaust opening protection, backdraft dampers and exceptions

If you’re installing an outdoor exhaust or dryer vent in California: use a corrosion‑resistant screen sized between 1/4" and 1/2" where the code requires screening (see **§ 502.1**), install a backdraft damper at exhaust terminations unless an explicit exception applies (see **§ 504.1.1**), and remember that clothes dryers are specially handled — they need a backdraft damper but must **not** have a screen at the termination.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The California Mechanical Code requires that outdoor exhaust openings be protected with a corrosion‑resistant screen sized between 1/4 inch (6.4 mm) and 1/2 inch (12.7 mm) openings per § 502.1. Exhaust ducts must terminate outdoors and generally be equipped with a backdraft damper (or motorized damper that closes when the system is not in use) per § 504.1.1. Several express exceptions apply (for example, clothes dryers and systems that operate continuously); those exceptions are stated in the same sections.

The single most important rule: outdoor exhaust openings need corrosion‑resistant screening of not less than 1/4" and not more than 1/2" openings, and exhaust terminations must have backdraft protection unless a listed exception applies.

Requirements in detail

Exhaust opening protection — what to install

  • Required: a corrosion‑resistant screen covering exhaust openings terminating to the outdoors. The screen openings must be not less than 1/4 inch (6.4 mm) and not more than 1/2 inch (12.7 mm). § 502.1 is the controlling provision.
  • Exception in the same subsection: clothes dryers are carved out from the screen requirement. § 502.1 explicitly lists that exception.

Backdraft protection — when a damper is required

  • Required: exhaust ducts that terminate outside the building shall be equipped with backdraft dampers or motorized dampers that automatically shut when the systems/spaces served are not in use. This is the rule in § 504.1.1.
  • Exceptions listed in § 504.1.1 where backdraft dampers are not required when the duct does not discharge into a common plenum and one of the following applies:
    • The exhaust fan runs continuously.
    • The exhaust duct serves spaces that are not mechanically heated or cooled (unconditioned spaces).
    • The space served is maintained at positive pressure.
  • Note: OSHPD projects have a related exception noted in the section text for continuous operation.

Clothes dryers — special rules

  • Clothes dryer exhausts are governed by § 504.4 (related to § 504.1.1). Requirements include that dryer ducts terminate outside in accordance with the termination rules and shall be equipped with a backdraft damper; screens shall not be installed at the dryer duct termination. Devices that obstruct flow (fire or smoke dampers) are prohibited at dryer terminations.

Decision‑relevant dimensions & values (quick reference table)

Decision factor Required value / limit Where to find it (CMC) Code Reference
Exhaust screen minimum opening 1/4 inch (6.4 mm) Exhaust opening protection § 502.1
Exhaust screen maximum opening 1/2 inch (12.7 mm) Exhaust opening protection § 502.1
Backdraft damper required? Yes — unless an exception applies Backdraft protection requirement § 504.1.1
Backdraft damper NOT required when Fan runs continuously, duct serves unconditioned spaces, or space is at positive pressure Exceptions to damper requirement § 504.1.1
Dryer duct termination: screen allowed? No — screens are not permitted at dryer terminations Clothes dryer exhaust rules § 504.4

Exceptions & special cases

  • Clothes dryers: Exempt from the general exhaust screen requirement in § 502.1, but dryer terminations are separately regulated in § 504.4, which requires a backdraft damper and expressly prohibits screens at the dryer termination. In short: dryer terminations should not have a screen but must have a backdraft damper.
  • Continuous‑run fans: If the fan must operate continuously, backdraft dampers are not required under § 504.1.1 (this exception is explicit and is repeated for OSHPD contexts).
  • Unconditioned spaces / Positive pressure: If the duct serves spaces that are not mechanically heated/cooled (unconditioned), or the served space is maintained at positive pressure, the backdraft damper requirement may be waived per § 504.1.1.
  • Appendix E / damper control exceptions: Appendix E provides additional allowances for nonmotorized (gravity) dampers and small‑capacity systems in certain climate zones and building heights — see E 503.4.6.4.1 for circumstances where nonmotorized dampers are permitted. (Appendix E is part of the CMC material and clarifies motorization and leakage requirements.)

If you need to apply these exceptions to a specific project (e.g., high‑rise, OSHPD, or a cooking hood), check the referenced sections and any project‑specific OSHPD or jurisdictional amendments because the CMC text includes targeted exceptions for those contexts.

Common mistakes

  • Installing a screen at a clothes dryer termination (many installers assume a screen is always required). The CMC exempts dryers from § 502.1 and § 504.4 explicitly prohibits screens at dryer terminations — screens can collect lint and create a fire hazard.
  • Using screen mesh outside the permitted size range: smaller than 1/4 inch (can clog) or larger than 1/2 inch (lets pests in) — both violate § 502.1.
  • Adding fire or smoke dampers or other flow‑obstructing devices at dryer terminations — the code forbids devices that obstruct dryer exhaust flow. § 504.4 is clear about this.
  • Installing backdraft dampers on systems that are required to run continuously — the damper would be unnecessary and could interfere with required continuous operation; when fans run continuously the backdraft damper requirement is waived under § 504.1.1.
  • Confusing termination/location clearances (distances from property lines, openings, grade, etc.) in § 502.2 with screen/damper requirements; termination separation distances are a separate set of rules and must be checked in addition to these damper/screen rules.

