CMC · California Mechanical Code
What are the indoor opening size and location requirements?
If your gas appliances will draw combustion air from adjacent indoor spaces, the CMC **§ 701.5** requires two permanent openings (one near the top and one near the bottom of the enclosure), and **each** opening must provide **1 square inch per 1,000 Btu/h** of the total appliance input (but **no smaller than 100 in²**), with no opening dimension smaller than **3 inches**; connected spaces on different stories have a separate total-area rule of **2 in² per 1,000 Btu/h**.
Last reviewed: July 6, 2026
What the code requires — plain English
Openings that connect indoor spaces to supply combustion air must be permanent openings sized so each opening provides at least 1 square inch per 1,000 Btu/h of the total appliance input (but not less than 100 square inches), and one opening must start within 12 inches (305 mm) of the top of the enclosure and one within 12 inches (305 mm) of the bottom. The minimum linear dimension of any required air opening is 3 inches (76 mm). These rules are the text of § 701.5 of the California Mechanical Code (CMC) .
For indoor combustion-air openings on the same story: provide two permanent openings (one near the top, one near the bottom), each sized at 1 in² per 1,000 Btu/h of total appliance input — but never smaller than 100 in² and never with any dimension less than 3 in. (See § 701.5.)
Requirements in detail
Key thresholds and locations (first mention bolded)
- Controlling section: § 701.5 (CMC) — indoor opening size and location.
- Sizing same story: 1 square inch per 1,000 Btu/h of the total input rating of all appliances in the space (each opening), but not less than 100 square inches. § 701.5(1).
- Location (same story): One permanent opening commencing within 12 inches (305 mm) of the top of the enclosure and one commencing within 12 inches (305 mm) of the bottom of the enclosure. § 701.5(1).
- Minimum linear dimension: 3 inches (76 mm). § 701.5(1).
- Combining different stories: Where spaces on different stories are used, treat their volumes as communicating only if connected by one or more permanent openings having a total minimum free area of 2 square inches per 1,000 Btu/h of the total input rating. § 701.5(2).
Decision-relevant dimensions — quick reference table
| Decision question | Required value (bold where first used) | Code Reference |
|---|---|---|
| Each indoor opening size (same story) | 1 in² per 1,000 Btu/h of total appliance input (each opening) | § 701.5(1) |
| Minimum allowed area for each opening | 100 in² (0.065 m²) (whichever is larger) | § 701.5(1) |
| Required location of the two openings | One opening starting within 12 in (305 mm) of the top, one within 12 in (305 mm) of the bottom of the enclosure | § 701.5(1) |
| Minimum linear dimension of openings | 3 in (76 mm) | § 701.5(1) |
| Openings connecting different stories (total) | 2 in² per 1,000 Btu/h of total appliance input (total free area of permanent openings) | § 701.5(2) |
Notes about terms and net area
- "Free area" means the net open area available for air passage (louvers/grilles/screens must be sized based on their net free area) — see § 701.10 for the requirement that required opening size is based on net free area. § 701.10 and the chapter headings show this relationship in the CMC.
Exceptions & special cases
- Combining spaces on different stories: you may treat separated volumes as communicating spaces only when connected by one or more permanent openings whose combined free area meets the 2 in² per 1,000 Btu/h requirement in § 701.5(2). Failure to meet that total means you cannot count the other story’s volume as part of the supply.
- The code requires permanent openings (openings that are not routinely closed). If you plan to use a mechanical or engineered approach (mechanical air supply, ducts sized per other sections), see § 701.9 and § 701.8 for engineered or mechanical alternatives. These are separate methods and have their own sizing/interlock requirements.
- When combining indoor and outdoor combustion air allowances, the outdoor opening sizing may be reduced using the ratio-of-volumes method in § 701.7.3 — see that section for the reduction factor calculation (and Appendix examples) if part of the combustion air will come from outdoors.
Common mistakes
- Assuming the 100 in² minimum is a total for both openings instead of the minimum for each opening (each required opening must meet the sizing rule). § 701.5(1).
- Designing openings with any linear dimension smaller than 3 inches (76 mm) — even if area-wise the opening is large enough. § 701.5(1).
- Using movable dampers or otherwise restricting required openings — dampers that reduce required free area are not allowed in combustion-air openings (see related provisions: § 701.12 lists dampers prohibited).
