CRC · California Residential Code
When is exterior combustion air required and how must intake ducts be installed?
Most fireplaces must have a dedicated outside-air supply unless the room is mechanically ventilated to neutral/positive pressure. Factory-built fireplaces must use a listed combustion‑air duct installed per the manufacturer, and exterior intakes must supply all combustion air from outside (or from ventilated attic/crawl spaces), avoid garages and basements, not be higher than the firebox for non‑listed factory units, and be covered with a corrosion‑resistant 1/4‑inch (6.4 mm) mesh screen (see §§ R1006.1, R1006.1.1, R1006.2) .
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
- Factory-built or masonry fireplaces must be supplied with an exterior air supply unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive (§ R1006.1) .
- For factory-built fireplaces, exterior combustion-air ducts must be a listed component and installed per the manufacturer’s instructions (§ R1006.1.1) .
- The exterior air intake must be able to supply all combustion air from outside (or from interior spaces ventilated with outdoor air such as nonmechanically ventilated crawl or attic spaces), must not be located in the garage or basement, and (for non‑listed factory-built fireplaces) must not be at an elevation higher than the firebox; the intake must be covered with a corrosion‑resistant screen of 1/4‑inch (6.4 mm) mesh (§ R1006.2) .
All fireplaces (factory-built or masonry) need outside air unless the room has mechanical ventilation that keeps the room neutral/positive pressure — and factory-built systems must use the listed combustion‑air duct supplied or approved by the manufacturer (see § R1006.1, § R1006.1.1, § R1006.2) .
Requirements in detail
When exterior combustion air is required
- Default requirement: an exterior air supply is required for every factory-built or masonry fireplace covered in Chapter 10. See § R1006.1 for the basic rule and the exception for properly controlled mechanical ventilation .
- Exception: if the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive, an exterior air supply is not required (that exception is stated in § R1006.1) .
How intake ducts / exterior air intakes must be installed (what CRC requires)
- For factory‑built fireplaces: combustion‑air ducts must be a listed component of the fireplace and installed per the fireplace manufacturer’s instructions (§ R1006.1.1) .
- For masonry fireplaces: listed combustion‑air ducts must be installed in accordance with their listing and the manufacturer’s instructions (see § R1006.1.2) .
- Intake location and source: the exterior air intake must be capable of supplying all combustion air from the exterior or from interior spaces ventilated with outdoor air (examples named: nonmechanically ventilated crawl or attic spaces) (§ R1006.2) .
- Prohibited locations: the intake shall not be located within the garage or basement of the dwelling unit (§ R1006.2) .
- Elevation rule (non‑listed factory-built fireplaces): the intake shall not be located at an elevation higher than the firebox (§ R1006.2) .
- Screening: the exterior intake shall be covered with a corrosion‑resistant screen of 1/4‑inch (6.4 mm) mesh (§ R1006.2) .
Quick reference table — decision‑relevant dimensions/values
| Decision / value | Required value or rule | Code Reference |
|---|---|---|
| Is exterior air required? | Yes, unless room is mechanically ventilated and pressure is neutral/positive | § R1006.1 |
| Factory‑built fireplace duct requirement | Duct must be a listed component and installed per manufacturer | § R1006.1.1 |
| Source of intake air | Exterior of dwelling, or interior spaces ventilated with outdoor air (e.g., nonmechanical crawl/attic) | § R1006.2 |
| Prohibited intake locations | Garage or basement (intake shall not be located here) | § R1006.2 |
| Elevation restriction (non‑listed factory-built) | Intake must not be higher than the firebox | § R1006.2 |
| Required intake screen | Corrosion‑resistant, 1/4‑inch (6.4 mm) mesh | § R1006.2 |
What the CRC does NOT specify here (so do not assume)
- The California Residential Code text in the retrieved sections does not provide detailed duct construction materials, minimum cross‑sectional duct area, slope, or specific termination clearances beyond the items listed above. Where the CRC refers to manufacturer instructions or listings, follow those documents. For detailed construction, sizing, and other installation rules, the Mechanical and Plumbing Codes and the appliance/manufacturer instructions are commonly applicable (see Related provisions below) — the CRC sections cited above do not supply those details themselves .