Worked example

Scenario: A single‑family laundry room installs a Type 1 (residential) clothes dryer. The dryer exhaust uses a 4‑inch rigid metal duct that runs horizontally 6 feet and terminates through an exterior wall.

Apply the code:

  • Screen requirement: § 502.1 requires outdoor exhaust openings to have a corrosion‑resistant screen sized between 1/4" and 1/2" openings — but clothes dryers are an explicit exception to § 502.1. Therefore you do not install a screen at the dryer termination.
  • Backdraft damper: § 504.1.1 and the dryer‑specific requirements in § 504.4 require that clothes dryer exhausts be equipped with a backdraft damper and terminate outside in accordance with termination rules. Install a listed backdraft damper in the duct before the termination.
  • Prohibited devices: Do not add any screen at the termination and do not place any flow‑obstructing fire or smoke damper at the dryer outlet (prohibited by § 504.4).
  • Termination location: Confirm the dryer discharge location meets the separation distances in § 502.2.1 (e.g., not terminating over a public way, minimum separation from openings/property line as required).

Result: 4" rigid metal duct → backdraft damper installed → terminate outdoors without screen → verify termination clearance per § 502.2.1.

Related provisions (CMC)

  • § 502.2 — Termination requirements and separation distances (distance to property lines, openings, public ways).
  • § 504.4 — Clothes dryer exhaust requirements (backdraft damper required; screens prohibited at dryer terminations; duct construction/length limits).
  • E 503.4.6.4.1 — Appendix E guidance on motorized vs. nonmotorized (gravity) dampers and exceptions by building height and climate zone. (Useful for applying damper motorization exceptions.)
  • § 504.2 — Independent exhaust systems; when exhaust systems must operate independently and negative pressure operation for combined systems.

If you need the exact wording of any of these sections for submittals or code analysis, refer to the cited CMC sections above. The article above synthesizes the controlling text from § 502.1 and § 504.1.1 and the related dryer rules in § 504.4.

Code references

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

  • CMC § 510.9. High relevance — show source text

    Exception: Type I Hood exhaust termination shall be in accordance with Section 510.9.

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    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 § 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.

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    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 § 503.4.6.4.1 High relevance — show source text

    E 503.4.6.4.1 Shutoff Damper Controls. Outdoor air intake and exhaust systems shall be equipped with motorized dampers that will automatically shut when the systems or spaces served are not in use. Outdoor air and exhaust or relief dampers shall be capable of and configured to automatically shut off during preoccupancy building warm-up, cooldown, and setback, except when the supply of outdoor air reduces energy costs or when outdoor air shall

    be supplied to comply with the code require ments.

    Exceptions:

    (1) Nonmotorized (gravity back draft) dampers shall be permitted for exhaust and relief in buildings less than three stories in height and for outdoor air intakes and exhaust and relief dampers in buildings of any height located in Climate Zones 0, 1, 2 and 3. Nonmotorized dampers for outdoor air intakes shall be protected from direct exposure to wind.

    (2) Nonmotorized dampers shall be permitted in systems with a design outdoor air intake or exhaust capacity of 300 ft [3] /min (0.142 m [3] /s) or less.

    (3) Dampers shall not be required in ventilation or exhaust systems serving unconditioned spaces.

    (4) Dampers shall not be required in exhaust systems serving Type 1 kitchen exhaust hoods.

    (5) Dampers are not required in systems intended to operate continuously. [ASHRAE 90.1:6.4.3.4.2]

    E 503.4.6.4.2 Dampers Leakage. Where outdoor air supply, and exhaust or relief dampers are required in Section E 503.4.6.4, they shall have a maximum leakage rate in accordance with Table E 503.4.6.4.2. [ASHRAE 90.1:6.4.3.4.3]

    E 503.4.6.4.3 Ventilation Fan Controls. Fans with motors more than 0.75 hp (0.56 kW) shall have automatic controls in accordance with Section E 503.4.6.3.1 that are capable of and configured to shut off fans when not required.

    Exception: HVAC systems intended to operate continuously. [ASHRAE 90.1:6.4.3.4.4]

    E 503.4.6.5 Parking Garage Ventilation Sys- tems. Parking garage ventilation systems shall meet all of the following:

    (1) Separate ventilation systems and control systems shall be provided for each parking garage section.

    (2) Control systems for each parking garage section shall automatically detect and control contaminant levels and shall be capable of and configured to reduce fan airflow to 20 percent or less of design capacity.

    (3) The ventilation system for each parking garage section shall have controls and devices that

    result in fan motor demand of no more than 30 percent of design wattage at 50 percent of the design airflow.

