CMC · California Mechanical Code
Clothes dryers — application, electric vs gas, venting basics
Homeowners: Use a listed dryer for the location, follow the manufacturer, keep proper clearances (6" listed Type 1; 18" if unlisted), vent the dryer with rigid 4" metal duct to the outdoors, provide required makeup air for closets, and do not exceed the 14 ft combined duct length unless the manufacturer or a listed booster allows otherwise.
Last reviewed: July 6, 2026
What the code requires — 2–4 sentences
Clothes dryers must be manufactured/listed to applicable standards and installed per the code and the manufacturer’s instructions. The California Mechanical Code requires that dryers be listed as Type 1 or Type 2 (see § 908.1) and that electric and gas-fired appliances meet the specified listing/installation requirements in § 908.2 and § 908.3 respectively. Gas dryer clearance and installation rules are in § 908.3.1, and all dryers required to exhaust to outdoors must do so in accordance with § 908.3.2 referencing § 504.4 .
Most important rule (plain English): Use a listed dryer for the intended location (Type 1 vs Type 2), follow the dryer manufacturer, provide required clearances and makeup air, and vent the dryer to the outdoors with metal ducting sized and routed per the code. See § 908.1 and § 908.3 .
Requirements in detail
1) Application and classification (what is required of the appliance)
The code requires dryers be listed to the applicable ANSI/CSA standards:
- Type 1 clothes dryers: listed to ANSI Z21.5.1/CSA 7.1.
- Type 2 clothes dryers: listed to ANSI Z21.5.2/CSA 7.2.
These listing requirements are stated in § 908.1 .
Defined terms to note (first mention bolded): Type 1, Type 2, unlisted — these affect clearances, installation location, and exhaust requirements under § 908.3.1 and related exhaust rules .
2) Electric vs. Gas (what standard applies)
- Electric clothes dryers must comply with the appropriate UL standard:
- Commercial electric dryers: UL 1240.
- Residential and coin-operated electric dryers: UL 2158. These are required by § 908.2 and installations must follow the appliance manufacturer’s instructions .
- Gas-fired clothes dryers must comply with the requirements in § 908.3 (including clearances, exhaust to outdoors, and additional rules for multiple-family/public installations) and subsections 908.3.1–908.3.3 .
3) Clearances and special installation limitations
- Minimum clearances for gas dryers (installation rule summary):
- Listed Type 1 clothes dryers: minimum 6 inches (152 mm) to adjacent combustible material unless the dryer is listed for reduced clearance; follow the appliance listing/manufacturer instructions.
- Listed Type 2: install to the clearances marked on the appliance/marking plate and manufacturer instructions; units marked “For use only in noncombustible locations” must only be installed in noncombustible locations.
- Unlisted clothes dryers: minimum 18 inches (457 mm) clearance to combustible materials and combustible floors under an unlisted dryer must be protected.
These are in § 908.3.1 .
4) Venting basics — duct material, diameter, termination, length limits, transition ducts, makeup air
- Exhaust to outdoors: Type 1 and Type 2 clothes dryers must discharge to outside air per § 908.3.2 and Section 504.4 (exhaust rules) .