Exceptions & special cases
- Mechanical ventilation exception: no exterior air required if the room is mechanically ventilated and controlled so that indoor pressure is neutral or positive (§ R1006.1) . Verify how the mechanical ventilation system affects pressure before omitting exterior intake.
- Manufacturer/listing controls: for factory‑built fireplaces, follow the listed combustion‑air duct supplied or specified by the manufacturer; do not substitute unlisted duct assemblies for the listed component (§ R1006.1.1) .
- Interior ventilated spaces allowed as source: the intake may draw from nonmechanically ventilated crawl or attic spaces that are ventilated with outdoor air — this is allowed as an alternative to a direct exterior opening (§ R1006.2) .
Common mistakes
- Installing a generic duct instead of the listed combustion‑air duct supplied or specified for a factory‑built fireplace (violates § R1006.1.1) .
- Locating the intake inside the garage or basement (both explicitly prohibited in § R1006.2) .
- Placing an intake for a non‑listed factory‑built fireplace at an elevation higher than the firebox (disallowed by § R1006.2) .
- Using improper screening (anything finer than 1/4‑inch (6.4 mm) mesh could clog more quickly; CRC requires corrosion‑resistant 1/4‑inch mesh in § R1006.2) .
- Assuming the CRC sections completely specify duct material, size, slope, or termination details — those details are typically in the manufacturer's instructions or other codes and are not spelled out in the cited CRC sections .
Worked example — concrete scenario
Scenario: A homeowner is installing a masonry fireplace in a living room that does not have mechanical ventilation controlling pressure.
- Rule application: Because the room is not mechanically ventilated to neutral/positive pressure, an exterior air supply is required (§ R1006.1) .
- Intake location: The installer plans an intake opening through an exterior wall, terminating at an exterior grille 8 inches above adjacent finished grade. This is acceptable from the CRC § R1006.2 standpoint because the intake is on the exterior (not in a garage or basement) and is lower than the firebox elevation (the intake is not higher than the firebox), and the intake will be covered with a corrosion‑resistant 1/4‑inch (6.4 mm) mesh screen as required by § R1006.2 .
- Duct selection: Because this is a masonry fireplace, any listed combustion‑air duct installed must be installed in accordance with its listing and manufacturer instructions (see § R1006.1.2) — the installer must verify the product listing and follow those instructions for sizing and attachment .
- Note: The CRC sections cited here do not give the exact duct diameter or material for every circumstance; the installer must follow the listed product documentation and applicable Mechanical/Plumbing Code instructions for detailed duct construction and sizing.
Related provisions
- § R1006.3 — Clearance for unlisted combustion‑air ducts (installation clearance to combustibles) .
- § R1006.4 — Combustion‑air passageway area limits (passageway not less than 6 square inches and not more than 55 square inches, except as permitted by manufacturer/listing) .
- § R1006.5 — Exterior air outlet location and closable outlet requirements (outlet permitted at back/side of firebox or within 24 inches (610 mm) of firebox opening; outlet shall be closable and prevent burning material entry to concealed spaces) .
(Those adjacent CRC sections provide installation detail that commonly applies together with § R1006.1, § R1006.1.1, and § R1006.2.)
Code references
Grounded in the retrieved California Residential Code — click a citation to read the verbatim passage:
CRC § 1.1 High relevance — show source text
SECTION R1006—EXTERIOR AIR SUPPLY
R1006.1 Exterior air. Factory-built or masonry fireplaces covered in this chapter shall be equipped with an exterior air supply to ensure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.
R1006.1.1 Factory-built fireplaces. Exterior combustion air ducts for factory-built fireplaces shall be a listed component of the fireplace and shall be installed in accordance with the fireplace manufacturer’s instructions.
R1006.1.2 Masonry fireplaces. Listed combustion air ducts for masonry fireplaces shall be installed in accordance with the terms of their listing and the manufacturer’s instructions.