    Exception: Garage ventilation systems serving a single parking garage section having a total ventilation system motor nameplate horsepower (kilowatts)

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    not exceeding 5 hp (3.7 kW) at fan system design conditions and where the parking garage section has no mechanical cooling or mechanical heating.

  • CMC § 3.1. Medium relevance — show source text

    For the purposes of this exception, a fully ducted HVAC system shall be a duct system for conveying supply, return or exhaust air as part of the structure’s HVAC system. Such a duct system shall be constructed of sheet steel not less than No. 26 gage thickness and shall be continuous from the air-handling appliance or equipment to the air outlet and inlet terminals. Nonmetal flexible air connectors shall be permitted in the following locations in accordance with Section 603 of the California Mechanical Code: 3.1. At the duct connection to the air handling unit or equipment located within the mechanical room. 3.2. From an overhead metal duct to a ceiling diffuser within the same room.

    [SFM] For Group A, E, H, I, L and R occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, ducts and air transfer openings of fire barriers shall be protected with approved fire and smoke dampers installed in accordance with their listing. Ducts and air transfer openings shall not penetrate exit enclosures and exit passageways except as permitted by Sections 1023.5 and 1024.6, respectively.

    Exceptions: 1. Fire dampers are not required at penetrations of fire barriers where penetrations are tested in accordance with ASTM E119 as part of the fire-resistance rated assembly.

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    2. Fire and smoke dampers are not required where ducts are used as part of an approved smoke control system in accordance with Section 909 and where the use of a fire or smoke damper would interfere with the operation of a smoke control system.

    717.5.2.1 Horizontal exits. A listed smoke damper designed to resist the passage of smoke shall be provided at each point a duct or air transfer opening penetrates a fire barrier that serves as a horizontal exit.

    717.5.3 Shaft enclosures. Shaft enclosures that are permitted to be penetrated by ducts and air transfer openings shall be protected with listed fire and smoke dampers installed in accordance with their listing.

    Exceptions:

    1. Fire dampers are not required at penetrations of shafts where any of the following criteria are met: 1.1. Steel exhaust subducts having a wall thickness of not less than 0.0187 inch (0.4712 mm) are extended not less than 22 inches (559 mm) vertically in exhaust shafts, and an exhaust fan is installed at the upper terminus of the shaft that is powered continuously in accordance with Section 909.11, so as to maintain a continuous upward airflow to the outdoors. 1.2. Penetrations are tested in accordance with ASTM E119 or UL 263 as part of the fire-resistance-rated assembly. 1.3. Ducts are used as part of an approved smoke control system designed and installed in accordance with Section 909 and where the fire damper will interfere with the operation of the smoke control system. 1.4. The penetrations are in parking garage exhaust or supply shafts that are separated from other building shafts by not less than 2-hour fire-resistance-rated construction.
  • CMC § 0.019 Medium relevance — show source text

    Exceptions:

    1. Smoke dampers are not required where the building is equipped throughout with an approved smoke control system in accordance with Section 909, and smoke dampers are not necessary for the operation and control of the system.
    2. Smoke dampers are not required in corridor penetrations where the duct is constructed of steel not less than 0.019 inch (0.48 mm) in thickness and there are no openings serving the corridor.

    [SFM] For Group A, E, H, I, L and R occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, a listed smoke damper designed to resist the passage of smoke shall also be provided at each point a duct or air transfer opening penetrates a fire-resistance rated corridor enclosure required to have smoke and draft doors in accordance with Section 716.2.2.1 or doors that provide an effective barrier to limit the transfer of smoke in accordance with Section 407.3.1.

    Exceptions: 1. Smoke dampers are not required where ducts are used as part of an approved mechanical smoke control system designed in accordance with Section 909 and where the smoke damper will interfere with the operation of the smoke control system. 2. Smoke dampers are not required in corridor penetrations where the duct is constructed of steel not less than 0.019 inch (0.48 mm) in thickness and there are no openings serving the corridor. 717.5.5 Smoke barriers. A listed smoke damper designed to resist the passage of smoke shall be provided at each point a duct or air transfer opening penetrates a smoke barrier. Smoke dampers and smoke damper actuation methods shall comply with Section 717.3.3.2.

    Exception: Smoke dampers are not required where the openings in ducts are limited to a single smoke compartment and the ducts are constructed of steel. � 717.5.6 Exterior walls. Ducts and air transfer openings in fire-resistance-rated exterior walls required to have protected openings in accordance with Section 705.11 shall be protected with listed fire dampers installed in accordance with their listing.

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    717.5.7 Smoke partitions. A listed smoke damper designed to resist the passage of smoke shall be provided at each point that an air transfer opening penetrates a smoke partition. Smoke dampers and smoke damper actuation methods shall comply with Section 717.3.3.2.

    Exception: Where the installation of a smoke damper will interfere with the operation of a required smoke control system in accordance with Section 909, approved alternative protection shall be utilized.