- Forgetting to size openings by net free area (the louver/grille manufacturer’s free-area rating), rather than gross dimensions. See § 701.10 on louvers/grilles and net free area.
- Counting the volume of another story without meeting the 2 in² per 1,000 Btu/h total free area requirement for the openings between stories (i.e., not meeting § 701.5(2)).
Worked example
Scenario: A mechanical room contains two gas appliances whose combined input is 140,000 Btu/h. The appliances are in the same story and the installer plans to provide combustion air from adjacent indoor spaces through permanent openings.
Step 1 — Sizing each required opening (same-story rule):
- Required area per opening = 1 in² per 1,000 Btu/h × 140,000 Btu/h = 140 in². Because 140 in² is greater than the minimum 100 in², each opening must be 140 in². § 701.5(1).
Step 2 — Location:
- Provide one permanent opening that commences within 12 in (305 mm) of the top of the enclosure and one that commences within 12 in (305 mm) of the bottom. § 701.5(1).
Step 3 — Check minimum linear dimension:
- Ensure neither opening has any linear side less than 3 in (76 mm). § 701.5(1).
Practical grille option:
- Choose a grille whose net free area is at least 140 in². If a grille/louver has a net free-area rating of 50% (manufacturer’s data), the gross opening would need to be 280 in² (for example, approx. 14" × 20"), because net free area is the metric the code uses for required sizes. See § 701.10 regarding net free area.
Related provisions (CMC sections)
- § 701.6 — Outdoor combustion air (methods and sizing when air comes from outdoors).
- § 701.7 — Combination of indoor and outdoor combustion air; § 701.7.1 references that indoor openings used under combination methods must comply with § 701.5.
- § 701.8 — Engineered installations (allows engineered determinations of combustion/dilution air).
- § 701.9 — Mechanical combustion air supply (alternate method using a mechanical air supply and required interlocks).
- § 701.10 — Louvers, grilles, and screens (required size based on net free area).
- § 701.11 — Combustion air ducts (where ducts are used rather than direct openings).
- § 701.12 — Dampers prohibited (restrictions on devices that reduce required openings).
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 701.4.2 High relevance — show source text
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 701.4.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 701.4.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
I other = All appliances other than fan-assisted input (Btu/h) (kW) I = Fan-assisted appliance input (Btu/h) (kW) fan ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h
( )
For SI units: 1 cubic foot = 0.0283 m [3], 1000 British thermal units per
hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 701.4.2(1) and Equation 701.4.2(2). [NFPA 54:9.3.2.2(3)]
701.5 Indoor Opening Size and Location. Openings used to connect indoor spaces shall be sized and located in accordance with the following: (1) Combining spaces on the same story. Each opening shall have a minimum free area of 1 square inch per 1000 Btu/h (0.002 m [2] /kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.065 m [2] ). One permanent opening shall commence within 12 inches (305 mm) of the top of the enclosure and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 701.5). The minimum dimension of air openings shall not be less than 3 inches (76 mm). (2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more permanent openings in doors or floors having a total minimum free area of 2 square inches per 1000 Btu/h (0.004 m [2] /kw) of total input rating of all appliances. [NFPA 54:9.3.2.3] 701.6 Outdoor Combustion Air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accor
2025 CALIFORNIA MECHANICAL CODE 147
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COMBUSTION AIR
dance with the methods in Section 701.6.1 or Section 701.6.2. The minimum dimension of air openings shall not be less than
FIGURE 701.5
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
CMC § 701.6.2 High relevance — show source text
The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 701.6.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2] 701.7 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 701.7.1 through Section 701.7.3. [NFPA 54:9.3.4] (see Appendix F for example calculations)
FIGURE 701.6.1(1) ALL COMBUSTION AIR FROM OUTDOORS – INLET
AIR FROM VENTILATED CRAWL SPACE AND OUTLET AIR TO
VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(a)]
FIGURE 701.6.1(2)
ALL COMBUSTION AIR FROM OUTDOORS
THROUGH VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(b)]
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COMBUSTION AIR
FIGURE 701.6.1(3) ALL COMBUSTION AIR FROM OUTDOORS
THROUGH HORIZONTAL DUCTS
[NFPA 54: FIGURE A.9.3.3.1(2)]
FIGURE 701.6.2
ALL COMBUSTION AIR FROM OUTDOORS THROUGH
SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
701.7.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 701.5. [NFPA 54:9.3.4(1)]
701.7.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 701.6. [NFPA 54:9.3.4(2)]
701.7.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 701.6, multiplied by the reduction factor. The minimum dimension of air openings shall not be less than 3 inches (76 mm).