R1006.2 Exterior air intake. The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling unit or from spaces within the dwelling unit ventilated with outdoor air such as nonmechanically ventilated crawl or attic spaces. The
10-12 2025 CALIFORNIA RESIDENTIAL CODE
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CHIMNEYS AND FIREPLACES
exterior air intake shall not be located within the garage or basement of the dwelling unit. The exterior air intake, for other than listed factory-built fireplaces, shall not be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of [1] / 4 -inch (6.4 mm) mesh.
R1006.3 Clearance. Unlisted c ombustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet. R1006.4 Passageway. The combustion air passageway shall be not less than 6 square inches (3870 mm [2] ) and not more than 55 square inches (0.035 m [2] ), except that combustion air systems for listed fireplaces shall be constructed in accordance with the fireplace manufacturer’s instructions.
R1006.5 Outlet. The exterior air outlet shall be located in the back or side of the firebox chamber or shall be located outside of the firebox, at the level of the hearth and not greater than 24 inches (610 mm) from the firebox opening. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces.
2025 CALIFORNIA RESIDENTIAL CODE 10-13
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Part IV—Energy Conservation
(Note: Part IV is not adopted. See California Energy Code, Title 24, Part 6.)
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Part V—Mechanical
(Note: Part V is not adopted. See California Mechanical Code, Title 24, Part 4.)
CRC § 25.4 High relevance — show source text
WOOD MANTEL
MASONRY
COMBUSTIBL
EDGE ABUTTI
12″ MIN FROM
2″ CLEAR
TO COMB
FRAME WCOMBUSTIBL
EDGE ABUTTI
12″ MIN FROM
2″ CLEAR
TO COMBCol3 COMBUSTIBL
EDGE ABUTTI
12″ MIN FROM
MASONRY
2″ CLEAR
TO COMB
FRAME WFor SI: 1 inch = 25.4 mm
2111 A .13 Fireplace fireblocking. All spaces between fireplaces and floors and ceilings through which fireplaces pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between wood joists, beams or headers shall be to a depth of 1 inch (25 mm) and shall only be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.
2111 A .14 Exterior air. Factory-built or masonry fireplaces covered in this section shall be equipped with an exterior air supply to ensure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.
2111 A .14.1 Factory-built fireplaces. Exterior combustion air ducts for factory-built fireplaces shall be listed components of the fireplace, and installed according to the fireplace manufacturer’s instructions.
21A-12 2025 CALIFORNIA BUILDING CODE
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MASONRY
2111 A .14.2 Masonry fireplaces. Listed combustion air ducts for masonry fireplaces shall be installed according to the terms of their listing and manufacturer’s instructions.
2111 A .14.3 Exterior air intake. The exterior air intake shall be capable of providing all combustion air from the exterior of the dwelling. The exterior air intake shall not be located within a garage, attic, basement or crawl space of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of [1] / 4 -inch (6.4 mm) mesh.
2111 A .14.4 Clearance. Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet. 2111 A .14.5 Passageway. The combustion air passageway shall be not less than 6 square inches (3870 mm [2] ) and not more than 55 square inches (0.035 m [2] ), except that combustion air systems for listed fireplaces or for fireplaces tested for emissions shall be constructed according to the fireplace manufacturer’s instructions.
2111 A .14.6 Outlet. The exterior air outlet is permitted to be located in the back or sides of the firebox chamber or within 24 inches (610 mm) of the firebox opening on or near the floor. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces.
SECTION 2112 A —MASONRY HEATERS
CRC § 0.52 High relevance — show source text
R1005.6 Medium-heat appliances. Factory-built chimneys for medium-heat appliances producing flue gases having a temperature above 1,000°F (538°C), measured at the entrance to the chimney, shall comply with UL 959.
R1005.7 Factory-built chimney offsets. Where a factory-built chimney assembly incorporates offsets, no part of the chimney shall be at an angle of more than 30 degrees (0.52 rad) from vertical at any point in the assembly and the chimney assembly shall not include more than four elbows.