    717.6 Horizontal assemblies. Penetrations by ducts and air transfer openings of a floor, floor/ceiling assembly or the ceiling membrane of a roof/ceiling assembly shall be protected by a shaft enclosure that complies with Section 713 or shall comply with Sections 717.6.1 through 717.6.3.

  • CMC § 504.4 Medium relevance — show source text

    (6) Range hoods shall discharge to the outdoors through a single wall duct and shall not terminate in an attic or crawl space.

    504.4 Clothes Dryers. A clothes dryer exhaust duct shall not be connected to a vent connector, gas vent, chimney, and shall not terminate into a crawl space, attic, or other concealed space. Exhaust ducts shall not be assembled with screws or other fastening means that extend into the duct and that are capable of catching lint, and that reduce the efficiency of the exhaust system. Exhaust ducts shall be constructed of rigid metallic material. Transition ducts used to connect the dryer to the exhaust duct shall be listed and labeled in accordance

    with UL 2158A, or installed in accordance with the clothes dryer manufacturer’s installation instructions. Clothes dryer exhaust ducts shall terminate to the outside of the building in accordance with Section 502.2.1 and shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination. Devices, such as fire or smoke dampers that will obstruct the flow of the exhaust shall not be used. Where joining of ducts, the male end shall be inserted in the direction of airflow.

    504.4.1 Provisions for Makeup Air. Makeup air shall be provided in accordance with the following:

    (1) Makeup air shall be provided for Type 1 clothes dryers in accordance with the manufacturer’s installa tion instructions. [NFPA 54:10.4.4.1] Where a closet is designed for the installation of a clothes dryer, an opening of not less than 100 square inches (0.065 m [2] ) for makeup air shall be provided in the door or by other approved means.

    (2) Provision for makeup air shall be provided for Type 2 clothes dryers, with a minimum free area of 1 square inch (0.0006 m [2] ) for each 1000 British thermal units per hour (Btu/h) (0.293 kW) total input rating of the dryer(s) installed. [NFPA 54:10.4.4.2]

    504.4.2 Domestic Clothes Dryers. Where a compartment or space for a Type 1 clothes dryer is provided, not less than a 4 inch diameter (102 mm) exhaust duct of approved material shall be installed in accordance with Section 504.0.

    Type 1 clothes dryer exhaust ducts shall be of rigid metal and shall have smooth interior surfaces. The diam eter shall be not less than 4 inches nominal (100 mm), and the thickness shall be not less than 0.016 of an inch

    (0.406 mm).

    504.4.2.1 Length Limitation. Unless otherwise permitted or required by the dryer manufacturer’s instructions and approved by the Authority Having Jurisdiction, domestic dryer moisture exhaust ducts

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    shall not exceed a total combined horizontal and ver tical length of 14 feet (4267 mm), including two 90 degree (1.57 rad) elbows. A length of 2 feet (610 mm) shall be deducted for each 90 degree (1.57 rad) elbow in excess of two.

  • CMC § 717.1.1 Medium relevance — show source text

    717.1.1 Ducts and air transfer openings. Ducts transitioning horizontally between shafts shall not require a shaft enclosure provided that the duct penetration into each associated shaft is protected with dampers complying with this section.

    717.1.2 Ducts that penetrate fire-resistance-rated assemblies without dampers. Ducts that penetrate fire-resistance-rated walls and are not required by this section to have fire dampers shall comply with the requirements of Sections 714.3 through 714.4.3. Ducts that penetrate horizontal assemblies not required to be contained within a shaft and not required by this section to have fire dampers shall comply with the requirements of Sections 714.5 through 714.6.2.

    717.1.2.1 Ducts that penetrate nonfire-resistance-rated assemblies. The space around a duct penetrating a nonfire-resistance-rated floor assembly shall comply with Section 717.6.3.

    717.2 Installation. Fire dampers, smoke dampers, combination fire/smoke dampers and ceiling radiation dampers located within air distribution and smoke control systems shall be installed in accordance with the manufacturer’s instructions, the dampers’ listing and Sections 717.2.1 through 717.2.4.

    717.2.1 Smoke control system. Where the installation of a fire damper will interfere with the operation of a required smoke control system in accordance with Section 909, approved alternative protection shall be utilized. Where mechanical systems including ducts and dampers utilized for normal building ventilation serve as part of the smoke control system, the expected performance of these systems in smoke control mode shall be addressed in the rational analysis required by Section 909.4.

    717.2.2 Hazardous exhaust ducts. Fire dampers for hazardous exhaust duct systems shall comply with the California Mechanical Code .

    717.2.3 Static dampers. Fire dampers and ceiling radiation dampers that are listed for use in static systems shall only be installed in heating, ventilation and air-conditioning systems that are automatically shut down in the event of a fire.

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    717.2.4 Mechanical, electrical and plumbing controls. Mechanical, electrical and plumbing controls shall not be installed in air duct systems.

    Exception: Controls where the wiring is directly associated with the air distribution system. The wiring shall comply with the requirements of Section 601 of the California Mechanical Code and be as short as practicable.