- Key venting dimensions and construction requirements (see table below for quick decision points):
| Decision dimension / value | Required value or threshold | Code Reference |
|---|---|---|
| Appliance listing for application | Type 1 → ANSI Z21.5.1/CSA 7.1; Type 2 → ANSI Z21.5.2/CSA 7.2 | § 908.1 |
| Minimum clearance — listed Type 1 | 6 inches (152 mm) to combustible material (unless listed for less) | § 908.3.1(1) |
| Minimum clearance — unlisted dryer | 18 inches (457 mm) to combustibles; protect combustible floor | § 908.3.1(3) |
| Exhaust duct minimum diameter (domestic) | 4 inches nominal rigid metal, smooth interior | § 504.4.2 |
| Exhaust duct construction | Rigid metallic material; no screws protruding into duct; no screens at termination | § 504.4 |
| Exhaust duct combined length limit (domestic) | 14 feet (4267 mm) total (horizontal + vertical), including two 90° elbows; deduct 2 ft per additional 90° elbow beyond two | § 504.4.2.1 |
| Transition duct length and concealment | Listed transition duct ≤ 6 feet (1829 mm) permitted; transition ducts must not be concealed | § 504.4.2.2 |
| Exhaust duct power ventilator | Where used, follow the ventilator manufacturer’s instructions (may allow longer runs) | § 504.4.2.1 (Exception) |
| Makeup air for dryer closets | 100 sq in opening to provide makeup air where closet provided for a Type 1 dryer | § 504.4.1(1) |
| Makeup air for Type 2 (commercial) | 1 sq in per 1000 Btu/h total input rating of installed dryers | § 504.4.1(2) |
| Common exhaust for multiple dryers | Allowed where the manufacturer permits; rigid metal, fire-resistant rated enclosure, cleanout access, continuous exhaust fan downstream of branches | § 504.4.4 |
- Additional venting rules to note:
- Exhaust ducts must terminate outdoors in accordance with § 502.2.1 and be equipped with a backdraft damper; screens are not allowed at the duct termination; devices that obstruct flow (fire/smoke dampers) are prohibited in dryer exhausts (§ 504.4) .
- When joining ducts, the male end must be inserted in the direction of airflow (§ 504.4) .
Exceptions & special cases
- Reduced clearances: If the dryer is listed for reduced clearances, install per the listing and manufacturer’s instructions — do not rely on default clearances in § 908.3.1 .
- Closet installations: Type 1 dryers installed in closets must be listed for closet installation and makeup-air openings are required when a closet is provided (see § 908.3.1 and § 504.4.1(1)) .
- Commercial / Type 2 / multiple-family: Multiple or commercial dryers (Type 2) have stricter duct construction, lint-control, continuous or interlocked exhaust fan operation, and makeup-air rules in § 504.4.3, § 504.4.3.1, and § 908.3.3 — review those specific subsections and the appliance manufacturer’s instructions for allowable common exhaust use .
Common mistakes
- Using flexible foil or plastic ducting instead of rigid metallic duct for the exhaust (code requires rigid metal for Type 1 domestic ducts) — see § 504.4.2 .
- Exceeding the 14 ft combined duct length or adding too many elbows without adjusting length per elbow rules (deduct 2 ft per extra 90° elbow beyond two) — see § 504.4.2.1 .
- Installing a dryer vent that terminates into an attic, crawl space, or other concealed space — prohibited by § 504.4 and referenced in § 908.3.2 .
- Blocking or screening the termination cap (screens not allowed) or using screws that enter the duct interior (lint-catching hazard) — see § 504.4 .
- Installing an unlisted dryer without providing the 18 in clearance to combustibles or without floor protection (for unlisted units) — see § 908.3.1(3) .
Worked example — concrete scenario
Scenario: Single-family residence, residential Type 1 electric dryer installed in a laundry closet. The dryer outlet is connected to a rigid metal exhaust run that contains three 90° elbows and a combined run length of 18 ft horizontal + 2 ft vertical.
Step through code checks:
- Appliance listing: The dryer is Type 1 and listed — satisfies § 908.1 and installation standard in § 908.2 for electric home dryers .
- Closet rules: A laundry closet is provided, so a 100 sq in makeup-air opening is required in the door or by other approved means for Type 1 dryer closets per § 504.4.1(1) .
- Exhaust duct diameter/material: Use 4 in nominal rigid metal, smooth interior, per § 504.4.2 .
- Length calculation: Base allowable combined length is 14 ft including two 90° elbows (§ 504.4.2.1). There are three 90° elbows — that’s one elbow in excess of two, so subtract 2 ft from the maximum: allowable length becomes 12 ft. The actual run is 20 ft (18 + 2) which exceeds the allowable 12 ft, so the installation as described is NOT code compliant unless:
- The dryer manufacturer’s instructions allow a longer run and the Authority Having Jurisdiction approves, or
- An exhaust duct power ventilator listed for this use is installed and the ventilator manufacturer’s instructions permit the longer run (see § 504.4.2.1 Exception and § 504.4.2.3) .