R1005.8 Insulation shield. Where factory-built chimneys pass through insulated assemblies, an insulation shield constructed of steel having a thickness of not less than 0.0187 inch (0.4712 mm) (No. 26 gage) shall be installed to provide clearance between the chimney and the insulation material. The clearance shall be not less than the clearance to combustibles specified by the chimney manufacturer’s installation instructions. Where chimneys pass through attic space, the shield shall terminate not less than 2 inches (51 mm) above the insulation materials and shall be secured in place to prevent displacement. Insulation shields provided as part of a listed chimney system shall be installed in accordance with the manufacturer’s installation instructions.
R1005.9 Factory-built chimney offsets. Where a fireplace manufacturer’s instructions do not address factory-built chimney offsets, no part of the chimney shall be at an angle of more than 30 degrees (0.52 rad) from vertical at any point in the assembly and the chimney assembly shall not include more than four elbows.
SECTION R1006—EXTERIOR AIR SUPPLY
R1006.1 Exterior air. Factory-built or masonry fireplaces covered in this chapter shall be equipped with an exterior air supply to ensure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.
R1006.1.1 Factory-built fireplaces. Exterior combustion air ducts for factory-built fireplaces shall be a listed component of the fireplace and shall be installed in accordance with the fireplace manufacturer’s instructions.
R1006.1.2 Masonry fireplaces. Listed combustion air ducts for masonry fireplaces shall be installed in accordance with the terms of their listing and the manufacturer’s instructions.
R1006.2 Exterior air intake. The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling unit or from spaces within the dwelling unit ventilated with outdoor air such as nonmechanically ventilated crawl or attic spaces. The
10-12 2025 CALIFORNIA RESIDENTIAL CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
CHIMNEYS AND FIREPLACES
exterior air intake shall not be located within the garage or basement of the dwelling unit. The exterior air intake, for other than listed factory-built fireplaces, shall not be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of [1] / 4 -inch (6.4 mm) mesh.
CRC § 506.8 High relevance — show source text
506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
2025 CALIFORNIA PLUMBING CODE 87
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WATER HEATERS
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
[NFPA 54:9.3.8.8]
(7) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
CRC § 14.3 High relevance — show source text
2111 A .14.3 Exterior air intake. The exterior air intake shall be capable of providing all combustion air from the exterior of the dwelling. The exterior air intake shall not be located within a garage, attic, basement or crawl space of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of [1] / 4 -inch (6.4 mm) mesh.
2111 A .14.4 Clearance. Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet. 2111 A .14.5 Passageway. The combustion air passageway shall be not less than 6 square inches (3870 mm [2] ) and not more than 55 square inches (0.035 m [2] ), except that combustion air systems for listed fireplaces or for fireplaces tested for emissions shall be constructed according to the fireplace manufacturer’s instructions.
2111 A .14.6 Outlet. The exterior air outlet is permitted to be located in the back or sides of the firebox chamber or within 24 inches (610 mm) of the firebox opening on or near the floor. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces.
SECTION 2112 A —MASONRY HEATERS
2112 A .1 Definition. A masonry heater is a heating appliance constructed of concrete or solid masonry, hereinafter referred to as “masonry,” which is designed to absorb and store heat from a solid fuel fire built in the firebox by routing the exhaust gases through internal heat exchange channels in which the flow path downstream of the firebox includes flow in either a horizontal or downward direction before entering the chimney and which delivers heat by radiation from the masonry surface of the heater.
2112 A .2 Installation. Masonry heaters shall be installed in accordance with this section and comply with one of the following:
- Masonry heaters shall comply with the requirements of ASTM E1602.
- Masonry heaters shall be listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
2112 A .3 Footings and foundation. The firebox floor of a masonry heater shall be a minimum thickness of 4 inches (102 mm) of noncombustible material and be supported on a noncombustible footing and foundation in accordance with Section 2113 A .2.
2112 A .4 Seismic reinforcing. In structures assigned to Seismic Design Category D, E or F, masonry heaters shall be anchored to the masonry foundation in accordance with Section 2113 A .3. Seismic reinforcing shall not be required within the body of a masonry heater with a height that is equal to or less than 3.5 times its body width and where the masonry chimney serving the heater is not supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section 2113 A .