CMC § 0.293 High relevance — show source text
per hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 506.2.2(1) and Equation 506.2.2(2).
[NFPA 54:9.3.2.2(3)] 506.3 Indoor Opening Size and Location. Openings used to connect indoor spaces shall be sized and located in accordance with the following:
(1) Combining spaces on the same story. Each opening shall have a minimum free area of 1 square inch per 1000 Btu/h (0.002 m [2] /kW) of the total input rating of all appliances in the space but not less than 100 square inches (0.065 m [2] ). One permanent opening shall commence within 12 inches (305 mm) of the top of the enclosure and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 506.3). The minimum dimension of air openings shall not be less than 3 inches (76 mm).
(2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more permanent openings in doors or floors having a total minimum free area of 2 square inches per 1000 Btu/h (0.004 m [2] /kW) of total input rating of all appliances. [NFPA 54:9.3.2.3]
FIGURE 506.3
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
THROUGH INDOOR COMBUSTION AIR OPENINGS
[NFPA 54: FIGURE A.9.3.2.3(1)]
506.4 Outdoor Combustion Air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with the methods in Section 506.4.1 or Section 506.4.2. The minimum dimension of air openings shall not be less than 3 inches (76 mm). [NFPA 54:9.3.3] 506.4.1 Two Permanent Openings Method. Two permanent openings, one commencing within 12 inches
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WATER HEATERS
FIGURE 506.4.1(1)
ALL COMBUSTION AIR FROM OUTDOORS –
INLET AIR FROM VENTILATED CRAWL SPACE AND
OUTLET AIR TO VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(a)]
For SI units: 1 foot = 304.8 mm
FIGURE 506.4.1(2)
ALL COMBUSTION AIR FROM OUTDOORS
THROUGH VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(b)]
FIGURE 506.4.1(3)
ALL COMBUSTION AIR FROM
OUTDOORS THROUGH HORIZONTAL DUCTS
[NFPA 54: FIGURE A.9.3.3.1(2)]
FIGURE 506.4.2
CMC § 1.5. High relevance — show source text
For openings less than or equal to 6 feet in
height that have both a head track and a sill track, the spans are permitted to be multiplied by a factor of 1.5.
d. Minimum Grade 33 ksi steel shall be used for 33 mil and 43 mil thicknesses. Minimum Grade 50 ksi steel shall be used for 54 and 68 mil thicknesses.|For SI: 1 mil = 0.0254 mm, 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s, 1 ksi = 1,000 psi = 6.895 MPa.
a. Deflection limit:L/240.
b. Head and sill track spans are based on components and cladding wind pressures and 48-inch tributary span.
c. For openings less than 4 feet in height that have both a head track and sill track, the spans are permitted to be multiplied by 1.75. For openings less than or equal to 6 feet in
height that have both a head track and a sill track, the spans are permitted to be multiplied by a factor of 1.5.
d. Minimum Grade 33 ksi steel shall be used for 33 mil and 43 mil thicknesses. Minimum Grade 50 ksi steel shall be used for 54 and 68 mil thicknesses.|For SI: 1 mil = 0.0254 mm, 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s, 1 ksi = 1,000 psi = 6.895 MPa.
a. Deflection limit:L/240.
b. Head and sill track spans are based on components and cladding wind pressures and 48-inch tributary span.
c. For openings less than 4 feet in height that have both a head track and sill track, the spans are permitted to be multiplied by 1.75. For openings less than or equal to 6 feet in
height that have both a head track and a sill track, the spans are permitted to be multiplied by a factor of 1.5.
d. Minimum Grade 33 ksi steel shall be used for 33 mil and 43 mil thicknesses. Minimum Grade 50 ksi steel shall be used for 54 and 68 mil thicknesses.|For SI: 1 mil = 0.0254 mm, 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s, 1 ksi = 1,000 psi = 6.895 MPa.