Related provisions
- § 908.0 — Clothes dryers (general heading)
- § 908.1 — Application and listing requirements (Type 1 vs Type 2)
- § 908.2 — Electric clothes dryer standard/installation requirements (UL 1240 / UL 2158)
- § 908.3 — Gas-fired clothes dryers and subsections 908.3.1–908.3.3 (clearance, exhaust to outdoors, multiple-family/public use)
- § 504.4 — Clothes dryer exhaust system requirements (duct construction, termination, dampers, prohibition on attic/crawl space termination)
- § 504.4.2 / § 504.4.2.1 — Domestic dryer duct diameter, construction, and 14 ft length limit (and elbow deductions)
- § 504.4.2.2 — Transition duct (≤ 6 ft) and concealment rules
- § 504.4.2.3 — Exhaust duct power ventilators (UL 705) — may allow longer runs per the ventilator manufacturer’s instructions
- § 504.4.3 / § 504.4.3.1 — Commercial dryer exhaust construction and lint control for Type 2 dryers
- § 504.4.4 — Common exhaust for multiple dryers (conditions and continuous/interlocked fan requirements)
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 10.10.13 High relevance — show source text
[NFPA 54:10.10.13]
906.14 Oil-Fired Floor Furnaces. Oil-fired floor furnaces shall comply with UL 729 and installed in accordance with the manufacturer’s installation instructions.
907.0 Wall Furnaces.
907.1 Application. Wall furnaces shall be listed in accordance with ANSI Z21.86/CSA 2.32. [NFPA 54:10.25.1]
907.2 Installation. Wall furnaces shall be installed in accor dance with the manufacturer’s installation instructions. Wall
furnaces installed in or attached to combustible material shall be listed for such installation. [NFPA 54:10.25.2.1]
907.2.1 Vented Wall Furnaces. Vented wall furnaces connected to a Type B-W gas vent system listed only for a single story shall be installed only in single-story buildings or the top story of multistory buildings. Vented wall furnaces connected to a Type B-W gas vent system listed for installation in multistory buildings shall be permitted to be installed in single-story or multistory buildings. Type B-W gas vents shall be attached directly to a solid header plate that serves as a firestop at that point and that
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INSTALLATION OF SPECIFIC APPLIANCES
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907.4 Combustion and Circulating Air. Combustion and circulating air shall be provided in accordance with Section 701.0. [NFPA 54:10.25.4]
907.5 Oil-Fired Wall Furnaces. Oil-fired wall furnaces
shall comply with UL 730 and installed in accordance with the manufacturer’s installation instructions.
908.0 Clothes Dryers.
908.1 Application. Clothes dryers shall be listed in accordance with ANSI Z21.5.1/CSA 7.1 for Type I clothes dryers or ANSI Z21.5.2/CSA 7.2 for Type II clothes dryers. [NFPA 54:10.4.1]
908.2 Electric Clothes Dryers. Commercial electric clothes dryers shall comply with UL 1240 and installed in accordance with the manufacturer’s installation instructions.
Residential and coin-operated electric clothes dryers shall comply with UL 2158 and installed in accordance with the manufacturer’s installation instructions.
908.3 Gas-Fired Clothes Dryers. Gas-fired clothes dryers shall comply with Section 908.3.1 through Section 908.3.3.
908.3.1 Clearance. The installation of clothes dryers shall comply with the following requirements:
(1) Listed Type 1 clothes dryers shall be installed with a clearance of not less than 6 inches (152 mm) from adjacent combustible material. Clothes dryers listed for installation at reduced clearances shall be
installed in accordance with their listing and the manufacturer’s installation instructions. Type 1 clothes dryers installed in closets shall be listed for such installation.
CMC § 907.4 High relevance — show source text
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907.4 Combustion and Circulating Air. Combustion and circulating air shall be provided in accordance with Section 701.0. [NFPA 54:10.25.4]
907.5 Oil-Fired Wall Furnaces. Oil-fired wall furnaces
shall comply with UL 730 and installed in accordance with the manufacturer’s installation instructions.