    717.2.4.1 Controls not permitted to be installed through dampers. Mechanical, electrical and plumbing controls shall not be installed through fire dampers, smoke dampers, combination fire/smoke dampers or ceiling radiation dampers unless otherwise permitted by the manufacturer and the listing.

    717.3 Damper testing, ratings and actuation. Damper testing, ratings and actuation shall be in accordance with Sections 717.3.1 through 717.3.3.

    717.3.1 Damper testing. Dampers shall be listed and labeled in accordance with the standards in this section.

    1. Fire dampers shall comply with the requirements of UL 555.
    2. Smoke dampers shall comply with the requirements of UL 555S.
    3. Combination fire/smoke dampers shall comply with the requirements of both UL 555 and UL 555S.
  • CMC § 1.1. Medium relevance — show source text

    (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.

    504.4 Clothes Dryers. A clothes dryer exhaust duct shall not be connected to a vent connector, gas vent, chimney, and shall not terminate into a crawl space, attic, or other concealed space. Exhaust ducts shall not be assembled with screws or other fastening means that extend into the duct and that are capable of catching lint, and that reduce the efficiency of the exhaust system. Exhaust ducts shall be constructed of rigid metallic material. Transition ducts used to connect the dryer to the exhaust duct shall be listed and labeled in accordance

    with UL 2158A, or installed in accordance with the clothes dryer manufacturer’s installation instructions. Clothes dryer exhaust ducts shall terminate to the outside of the building in accordance with Section 502.2.1 and shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination. Devices, such as fire or smoke dampers that will obstruct the flow of the exhaust shall not be used. Where joining of ducts, the male end shall be inserted in the direction of airflow.

    504.4.1 Provisions for Makeup Air. Makeup air shall be provided in accordance with the following:

    (1) Makeup air shall be provided for Type 1 clothes dryers in accordance with the manufacturer’s installa tion instructions. [NFPA 54:10.4.4.1] Where a closet is designed for the installation of a clothes dryer, an opening of not less than 100 square inches (0.065 m [2] ) for makeup air shall be provided in the door or by other approved means.

  • CMC § 8.13.6 Medium relevance — show source text

    [ASHRAE 15:8.13.6]

    »

    (1) Operated, where occupied, to supply not less than 0.5 CFM/ft [2] [2.54 L/(s•m [2] )] of machinery room area or 20 cubic feet per minute (9.44 L/s) per person.

    (2) Operable, where occupied at a volume required to not exceed the higher of a temperature rise of 18°F (10°C) above inlet air temperature or a maximum temperature of 122°F (50°C).

    1106.4 Natural Ventilation. When a refrigerating system is located outdoors more than 20 feet (6096 mm) from building openings and is enclosed by a penthouse, lean-to, or other open structure, natural or mechanical ventilation shall be provided. The requirements for such natural ventilation shall be in accordance with the following:

    (1) The free-aperture cross section for the ventilation of a machinery room shall be not less than as determined in accordance with Equation 1106.4.

    F = √ G (Equation 1106.4)

    Where:

    F = The free opening area, ft [2] (m [2] ).

    G = The mass of refrigerant in the largest system, any part of which is located in the machinery room, pounds.

    For SI units: 1 square foot = 0.0929 m [2], 1 pound = 0.453 kg

    (2) Locations of the gravity ventilation openings shall be based on the relative density of the refrigerant to air.

    [ASHRAE 15:8.14]

    1106.5 Combustion Air. No open flames that use combustion air from the machinery room shall be installed where refrigerant is used. Combustion equipment shall not be installed in the same machinery room with refrigerant-containing equipment except under one of the following conditions:

    (1) Combustion air shall be ducted from outside the machinery room and sealed in such a manner as to prevent refrigerant leakage from entering the combustion chamber.

    (2) A refrigerant detector, that is in accordance with Section 1106.2.5, shall be installed to automatically shut down the combustion process in the event of refrigerant leak age.

    Exception: Machinery rooms where carbon dioxide (R-744) or water (R-718) is the refrigerant.

    1106.6 Ventilation Intake. Makeup air intakes to replace the exhaust air shall be provided to the refrigeration machinery room directly from outside the building. Intakes shall be located as required by other sections of the code and fitted with backdraft dampers or other approved flow-control means to prevent reverse flow. Distribution of makeup air shall be arranged to provide thorough mixing within the refrigeration machinery room to prevent short circuiting of the makeup air directly to the exhaust.

    234 2025 CALIFORNIA MECHANICAL CODE

    ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

    REFRIGERATION

    1106.11.6.1 Mechanical Ventilation. The machinery room shall have a mechanical ventilation system in accordance with Section 1106.11.11. The mechanical ventilation system shall:

    (1) Run continuously, and failure of the mechanical ventilation system actuates an alarm, or

  • CMC § 714.5.2 Medium relevance — show source text

    1. A shaft enclosure in accordance with Section 713.

    2. A listed ceiling radiation damper installed at the ceiling line where a duct penetrates the ceiling of a fire-resistance-rated floor/ceiling or roof/ceiling assembly. Exceptions:

    3. A fire-resistance-rated assembly tested in accordance with ASTM E119 or UL 263 showing that ceiling radiation dampers are not required in order to maintain the fire-resistance rating of the assembly.