a. Deflection limit:L/240.
b. Head and sill track spans are based on components and cladding wind pressures and 48-inch tributary span.
c. For openings less than 4 feet in height that have both a head track and sill track, the spans are permitted to be multiplied by 1.75. For openings less than or equal to 6 feet in
height that have both a head track and a sill track, the spans are permitted to be multiplied by a factor of 1.5.
d. Minimum Grade 33 ksi steel shall be used for 33 mil and 43 mil thicknesses.CMC § 506.4.2 High relevance — show source text
The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 506.4.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2]
506.5 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 506.5.1 through Section 506.5.3. [NFPA 54:9.3.4] (See Appendix J for example calculations) 506.5.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 506.3. [NFPA 54:9.3.4(1)] 506.5.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 506.4. [NFPA 54:9.3.4(2)] 506.5.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 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]
CMC § 4.6 High relevance — show source text
(b) Vertical half-scale wall furnace. The furnace exposure panel or door shall consist of an insulated steel restraining frame having an available opening of not less than 50 square feet (4.6 m [2] ) for the test sample. Neither dimension of the furnace opening shall be less than 7 feet (2133 mm).
(c) Horizontal large-scale floor furnace. The furnace exposure panel shall consist of an insulated steel restraining frame having an available opening of not less than 180 square feet (16.7 m [2] ) for the test sample. Neither dimension of the furnace opening shall be less than 12 feet (3657 mm).
(d) Horizontal small-scale furnace. The furnace exposure panel shall consist of an insulated frame having an available opening of not less than 35 square feet (3 m [2] ) for the test sample. Neither dimension of the furnace opening shall be less than 5 feet (1524 mm).
(e) Horizontal large-scale beam furnace. The furnace exposure panel shall consist of an insulated steel restraining frame having an available opening of not less than 180 square feet (16.7 m [2] ) for the test sample. Neither dimension of the furnace opening shall be less than 5 feet (1524 mm).
(f) Horizontal small-scale beam furnace. The furnace exposure panel for the “Alternate Test of Protection for Structural Steel Beams, Girders and Trusses” shall consist of an insulated steel frame having an available opening of not less than 35 square feet (3 m [2] ) for the test sample. Neither dimension of the furnace opening shall be less than 5 feet (1524 mm).
(g) Column furnace. The column furnace shall be of such dimensions as to provide an opening for column sections not less than 8 feet (2438 mm) in clear length.
(h) Protection of equipment and test specimen. The testing furnaces, equipment and test specimen undergoing the fire test shall be protected from any condition of wind or weather, that might lead to abnormal results. The ambient air temperature of the testing room at the beginning of the test shall be within the range of 50°F to 90°F (10°C to 32°C). Velocity of air across the unexposed face of the test specimen shall not exceed 4.4 feet per second, as determined by an anemometer placed at right angles to the unexposed surface, measured before or during the test. If mechanical ventilation is employed during the test, an airstream shall not be directed across the surface of the specimen.
BURNERS AND FUEL
Sec. 12-7-302.
(a) Burners.
- In vertical furnaces, burners shall be placed in the back wall of the furnace. The location of the burners and provisions for combustion air shall be such as to provide an even flame exposure to the entire exposed face of the test specimen. Combustion air openings shall be provided in such a manner as to normally prevent induction of combustion air through any opening in the test specimen.
- In horizontal furnaces, burners shall be placed in the floor or side walls. Burrs and the provisions for combustion air shall be so arranged as to provide a uniform exposure to the entire exposed face of the test specimen.
- In column furnaces, burners shall be placed in the four walls to provide an even luminous flame exposure to all sides of the test sample.
(b) Fuel. Furnaces shall be supplied with natural, manufactured or bottled gas.