908.0 Clothes Dryers.
908.1 Application. Clothes dryers shall be listed in accordance with ANSI Z21.5.1/CSA 7.1 for Type I clothes dryers or ANSI Z21.5.2/CSA 7.2 for Type II clothes dryers. [NFPA 54:10.4.1]
908.2 Electric Clothes Dryers. Commercial electric clothes dryers shall comply with UL 1240 and installed in accordance with the manufacturer’s installation instructions.
Residential and coin-operated electric clothes dryers shall comply with UL 2158 and installed in accordance with the manufacturer’s installation instructions.
908.3 Gas-Fired Clothes Dryers. Gas-fired clothes dryers shall comply with Section 908.3.1 through Section 908.3.3.
908.3.1 Clearance. The installation of clothes dryers shall comply with the following requirements:
(1) Listed Type 1 clothes dryers shall be installed with a clearance of not less than 6 inches (152 mm) from adjacent combustible material. Clothes dryers listed for installation at reduced clearances shall be
installed in accordance with their listing and the manufacturer’s installation instructions. Type 1 clothes dryers installed in closets shall be listed for such installation.
(2) Listed Type 2 clothes dryers shall be installed with clearances of not less than those shown on the mark ing plate and in the manufacturer’s instructions. Type 2 clothes dryers designed and marked, “For use only in noncombustible locations,” shall not be installed elsewhere.
(3) Unlisted clothes dryers shall be installed with clearances to combustible material of not less than 18
inches (457 mm). Combustible floors under unlisted clothes dryers shall be protected in an approved manner. {NFPA 54:10.4.2}
908.3.2 Exhausting to the Outdoors. Type 1 and Type 2 clothes dryers shall be exhausted to the outside air in accordance with Section 504.4. {NFPA 54:10.4.3}
908.3.3 Multiple-Family or Public Use. All clothes dryers installed for multiple-family or public use shall be installed as specified for a Type 2 clothes dryer under Section 504.4.3.1. [NFPA 54:10.4.7]
909.0 Conversion Burners.
909.1 General. Installation of conversion burners shall con form to CSA Z21.8. [NFPA 54:10.5]
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shall be permitted to be an integral part of the vented wall furnace, as illustrated in Figure 907.2.1. The stud space in which the vented wall furnace is installed shall be ventilated at the first ceiling level by installation of the ceiling plate spacers furnished with the gas vent. Firestop spacers shall be installed at each subsequent ceiling or floor level penetrated by the vent. [NFPA 54:10.25.2.2]
CMC § 8-1 Medium relevance — show source text
CHAPTER 8 INTERIOR FINISHES. . . . . . . . . . . . . . . . . . . . . .8-1
801 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3
802 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3
803 Wall and Ceiling Finishes . . . . . . . . . . . . . . . . . . . . . . . . 8-3
804 Interior Floor Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6
805 Combustible Materials in Types I and II Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7
806 Decorative Materials and Trim . . . . . . . . . . . . . . . . . . . 8-7
807 Insulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8
808 Acoustical Ceiling Systems . . . . . . . . . . . . . . . . . . . . . . 8-8
CHAPTER 9 FIRE PROTECTION AND LIFE
SAFETY SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1
901 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-7
902 Fire Pump and Riser Room Size . . . . . . . . . . . . . . . . . . 9-7
903 Automatic Sprinkler Systems . . . . . . . . . . . . . . . . . . . . 9-8
904 Alternative Automatic Fire-Extinguishing Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-17
905 Standpipe Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-20
906 Portable Fire Extinguishers . . . . . . . . . . . . . . . . . . . . . 9-22
907 Fire Alarm and Detection Systems . . . . . . . . . . . . . . . 9-25
908 Emergency Alarm Systems . . . . . . . . . . . . . . . . . . . . . 9-43
909 Smoke Control Systems . . . . . . . . . . . . . . . . . . . . . . . . 9-43
910 Smoke and Heat Removal . . . . . . . . . . . . . . . . . . . . . . 9-51
CMC § 303.2 Medium relevance — show source text
Central heating
furnaces and boilers. . . . . . . . . . . . . . . . 303.2, 904.2
Clothes dryers . . . . . . . . . . . . . . . . . . . 504.4.1, 908.3.1
CLOTHES DRYER
Clearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 908.3.1
Commercial use. . . . . . . . . . . . . . . . . . . . . . . . . 504.4.3
Common exhaust. . . . . . . . . . . . . . . . . . . . . . . . 504.4.4
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0