CRC § 9.3.8.1 High relevance — show source text
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
(7) The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
[NFPA 54:9.3.8.8]
701.12 Dampers Prohibited. Combustion air ducts or plenums shall not be installed so as to require openings in or penetrations through construction where fire dampers are required. Manually operated dampers shall not be installed in combustion air openings. With prior approval, power-actuated movable louvers admitting combustion air shall be permitted to be used and, where installed, shall be electrically interlocked with the main burner fuel-supply valve so as to prevent fuel delivery unless the louvers are in the fully open position.
702.0 Extra Device or Attachment.
702.1 General. No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas. [NFPA 54:9.1.15]
150 2025 CALIFORNIA MECHANICAL CODE
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CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CHAPTER 8 – CHIMNEYS AND VENTS
(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)
CRC § 25.4 High relevance — show source text
2 ″ CLEARANCE (AIRSPACE) TO COMBUSTIBLE FRAMING
For SI: 1 inch = 25.4 mm
2111.13 Fireplace fireblocking. All spaces between fireplaces and floors and ceilings through which fireplaces pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between wood joists, beams or headers shall be to a depth of 1 inch (25 mm) and shall only be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.
2111.14 Exterior air. Factory-built or masonry fireplaces covered in this section shall be equipped with an exterior air supply to ensure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.
2111.14.1 Factory-built fireplaces. Exterior combustion air ducts for factory-built fireplaces shall be listed components of the fireplace, and installed according to the fireplace manufacturer’s instructions.
MASONRY
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EDGE ABUTTI
12″ MIN FROM
2″ CLEAR
TO COMB
FRAME W
WOOD MANT
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r masonry fireplaces covered in this sect
s the room is mechanically ventilated an
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COMBUSTIBL
EDGE ABUTTI
12″ MIN FROM
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2″ CLEAR
TO COMB
FRAME W
WOOD MANT2111.14.2 Masonry fireplaces. Listed combustion air ducts for masonry fireplaces shall be installed according to the terms of their listing and manufacturer’s instructions.
2111.14.3 Exterior air intake. The exterior air intake shall be capable of providing all combustion air from the exterior of the dwelling. The exterior air intake shall not be located within a garage, attic, basement or crawl space of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of [1] / 4 -inch (6.4 mm) mesh.
2111.14.4 Clearance. Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet. 2111.14.5 Passageway. The combustion air passageway shall be not less than 6 square inches (3870 mm [2] ) and not more than 55 square inches (0.035 m [2] ), except that combustion air systems for listed fireplaces or for fireplaces tested for emissions shall be constructed according to the fireplace manufacturer’s instructions.
CRC § 9.3.8.7 High relevance — show source text
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
[NFPA 54:9.3.8.8]
701.12 Dampers Prohibited. Combustion air ducts or plenums shall not be installed so as to require openings in or penetrations through construction where fire dampers are required. Manually operated dampers shall not be installed in combustion air openings. With prior approval, power-actuated movable louvers admitting combustion air shall be permitted to be used and, where installed, shall be electrically interlocked with the main burner fuel-supply valve so as to prevent fuel delivery unless the louvers are in the fully open position.
702.0 Extra Device or Attachment.
702.1 General. No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas. [NFPA 54:9.1.15]
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), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CHAPTER 8 – CHIMNEYS AND VENTS
(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)
Adopting Agency BSC BSC-
CGSFM HCD Col6 Col7 DSA Col9 Col10 OSHPD Col12 Col13 Col14 Col15 Col16 Col17 BSCC DPH AGR DWR CEC CA SL SLC Adopting Agency BSC BSC-
CGSFM 1 2 1-AC AC ** SS** ** SS/CC** 1 1R 2 3 4 5 6 6 6 6 6 6 6 6 6 Adopt Entire Chapter X X X X X X X X X X X X Adopt Entire Chapter as
amended (amended sections
listed below)Adopt only those sections
that are listed belowChapter/Section CRC § 2111.14.2 High relevance — show source text
The fireblock
nd shall only be placed on strips of metal
r masonry fireplaces covered in this sect
s the room is mechanically ventilated an
es. Exterior combustion air ducts for fact
o the fireplace manufacturer’s instruction
COMBUSTIBL
EDGE ABUTTI
12″ MIN FROM
MASONRY
2″ CLEAR
TO COMB
FRAME W
WOOD MANT|||2111.14.2 Masonry fireplaces. Listed combustion air ducts for masonry fireplaces shall be installed according to the terms of their listing and manufacturer’s instructions.