2025 CALIFORNIA REFERENCED STANDARDS CODE 25
CMC § 608.9 High relevance — show source text
608.9 Floor Registers . . . . . . . . . . . . . . . 143
608.10 Exterior Wall and Interior Stud
Partitions. . . . . . . . . . . . . . . . . . . . 143
608.11 Wall Register . . . . . . . . . . . . . . . . 143
608.12 Distance from Combustible . . . . . 143
608.13 Vapor Barrier . . . . . . . . . . . . . . . . 143
608.14 Prohibited . . . . . . . . . . . . . . . . . . . 143
609.0 Automatic Shutoffs . . . . . . . . . . . . 143
609.1 Air-Moving Systems and Smoke Detectors . . . . . . . . . . . . . . . . . . . 143
609.2 Air-Moving Systems and Smoke Detectors in Group I-2 Occupancies . . . . . . . . . . . . . . . . 144
CHAPTER 7 COMBUSTION AIR . . . . . . . . . . . 145
701.0 General. . . . . . . . . . . . . . . . . . . . . 147
701.1 Applicability. . . . . . . . . . . . . . . . . . 147
701.2 Pressure Difference . . . . . . . . . . . 147
701.3 Makeup Air . . . . . . . . . . . . . . . . . . 147
701.4 Indoor Combustion Air . . . . . . . . . 147
701.5 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . 147
701.6 Outdoor Combustion Air . . . . . . . . 147
701.7 Combination Indoor and Outdoor
Combustion Air . . . . . . . . . . . . . . . 148
701.8 Engineered Installations . . . . . . . . 149
701.9 Mechanical Combustion Air
Supply. . . . . . . . . . . . . . . . . . . . . . 149
701.10 Louvers, Grilles, and Screens . . . 149
701.11 Combustion Air Ducts. . . . . . . . . . 150
701.12 Dampers Prohibited . . . . . . . . . . . 150
702.0 Extra Device or Attachment . . . . . 150
702.1 General. . . . . . . . . . . . . . . . . . . . . 150
CHAPTER 8 CHIMNEYS AND VENTS. . . . . . . 151
801.0 General. . . . . . . . . . . . . . . . . . . . . 153
801.1 Applicability. . . . . . . . . . . . . . . . . . 153
801.2 Venting of Gas Appliances . . . . . . 153
801.3 Appliances Fueled by Other Fuels. 153
802.0 Venting of Appliances. . . . . . . . . . 153
xxxiv
CMC § 314.2 High relevance — show source text
1|314.2| |20 x 24|161⁄2 x 201⁄2|22.0|380.1| |24 x 24|201⁄4 x 201⁄4|22.1|380.1| |24 x 24|201⁄4 x 201⁄4|24.0|452.3| |24 x 28|201⁄4 x 241⁄4|24.1|456.2| |28 x 28|241⁄4 x 241⁄4|26.4|543.3| |28 x 28|241⁄4 x 241⁄4|27.0|572.5| |30 x 30|251⁄2 x 251⁄2|27.9|607.0| |30 x 30|251⁄2 x 251⁄2|30.0|706.8| |30 x 36|251⁄2 x 311⁄2|30.9|749.9| |30 x 36|251⁄2 x 311⁄2|33.0|855.3| |36 x 36|311⁄2 x 311⁄2|34.4|929.4| |36 x 36|311⁄2 x 311⁄2|36.0|1017.9|
For SI units, 1 inch. = 25.4 mm, 1 square inch = 0.000645 m [2]
- Where liner sizes differ dimensionally from those shown in this table, equivalent diameters can be determined from published tables for square and rectangular ducts of equivalent carrying capacity or by other engineering methods.
542 2025 CALIFORNIA MECHANICAL CODE
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APPENDIX F
(2) Determine the total required volume.
The Standard Method to determine combustion air is used to calculate the required volume.
The combined input for the appliances located in the basement is calculated as follows:
100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
The Standard Method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m [3] /kW) .
Using Table F 103.0 the required volume for a 140 000 Btu/h (41 kW) combined input is 7000 cubic feet (198.22 m [3] ) .
Conclusion:
The indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m [3] ) does not meet the required volume of 7000 cubic feet (198.22 m [3] ) . Therefore, additional combustion air must be provided from the outdoors.
(3) Determine the ratio of the available volume to the required volume:
3600 ft [3]
= 0.51 7000 ft [3]
(4) Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on ratio of indoor spaces:
1.00 – 0.51 (from Step 3) = 0.49
(5) Determine the single outdoor combustion air opening size as though all combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors:
CMC § 101.0 High relevance — show source text
2025 CALIFORNIA PLUMBING CODE 487
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488 2025 CALIFORNIA PLUMBING CODE
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APPENDIX J
COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND
VENTILATION OPENING DESIGN
The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.