Domestic type . . . . . . . . . . . . . . . . . . . . . . . . . . 504.4.2
Electric type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 908.2
Exhaust duct . . . . . . . . . . . . . . . . . . . . . . 504.4-504.4.6
2025 CALIFORNIA MECHANICAL CODE 575
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INDEX
For multiple family use . . . . . . . . . . . . . . . . . . . . 908.3.3
For public use . . . . . . . . . . . . . . . . . . . . . . . . . . 908.3.3
Makeup air. . . . . . . . . . . . . . . . . . . . . . . . 504.4.1, 701.3
Termination . . . . . . . . . . . . . . . . . . . . . . . 502.2.1, 504.4
Transition ducts. . . . . . . . . . . . . . . . . . . . . . . . 504.4.2.2
Type 1 . . . . . . . . . . . . . 205.0, 504.4.1, 504.4.2, 908.3.1
Type 2 . . . . . . . . . . . 205.0, 504.4.1, 504.4.3.1, 908.3.1
Unlisted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 908.3.1
CODE
Conflicts between . . . . . . . . . . . . . . . . . . . . . . . . . 102.1
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0
Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.3
CMC § 2.6 Medium relevance — show source text
(d) Electric Clothes Dryer Ready. Clothes dryer locations with gas or propane plumbing shall include the following:
- Systems serving individual dwelling units shall include: A. A dedicated 240 volt branch circuit wiring shall be installed within 3 feet from the clothes dryer location and accessible to the clothes dryer location with no obstructions. The branch circuit conductors shall be rated at 30 amps minimum. The blank cover shall be identified as “240V ready.” All electrical components shall be installed in accordance with the California Electrical Code .
B. The main electrical service panel shall have a reserved space to allow for the installation of a double pole circuit breaker for a future electric clothes dryer installation. The reserved space shall be permanently marked as “For Future 240V use.”
- Systems in common use areas shall include: A. Conductors or raceway shall be installed with termination points at the main electrical panel, via subpanels panels if applicable, to a location no more than 3 feet from each gas outlet or a designated location of future electric replacement equipment. Both ends of the conductors or raceway shall be labelled “Future 240V Use.” Gas flow rates shall be determined in accordance with the California Plumbing Code . Capacity shall be one of the following: i. 24 amps at 208/240 volts per clothes dryer; ii. 2.6 kVA for each 10,000 Btu per hour of rated gas input or gas pipe capacity; or iii. The electrical power required to provide equivalent functionality of the gas-powered equipment as calculated and documented by the responsible person associated with the project.
(e) Individual Heat Pump Water Heater Ready. Systems using gas or propane water heaters to serve individual dwelling units shall include the following components for each gas or propane water heater:
A dedicated 125 volt, 20 amp electrical receptacle that is connected to the electric panel with a 120/240 volt 3 conductor branch circuit rated to 30 amps minimum, within 3 feet from the water heater and accessible to the water heater with no obstructions. In addition, all the following: A. Both ends of the unused conductor shall be labeled with the word “spare” and be electrically isolated; and B. A reserved single pole circuit breaker space in the electrical panel adjacent to the circuit breaker for the branch circuit in Item A and labeled with the words “Future 240V Use”; and
A condensate drain that is no more than 2 inches higher than the base of the installed water heater and allows natural draining without pump assistance; and
The construction drawings shall designate a space at least 39 inches by 39 inches and 96 inches tall for the future location of a heat pump water heater (HPWH).
A ventilation method meeting one of the following: A. The designated space for the future heat pump water heater shall have a minimum volume of 700 cubic feet; or B. If the future HPWH space is designed to vent indoors, the designated space for the future heat pump water heater shall vent to a communicating space in the same pressure boundary. The total combined volume connected shall be 700 cubic feet or larger and vent to the interior via: i. Fully louvered doors with fixed louvers consisting of a single layer of fixed flat slats and a minimum total net free area (NFA) of 250 square inches; or ii. Two permanent openings of equal area with a minimum total NFA of 250 square inches located within 12 inches from the enclosure top and bottom; or iii. Two 8-inch ducts to a communicating space.