2111.14.3 Exterior air intake. The exterior air intake shall be capable of providing all combustion air from the exterior of the dwelling. The exterior air intake shall not be located within a garage, attic, basement or crawl space of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of [1] / 4 -inch (6.4 mm) mesh.
2111.14.4 Clearance. Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet. 2111.14.5 Passageway. The combustion air passageway shall be not less than 6 square inches (3870 mm [2] ) and not more than 55 square inches (0.035 m [2] ), except that combustion air systems for listed fireplaces or for fireplaces tested for emissions shall be constructed according to the fireplace manufacturer’s instructions.
2111.14.6 Outlet. The exterior air outlet is permitted to be located in the back or sides of the firebox chamber or within 24 inches (610 mm) of the firebox opening on or near the floor. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces.
SECTION 2112—MASONRY HEATERS
2112.1 Definition. A masonry heater is a heating appliance constructed of concrete or solid masonry, hereinafter referred to as “masonry,” which is designed to absorb and store heat from a solid fuel fire built in the firebox by routing the exhaust gases through internal heat exchange channels in which the flow path downstream of the firebox includes flow in either a horizontal or downward direction before entering the chimney and which delivers heat by radiation from the masonry surface of the heater.
2112.2 Installation. Masonry heaters shall be installed in accordance with this section and comply with one of the following:
- Masonry heaters shall comply with the requirements of ASTM E1602.
- Masonry heaters shall be listed and labeled in accordance with UL 1482 or EN 15250 and installed in accordance with the manufacturer’s instructions.
2112.3 Footings and foundation. The firebox floor of a masonry heater shall be a minimum thickness of 4 inches (102 mm) of noncombustible material and be supported on a noncombustible footing and foundation in accordance with Section 2113.2.
2112.4 Seismic reinforcing. In structures assigned to Seismic Design Category D, E or F, masonry heaters shall be anchored to the masonry foundation in accordance with Section 2113.3. Seismic reinforcing shall not be required within the body of a masonry heater with a height that is equal to or less than 3.5 times its body width and where the masonry chimney serving the heater is not
CRC § 9.3.7.1 High relevance — show source text
Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
2025 CALIFORNIA MECHANICAL CODE 149
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COMBUSTION AIR
701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
(7) The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
CRC § 506.4 Medium relevance — show source text
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air
openings shall not be less than 3 inches (76 mm).
[NFPA 54:9.3.4(3)]
506.6 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5]
506.7 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
506.7.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]
506.7.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]
506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Frequently asked questions
Do I always have to run a metal duct to the outside?
No — the CRC requires an exterior air supply and, for factory‑built fireplaces, that the combustion‑air duct be a listed component installed per the manufacturer (§ R1006.1.1) . The CRC text retrieved does not mandate a specific duct material in those controlling sections; follow the listed component/manufacturer instructions and applicable Mechanical/Plumbing Code provisions for duct construction.
Can the intake open into my garage or basement?
No. The exterior air intake shall not be located within the garage or basement of the dwelling unit (this is explicit in § R1006.2) .
If my house has an exhaust fan, can I skip the exterior intake?
Only if the room is mechanically ventilated and controlled so that indoor pressure is neutral or positive; that is the exception in § R1006.1. Don’t assume all exhaust fans create neutral/positive pressure — verify the ventilation system’s performance first .
How fine must the intake screen be?
The CRC requires a corrosion‑resistant screen of 1/4‑inch (6.4 mm) mesh over the exterior intake (§ R1006.2) .
Can I run the intake into an attic or crawl space?
Yes — the intake may draw from spaces within the dwelling unit ventilated with outdoor air such as nonmechanically ventilated crawl or attic spaces, per § R1006.2 — but ensure those spaces actually receive outdoor ventilation and check other applicable code provisions and manufacturer instructions .
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