J 101.0 General.
J 101.1 Applicability. This appendix provides general guidelines for the sizing of combination indoor and outdoor combustion and ventilation air openings. J 101.2 Example of Combination Indoor and Outdoor Combustion Air Opening Design. Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a drafthood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces can be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 British thermal units per hour (Btu/h) (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
(1) Determine the total available room volume. Appliance room volume:
15 feet by 30 feet (4572 mm by 9144 mm) with an 8 foot (2438 mm) ceiling = 3600 cubic feet (101.94 m [3] )
(2) Determine the total required volume. The standard method to determine combustion air is used to calculate the required volume. The combined input for the appliances located in the basement is calculated as follows:
100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
The standard method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m [3] /kW). Using Table J 101.2, the required volume for a 140 000 Btu/h (41 kW) combined input is 7000 cubic feet (198.22 m [3] ).
Conclusion: The indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m [3] ) does not meet the required volume of 7000 cubic feet (198.22 m [3] ). Therefore, additional combustion air shall be provided from the outdoors.
(3) Determine ratio of the available volume to the required
volume: Col2 3600 cubic feet = 0.51 7000 cubic feet 7000 cubic feet (4) Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on the ratio of indoor spaces:
1.00 – 0.51 (from Step 3) = 0.49
(5) Determine the single outdoor combustion air opening size as though all combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors:
CMC § 505.1 Medium relevance — show source text
505.1 Water Heaters . . . . . . . . . . . . . . . .84
505.2 Safety Devices . . . . . . . . . . . . . . . .84
505.3 Oil-Fired Water Heaters . . . . . . . . .84
505.4 Indirect-Fired Water Heaters . . . . .84
506.0 Air for Combustion and
Ventilation . . . . . . . . . . . . . . . . . . . .84
506.1 General . . . . . . . . . . . . . . . . . . . . . .84
506.2 Indoor Combustion Air . . . . . . . . . .85
506.3 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . .85
506.4 Outdoor Combustion Air . . . . . . . . .85
506.5 Combination Indoor and
Outdoor Combustion Air . . . . . . . . .87
506.6 Engineered Installations . . . . . . . . .87
506.7 Mechanical Combustion Air Supply . . . . . . . . . . . . . . . . . . . . . . .87
506.8 Louvers, Grilles, and Screens . . . .87
506.9 Combustion Air Ducts . . . . . . . . . . .87
507.0 Appliance and Equipment Installation Requirements . . . . . . . .88
507.1 Dielectric Insulator . . . . . . . . . . . . .88
507.2 Seismic Provisions . . . . . . . . . . . . .88
507.3 Appliance Support . . . . . . . . . . . . .88
507.4 Ground Support . . . . . . . . . . . . . . .88
507.5 Drainage Pan . . . . . . . . . . . . . . . . .88
507.6 Added or Converted
Equipment or Appliances . . . . . . . .88
507.7 Type of Gas(es) . . . . . . . . . . . . . . .88
507.8 Safety Shutoff Devices for Unlisted LP-Gas Appliance Used Indoors . . . . . . . . . . . . . . . . .89
507.9 Use of Air or Oxygen Under Pressure . . . . . . . . . . . . . . . . . . . . .89
507.10 Protection of Gas Appliances from Fumes or Gases other
than Products of Combustion . . . . .89
507.11 Process Air . . . . . . . . . . . . . . . . . . .89
507.12 Flammable Vapors . . . . . . . . . . . . .89
507.13 Installation in Residential Garages . . . . . . . . . . . . . . . . . . . . .89
507.14 Installation in Commercial Garages . . . . . . . . . . . . . . . . . . . . .89
507.15 Installation in Aircraft Hangars . . . .89
507.16 Venting of Flue Gases . . . . . . . . . .89
507.17 Extra Device or Attachment . . . . . .89
507.18 Addition to Existing System . . . . . .89
CMC § 506.4.2 Medium relevance — show source text
FIGURE 506.4.2
ALL COMBUSTION AIR FROM OUTDOORS THROUGH
SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
86 2025 CALIFORNIA PLUMBING CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
WATER HEATERS
(305 mm) of the top of the enclosure and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors, as follows:
(1) Where directly communicating with the outdoors or where communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4000 Btu/h (0.0005 m [2] /kW) of total input rating of all appliances in the enclosure. [See Figure 506.4.1(1) and Figure 506.4.1(2)]
(2) Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 square inch per 2000 Btu/h (0.001 m [2] /kW) of total input rating of all appliances in the enclosure. [NFPA 54:9.3.3.1] [See Figure 506.4.1(3)] 506.4.2 One Permanent Opening Method. One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25.4 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 506.4.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2]
506.5 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 506.5.1 through Section 506.5.3. [NFPA 54:9.3.4] (See Appendix J for example calculations) 506.5.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 506.3. [NFPA 54:9.3.4(1)] 506.5.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 506.4. [NFPA 54:9.3.4(2)] 506.5.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
CMC § 501.1 Medium relevance — show source text
Table 501.1(1) Water Heaters . . . . . . . . . . . . . . . .83
501.2 California Energy Code Water Heating System Requirements . . . . . . . . . . . . . . . . .83
502.0 Permits . . . . . . . . . . . . . . . . . . . . . .83
502.1 General . . . . . . . . . . . . . . . . . . . . . .83
503.0 Inspection . . . . . . . . . . . . . . . . . . . .83
503.1 Inspection of Chimneys or Vents . . .83
503.2 Final Water Heater Inspection . . . .83
504.0 Water Heater Requirements . . . . . .83
504.1 Location . . . . . . . . . . . . . . . . . . . . .83
Table 501.1(2) First Hour Rating . . . . . . . . . . . . . .83
504.2 Vent . . . . . . . . . . . . . . . . . . . . . . . .84
504.3 Clearance . . . . . . . . . . . . . . . . . . . .84
504.4 Pressure-Limiting Devices . . . . . . .84
504.5 Temperature-Limiting Devices . . . .84
504.6 Temperature, Pressure, and Vacuum Relief Devices . . . . . . . . .84
504.7 Lead Content . . . . . . . . . . . . . . . . .84
505.0 Oil-Burning and Other Water Heaters . . . . . . . . . . . . . . . . . . . . . .84
505.1 Water Heaters . . . . . . . . . . . . . . . .84
505.2 Safety Devices . . . . . . . . . . . . . . . .84
505.3 Oil-Fired Water Heaters . . . . . . . . .84
505.4 Indirect-Fired Water Heaters . . . . .84
506.0 Air for Combustion and
Ventilation . . . . . . . . . . . . . . . . . . . .84
506.1 General . . . . . . . . . . . . . . . . . . . . . .84
506.2 Indoor Combustion Air . . . . . . . . . .85
506.3 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . .85
506.4 Outdoor Combustion Air . . . . . . . . .85
506.5 Combination Indoor and
Outdoor Combustion Air . . . . . . . . .87
506.6 Engineered Installations . . . . . . . . .87
506.7 Mechanical Combustion Air Supply . . . . . . . . . . . . . . . . . . . . . . .87
506.8 Louvers, Grilles, and Screens . . . .87
506.9 Combustion Air Ducts . . . . . . . . . . .87
507.0 Appliance and Equipment Installation Requirements . . . . . . . .88
507.1 Dielectric Insulator . . . . . . . . . . . . .88
Frequently asked questions
Do both openings (top and bottom) have to be the same size?
Yes — under the same-story method each opening must meet the sizing rule (1 in² per 1,000 Btu/h or not less than 100 in²), so both openings must individually meet the requirement in § 701.5(1).
Can I use a single opening instead of top-and-bottom?
Not under the same-story two-opening method in § 701.5(1). A single-opening method is a different approach used when outdoor openings are provided (see § 701.6 and combination rules in § 701.7); check those sections for permitted single-opening methods and sizes.
How do I account for louvers or insect screens?
Size required openings based on the net free area (the actual unobstructed area allowed by the louver/grille/screen). The CMC requires using net free area for required opening sizing (see § 701.10).
Can I use exhaust fans in the same space with these openings?
If exhaust fans are present, additional air must be provided to replace exhausted air. For mechanical combustion-air systems, interlocks and minimum supply rates are specified in § 701.9.
If openings connect different stories, are the sizing rules different?
Yes. When using openings between different stories to combine volumes, the required total free area is 2 in² per 1,000 Btu/h of total input for the appliances — see § 701.5(2).
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