CMC § 0.0006 Medium relevance — show source text
(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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EXHAUST SYSTEMS
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.
Exception: Where an exhaust duct power ventilator, in accordance with Section 504.4.2.3, is used, the maximum length of the dryer exhaust duct shall be permitted to be in accordance with the dryer exhaust duct power ventilator manufacturer’s installation instructions.
504.4.2.2 Transition Ducts. Listed clothes dryer transition ducts not more than 6 feet (1829 mm) in length shall be permitted to be used to connect the Type 1 dryer to the exhaust ducts. Transition ducts and flexible clothes dryer transition ducts shall not be concealed within construction, and shall be installed in accordance with the manufacturer’s
installation instructions.
504.4.2.3 Exhaust Duct Power Ventilators.
Dryer exhaust duct power ventilators for single residential clothes dryers shall be listed and labeled in accordance with UL 705 and installed in accor dance with the manufacturer’s installation instruc tions.
504.4.3 Commercial Clothes Dryers. Commercial dryer exhaust ducts shall be installed in accordance with their listings. The installation of commercial clothes dryer exhaust ducts shall comply with the appliance manufacturer’s installation instructions.
504.4.3.1 Exhaust Ducts for Type 2 Clothes Dryers. Exhaust ducts for Type 2 clothes dryers shall comply with the following:
(1) Exhaust ducts for Type 2 clothes dryers shall comply with Section 504.4. [NFPA 54:10.4.6.1]
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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EXHAUST SYSTEMS
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 § 1220.4.2 Medium relevance — show source text
8|1220.4.2 – 1220.4.6| |1221.7|1221.6| |1308.2 – 1308.4.1|1308.3 – 1308.5.1| |Table 1308.3.1|Table 1308.4.1| |1308.4.2|1308.5.2| |1308.4.2.2 – 1308.6.4|1308.5.2.1 – 1308.7.3| |Table 1308.4.6.2|Table 1308.5.6.2| |1308.6.5 – 1308.13.1|1308.7.7 – 1308.14.1| |1312.1.2 – 1312.1.3|1312.1.1 – 1312.1.2| |Chapter 17|Appendix F| |Chapter 18|Chapter 17| |D 103.2|D 103.1| |E 503.5.6.7|E 503.5.6.6| |E 503.5.7.2 – E 503.5.7.6|E 503.5.7.1 – E 503.5.7.5| |Table E 503.5.7.2|Table E 503.5.7.1| |Table E 503.5.7.6|Table E 503.5.7.5| |Table E 503.5.10.1.2(1)|Table E 503.5.10(1)| |Table E 503.5.10.1.2(2)|Table E 503.5.10(2)| |E 503.5.10.1.2|E 503.5.10| |E 503.5.10.2 – E 503.5.10.2.2|E 503.5.10.1 – E 503.5.10.1.2| |E 503.5.12.1|E 503.5.12| |E 503.5.12.2|E 503.5.12.1| |E 503.6.5.4|E 503.6.5.4.1|
xxii 2025 CALIFORNIA MECHANICAL CODE
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CMC § 90.1 Medium relevance — show source text
[ASHRAE 90.1: TABLE 6.4.3.4.3]
Col1 OUTDOOR AIR INTAKE (CFM/ft2) Col3 EXHAUST/RELIEF (CFM/ft2) Col5 CLIMATE ZONE NONMOTORIZED1 MOTORIZED NONMOTORIZED3 MOTORIZED 0, 1, 2 0, 1, 2 0, 1, 2 0, 1, 2 0, 1, 2 Any height 20 4 20 4 3 3 3 3 3 Any height 20 10 20 10 4, 5B, 5C 4, 5B, 5C 4, 5B, 5C 4, 5B, 5C 4, 5B, 5C Fewer than three stories 204 10 20 10 Three or more stories 204 10 204 10 5A, 6, 7, 8 5A, 6, 7, 8 5A, 6, 7, 8 5A, 6, 7, 8 5A, 6, 7, 8 Fewer than three stories 204 4 20 4 Three or more stories 204 4 204 4 For SI units: 1 square foot = 0.0929 m [2], 1 cubic foot per minute = 0.00047 m [3] /s, 1 cubic foot per minute = 0.4719 L/s, 1 cubic foot per minute per square foot = 5.08 [(L/s)/m [2] ], 1 inch water gauge = 0.249 kPa
Notes:
1 When tested in accordance with AMCA 500D.
2 Dampers smaller than 12 inches (305 mm) in height, width, or diameter need not be tested but shall be of the same design and construction as the smallest tested damper meeting the listed leakage rate requirement.
3 Nonmotorized dampers smaller than 24 inches (610 mm) in height, width, or diameter shall be permitted to have a leakage rate of 40 CFM/ft 2 [0.203 (m 3 /s)/m 2 ].
4 Where permitted by Section E 503.4.6.4.1, exception 2.
2025 CALIFORNIA MECHANICAL CODE 425
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APPENDIX E
(2) Systems serving zones where humidity levels are required to be maintained with precision of not more than ±5 percent relative humidity to comply with applicable codes or accreditation standards or as approved by the Authority Having Jurisdiction.
[ASHRAE 90.1:6.4.3.6.2]
CMC § 1112.12.4. Medium relevance — show source text
( P [2] 0 P 22 ) d - ln
(P0P2 )f - C [2] r 6 - f
2025 CALIFORNIA MECHANICAL CODE 243
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REFRIGERATION
expected to operate simultaneously, connect to a common discharge pipe, the common pipe shall be sized large enough to prevent the back pressure at each pressure-relief device from exceeding the maximum allowable back pressure in accordance with Section 1112.12.4.
[ASHRAE 15:9.7.9.3.3]
1112.13 Rating of Pressure-Relief Device. The rated discharge capacity of a pressure-relief device expressed in pounds of air per minute (kg/s), shall be determined in accordance with ASME BPVC Section VIII.1. Pipe and fittings between the pressure-relief valve and the parts of the system it protects shall have not less than the area of the pressurerelief valve inlet area. [ASHRAE 15:9.7.6]
1112.14 Rating of Rupture Members and Fusible Plugs. The rated discharge capacity of a rupture member or fusible plug discharging to the atmosphere under critical flow conditions, in pounds of air per minute (kg/s), shall be determined in accordance with the following formulas:
C = 0.64 P l d [2] [Equation 1112.14(1)]
d = 1.25
√C / P l [Equation 1112.14(2)]Where:
C = Rated discharge capacity expressed as mass flow of air, pounds per minute.
d = Smallest of the internal diameter of the inlet pipe, retaining flanges, fusible plug, and rupture member , inches.
For rupture members:
[Equation 1112.14(3)]
P1 = (rated pressure in psig x 1.1) + 14.7 psia
For fusible plugs:
P1 = Absolute saturation pressure corresponding to the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller, pound-force per square inch atmosphere, psia. [ASHRAE 15:9.7.7]
For SI units:1 pound per minute = 0.00756 kg/s
1113.0 Overpressure Protection.
1113.1 General. Pressure vessels shall be provided with overpressure protection in accordance with ASME BPVC Section VIII.1. Pressure vessels containing liquid refrigerant that are capable of being isolated by stop valves from other parts of a refrigerating system shall be provided with overpressure protection. Pressure relief devices or fusible plugs shall be sized in accordance with Section 1113.5. [ASHRAE 15:9.7.1, 9.7.2]
»
Unless the maximum allowable back pressure ( P0 ) is specified by the relief valve manufacturer, the following maximum allowable back pressure values shall be used for P0, where P is the set pressure and Pa is atmospheric pressure at the nominal elevation of the installation (see Table 1112.12.4):
CMC § 304.8 Medium relevance — show source text
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
1112.12.5 Simultaneous Operation. When outlets of two or more relief devices or fusible plugs, which are
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
244 2025 CALIFORNIA MECHANICAL CODE
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REFRIGERATION
1113.2 Type of Protection. Pressure vessels with an internal gross volume of 3 cubic feet (0.1 m [3] ) or less shall use one or more pressure relief devices or a fusible plug. Pressure vessels of more than 3 cubic feet (0.1 m [3] ) but less than 10 cubic feet (0.28 m [3] ) internal gross volume shall use one or more pressure relief devices. Fusible plugs shall not be used.