    4. Where exhaust duct or outdoor air duct penetrations protected in accordance with Section 714.5.2 are located within the cavity of a wall and do not pass through another dwelling unit or tenant space.

    5. Where duct and air transfer openings are protected with a duct outlet penetration system tested as part of a fire-resistance-rated assembly in accordance with ASTM E119 or UL 263.

    6. A listed ceiling radiation damper installed at the ceiling line where a diffuser with no duct attached penetrates the ceiling of a fire-resistance-rated floor/ceiling or roof/ceiling assembly. Exceptions:

    7. A fire-resistance-rated assembly tested in accordance with ASTM E119 or UL 263 showing that ceiling radiation dampers are not required in order to maintain the fire-resistance rating of the assembly.

    8. Where duct and air transfer openings are protected with a duct outlet penetration system tested as part of a fire-resistance-rated assembly in accordance with ASTM E119 or UL 263.

    717.6.2.1 Ceiling radiation dampers testing and installation. Ceiling radiation dampers shall be tested in accordance with Section 717.3.1. Ceiling radiation dampers shall be installed in accordance with the details specified in the fire-resistance-rated assembly and the manufacturer’s instructions and the listing.

    717.6.2.1.1 Dynamic systems. Only ceiling radiation dampers labeled for use in dynamic systems shall be installed in heating, ventilation and air-conditioning systems designed to operate with fans on during a fire.

    717.6.2.1.2 Static systems. Static ceiling radiation dampers shall be provided with systems that are not designed to operate during a fire.

    Exceptions:

    1. Where a static ceiling radiation damper is installed at the opening of a duct, a smoke detector shall be installed inside the duct or outside the duct with sampling tubes protruding into the duct. The detector or tubes in the duct shall be within 5 feet (1524 mm) of the damper. Air outlets and inlets shall not be located between the detector or tubes and the damper. The detector shall be listed for the air velocity, temperature and humidity

    2025 CALIFORNIA BUILDING CODE 7-39

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

    FIRE AND SMOKE PROTECTION FEATURES

    anticipated at the point where it is installed. Other than in mechanical smoke control systems, dampers shall be closed upon fan shutdown where local smoke detectors require a minimum velocity to operate. 2. Where a static ceiling radiation damper is installed in a ceiling, the ceiling radiation damper shall be permitted to be controlled by a smoke detection system installed in the same room or area as the ceiling radiation damper. 3. A static ceiling radiation damper shall be permitted to be installed in a room where an occupant sensor is provided within the room that will shut down the system.

    717.6.3 Nonfire-resistance-rated floor assemblies. Duct systems constructed of approved materials in accordance with the California Mechanical Code that penetrate nonfire-resistance-rated floor assemblies shall be protected by any of the following methods:

  • CMC § 1.4. Medium relevance — show source text

    1.4. The penetrations are in parking garage exhaust or supply shafts that are separated from other building shafts by not less than 2-hour fire-resistance-rated construction. 2. In Group B and R occupancies equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1, smoke dampers are not required at penetrations of shafts where all of the following criteria are met: 2.1. Kitchen, clothes dryer, bathroom and toilet room exhaust openings are installed with steel exhaust subducts, having a wall thickness of not less than 0.0187 inch (0.4712 mm). 2.2. The subducts extend not less than 22 inches (559 mm) vertically. 2.3. An exhaust fan is installed at the upper terminus of the shaft that is powered continuously in accordance with the provisions of Section 909.11, so as to maintain a continuous upward airflow to the outdoors. 3. Smoke dampers are not required at penetration of exhaust or supply shafts in parking garages that are separated from other building shafts by not less than 2-hour fire-resistance-rated construction. 4. Smoke dampers are not required at penetrations of shafts where ducts are used as part of an approved mechanical smoke control system designed in accordance with Section 909 and where the smoke damper will interfere with the operation of the smoke control system. 5. Fire dampers and combination fire/smoke dampers are not required in kitchen and clothes dryer exhaust systems where dampers are prohibited by the California Mechanical Code .

    717.5.3.1 Continuous upward airflow. Fire dampers and smoke dampers shall not be installed in shafts that are required to maintain a continuous upward airflow path where closure of the damper would result in the loss of the airflow.

    717.5.4 Fire partitions. In other than Group A, E, I and R occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, ducts and air transfer openings that penetrate fire partitions shall be protected with listed fire dampers installed in accordance with their listing.

    Exceptions: In occupancies other than Group H and L, fire dampers are not required where any of the following apply:

    1. Corridor walls in buildings equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1 or 903.3.1.2 and the duct is protected as a through penetration in accordance with Section 714.
    2. Tenant partitions in covered and open mall buildings where the walls are not required by provisions elsewhere in the code to extend to the underside of the floor or roof sheathing, slab or deck above.
    3. The duct system is constructed of approved materials in accordance with the California Mechanical Code and the duct penetrating the wall complies with all of the following requirements: 3.1. The duct shall not exceed 100 square inches (0.06 m [2] ). 3.2. The duct shall be constructed of steel not less than 0.0217 inch (0.55 mm) in thickness.

    3.3. The duct shall not have openings that communicate the corridor with adjacent spaces or rooms. 3.4. The duct shall be installed above a ceiling. 3.5. The duct shall not terminate at a wall register in the fire-resistance-rated wall. 3.6. A minimum 12-inch-long (305 mm) by 0.060-inch-thick (1.52 mm) steel sleeve shall be centered in each duct opening.

  • CMC § 1106.2.5 Medium relevance — show source text

    (1) Combustion air shall be ducted from outside the machinery room and sealed in such a manner as to prevent refrigerant leakage from entering the combustion chamber.

    (2) A refrigerant detector, that is in accordance with Section 1106.2.5, shall be installed to automatically shut down the combustion process in the event of refrigerant leak age.

    Exception: Machinery rooms where carbon dioxide (R-744) or water (R-718) is the refrigerant.

    1106.6 Ventilation Intake. Makeup air intakes to replace the exhaust air shall be provided to the refrigeration machinery room directly from outside the building. Intakes shall be located as required by other sections of the code and fitted with backdraft dampers or other approved flow-control means to prevent reverse flow. Distribution of makeup air shall be arranged to provide thorough mixing within the refrigeration machinery room to prevent short circuiting of the makeup air directly to the exhaust.

    234 2025 CALIFORNIA MECHANICAL CODE

    ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

    REFRIGERATION

    1106.11.6.1 Mechanical Ventilation. The machinery room shall have a mechanical ventilation system in accordance with Section 1106.11.11. The mechanical ventilation system shall:

    (1) Run continuously, and failure of the mechanical ventilation system actuates an alarm, or

    (2) Be activated by one or more refrigerant detectors, conforming to requirements of Section 1106.11.8. [ASHRAE 15:8.13.6.1] 1106.11.6.2 Detection System. Detection of refrigerant concentration that exceeds 25 percent of the LFL or the upper detection limit of the refrigerant detector, whichever is lower, shall automatically de-energize the following equipment in the machin ery room:

    (1) Refrigerant compressors

    (2) Refrigerant pumps

    (3) Normally closed automatic refrigerant valves

    (4) Other unclassified electrical sources of ignition with apparent power rating greater than 1 kVA, where the apparent power is the product of the circuit voltage and current rating. [ASHRAE 15:8.13.6.2] 1106.11.7 Mechanical Equipment Control. Remote control of the mechanical equipment in the refrigerating machinery room shall be provided immediately outside the machinery room door solely for the purpose of shutting down the equipment in an emergency. Ventilation fans shall be on a separate electrical circuit and have a control switch located immediately outside the machinery room door. [ASHRAE 15:8.13.7] 1106.11.8 Refrigerant Detectors. Each refrigerating machinery room in accordance with Section 1106.11 shall contain one or more refrigerant detectors in accordance with Section 1106.11.9. The detector(s) sensing element shall be located in areas where refrigerant from a leak will concentrate, with one or more set points that activate responses in accordance with Section 1106.11.10 for alarms and Section 1106.11.11 for mechanical ventilation. Multiport-type devices shall be prohibited. {ASHRAE 15:8.13.8} 1106.11.9 Refrigerant Detectors Requirements. Refrigerant detectors required by Section 1106.11 shall meet all of the following conditions:

  • CMC § 503.4.6.3.5.1 High relevance — show source text

    E 503.4.6.3.5.1 Guest Room HVAC Set-Point Control. Within 30 minutes of all occupants leaving the guest room, HVAC set points shall be automatically raised by not less than 4°F (2°C) from the occupant set point in the cooling mode and automatically lowered by at least 4°F (2°C) from the occupant set point in the heating mode. When the guest room is unrented and unoccupied, HVAC set points shall be automatically reset to 80°F (27°C) or higher in the cooling mode and to 60°F (16°C) or lower in the heating mode. Unrented and unoccupied guest rooms shall be determined by either of the following:

    (1) The guest room has been continuously unoccupied for up to 16 hours.

    (2) A networked guest room control system indicates the guest room is unrented and the guest room is unoccupied for no more than 30 minutes.

    Exceptions:

    (1) A networked guest room control system shall be permitted to return the thermostat set points to their default occupied set points 60 minutes prior to the time the room is scheduled to be occupied.

    (2) Cooling for humidity control shall be permitted during unoccupied periods.

    E 503.4.6.4 Stair and Elevator Shaft Vent Dampers. Where stair and elevator shafts have vents, they shall be equipped with motorized dampers that are capable of and configured to automatically close during normal building operation and are interlocked to only open as required by fire and smoke detection systems, or by thermostatic control systems.