[ASHRAE 15:9.7.2.1, 9.7.2.2]
1113.3 Discharging into Lowside of System. For pressure-relief valves discharging into the lowside of the system, a single relief valve (not rupture member) of the required relieving capacity shall not be used on vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume except under the conditions permitted in Section 1112.11.3. [ASHRAE 15:9.7.3]
1113.4 Parallel Pressure-Relief Devices. Two or more
pressure-relief devices in parallel to obtain the required capacity shall be considered as one pressure-relief device. The discharge capacity shall be the sum of the capacities required for each pressure vessel being protected.
1113.5 Discharge Capacity. The minimum required discharge capacity of the pressure-relief device or fusible plug for a pressure vessel shall be determined in accordance with ASHRAE 15.
1113.6 Three-Way Valve. Pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume shall use one or more rupture member(s) or dual pressure-relief valves where discharging to the atmosphere. Dual pressure-relief valves shall be installed with a three-way valve to allow testing or repair. Where dual relief valves are used, the valve shall comply with Section 1113.5.
Exception: A single relief valve shall be permitted on pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume where in accordance with the following conditions:
(1) The relief valves are located on the lowside of the sys tem.
(2) The vessel is provided with shutoff valves designed to allow pumpdown of the refrigerant charge of the pressure vessel.
(3) Other pressure vessels in the system are separately protected in accordance with Section 1113.1. [ASHRAE 15:9.7.2.3]
1114.0 Special Discharge Requirements.
CMC § 8-5 Medium relevance — show source text
803 Fire Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5
804 Means of Egress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5
805 Structural. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9
806 Electrical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10
807 Mechanical. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10
808 Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10
809 Energy Conservation. . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10
CHAPTER 9 ALTERATIONS—LEVEL 3 . . . . . . . . . . . . . . . . . .9-3
901 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-3
902 Special Use and Occupancy. . . . . . . . . . . . . . . . . . . . . . 9-3
903 Building Elements and Materials . . . . . . . . . . . . . . . . . 9-3
904 Fire Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4
905 Means of Egress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4
906 Structural. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4
907 Energy Conservation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4
908 Emergency Responder Communications Enhancement System Coverage. . . . . . . . . . . . . . . . 9-5
CHAPTER 10 CHANGE OF OCCUPANCY . . . . . . . . . . . . . . .10-3
1001 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3
1002 Special Use and Occupancy. . . . . . . . . . . . . . . . . . . . . 10-3
1003 Building Elements and Materials . . . . . . . . . . . . . . . . 10-3
Frequently asked questions
Can I use a flexible foil vent for my dryer?
No. For domestic Type 1 dryer exhaust the code requires rigid metallic duct with a smooth interior and a minimum nominal 4 in diameter; flexible foil or plastic is not acceptable per § 504.4.2 .
My dryer vent run is long; can I add an inline booster fan?
An exhaust duct power ventilator listed and installed per its manufacturer can be used; where such a listed ventilator is used, the maximum length may be permitted per the ventilator instructions (see § 504.4.2.1 Exception and § 504.4.2.3) .
Do I need makeup air for a dryer in a closet?
Yes. If a closet is provided for a Type 1 dryer, provide at least 100 square inches of opening for makeup air in the door or other approved means per § 504.4.1(1) .
Can multiple residential dryers tie into a common exhaust?
Only if the dryer manufacturer permits a common exhaust and the common duct meets the code (rigid metal, fire‑rated enclosure, cleanout, continuous exhaust fan downstream of branches, monitoring) — see § 504.4.4 and manufacturer instructions .
Are screen caps allowed on the exterior termination?
No. Screens are not allowed at dryer duct terminations and devices that obstruct airflow (like fire or smoke dampers) are prohibited in dryer exhaust systems per § 504.4 .
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