    Exception: Nonmotorized gravity back draft dampers are acceptable in buildings less than three stories in height and for buildings of any height located in Climate Zones 0, 1, 2, and 3. [ASHRAE 90.1:6.4.3.4.1]

    E 503.4.6.4.1 Shutoff Damper Controls. Outdoor air intake and exhaust systems shall be equipped with motorized dampers that will automatically shut when the systems or spaces served are not in use. Outdoor air and exhaust or relief dampers shall be capable of and configured to automatically shut off during preoccupancy building warm-up, cooldown, and setback, except when the supply of outdoor air reduces energy costs or when outdoor air shall

    be supplied to comply with the code require ments.

    Exceptions:

    (1) Nonmotorized (gravity back draft) dampers shall be permitted for exhaust and relief in buildings less than three stories in height and for outdoor air intakes and exhaust and relief dampers in buildings of any height located in Climate Zones 0, 1, 2 and 3. Nonmotorized dampers for outdoor air intakes shall be protected from direct exposure to wind.

    (2) Nonmotorized dampers shall be permitted in systems with a design outdoor air intake or exhaust capacity of 300 ft [3] /min (0.142 m [3] /s) or less.

    (3) Dampers shall not be required in ventilation or exhaust systems serving unconditioned spaces.

    (4) Dampers shall not be required in exhaust systems serving Type 1 kitchen exhaust hoods.

    (5) Dampers are not required in systems intended to operate continuously. [ASHRAE 90.1:6.4.3.4.2]

  • CMC § 504.0 Medium 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

    «

    »

    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 § 1703.2 Medium relevance — show source text

    Ground source loop. . . . . . . Table 1703.2, Table 1703.3

    Hydronics . . . . . . . . . . . . . . . . . . . 1210.0, Table 1210.1,

    Table 1220.4.2

    Joining and connections. . . . . . . . . . . . . . . . (see Joints

    and connections)

    Oxygen diffusion . . . . . . . . . . . . . . . . . . . . . . . . . 1210.4

    CROSS-LINKED POLYETHYLENE-ALUMINUM-CROSS-

    LINKED POLYETHYLENE (PEX-AL-PEX)

    PIPING OR TUBING

    Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218.0

    Hydronics . . . . . . . . . . . . . . . . . . . 1210.0, Table 1210.1

    Joining and connections. . . . . . . . . . . . . . . . (see Joints

    and connections)

    – D –

    DAMPERS

    Automatically

    operated . . . . . . . 606.8, 802.14, 802.14.1, E 502.10.2

    Backdraft. . . . . . . . . . . . . . . . . . . 504.1.1, 504.4, 1106.6

    Balancing. . . . . . . . . . . . . . . . . . . . . . . . . . . . .E 502.3.1

    Ceiling radiation . . . . . . . . . . . . . . . . . . . . . 206.0, 606.3

    Combination fire and smoke . . . . . . . . . . . 206.0, 606.4

    Corridor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606.5

    Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.0

    Economizers . . . . . . . . . . . . . . . . . . . . . . . . E 503.5.1.1

    Fire . . . . . . . . . . . . . . . . . 206.0, 508.3.5.3, 508.3.5.3.1,

    511.4.2, 516.2.6,

    606.2, 606.8, 932.1

    Fire and smoke, access to . . . . . . . . . . . . . . . . . . 606.8

    Freedom from interference . . . . . . . . . . . . . . . . . . 606.9

    Heating, ventilation,

    and air conditioning . . . . . . . . . . . . . . . . . . . E 502.12

    HVAC system tests of . . . . . . . . . . . . . . . . . . . . E 805.0

Frequently asked questions

Can I put a screen on a dryer vent to keep pests out?

No. Although § 502.1 generally requires screens on exhaust openings, clothes dryers are an exception, and § 504.4 explicitly forbids screens at dryer terminations because lint accumulation and flow obstruction are fire and performance hazards.

When may I omit a backdraft damper?

You may omit a backdraft damper where the exhaust duct does not discharge into a common plenum and one of the § 504.1.1 exceptions applies — for example, the exhaust fan runs continuously, the duct serves unconditioned (not mechanically heated/cooled) spaces, or the served space is at positive pressure.

What mesh sizes are allowed for required exhaust screens?

Screens must be corrosion‑resistant and have openings not less than 1/4" (6.4 mm) and not more than 1/2" (12.7 mm) per § 502.1.

Are gravity (nonmotorized) dampers ever acceptable?

Yes — Appendix E (E 503.4.6.4.1) permits nonmotorized gravity backdraft dampers in certain buildings (for example, buildings less than three stories or small‑capacity systems and specific climate zones) and provides related control guidance. Check E 503.4.6.4.1 for the precise conditions.

Do these rules apply to commercial kitchen Type I hoods?

Type I hoods have specialized termination and protection requirements (Type I termination is handled under other sections, e.g., references to Section 510.9). The general backdraft/damper rules in § 504.1.1 may have exceptions for Type I systems; always check the Type I hood requirements and § 510.9.

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