CMC · California Mechanical Code
Mechanical & engineered combustion air
Chapter 7 of the CMC governs how combustion air is supplied — indoors, outdoors, mechanically, or by engineered design — with specific rules at §701.8–§701.12 and worked examples in the appendices.
Last reviewed: July 6, 2026
Overview
This topic covers the CMC requirements for supplying combustion, dilution, and makeup air to fuel‑burning appliances — when air may be taken from indoors, from outdoors, or must be provided by a mechanical or engineered system. Chapter 7 (Combustion Air) is the primary chapter; it defines methods to ensure safe combustion and dilution of flue products and explains when appliances need outdoor, indoor, combination, mechanical, or engineered air sources .
Key rules you’ll consult here include the engineered‑installation requirements in §701.8, the mechanical supply criteria in §701.9 (including the minimum forced‑supply rate), and the installation details for louvers, grilles, ducts, and prohibited devices such as manual dampers in §§701.10–701.12. The code also supplies worked examples and sizing tables (Appendix examples) to apply the standard and combination methods for calculating required combustion‑air openings .
In this section
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 701.6.1 High relevance — show source text
148 2025 CALIFORNIA MECHANICAL CODE
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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).
[NFPA 54:9.3.4(3)] 701.8 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] 701.9 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 not less than 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]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 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] 701.9.3 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] 701.10 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.
CMC § 9.3.4 High relevance — show source text
[NFPA 54:9.3.4(3)] 701.8 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] 701.9 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 not less than 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]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 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] 701.9.3 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] 701.10 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]
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]
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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.
CMC § 2025 High relevance — show source text
Systems that carry nonabrasive exhaust, such as smoke, moderate abrasives such as sawdust, and high abrasives such as manganese or acid vapors use product-conveying ducts. Part II provides the minimum fire safety requirements related to the design, installation, inspection, and maintenance of grease-type operations, such as cooking, for both fuel-gas and solid fuel. Cooking produces a significant amount of smoke, fumes, vapors, heat, and other pollutants. Therefore, acceptable kitchen ventilation is necessary to prevent fires from the build-up of grease and to provide the occupants protection from smoke, unpleasant odors, pollutants, and dangerous gases. There are two types of exhaust hoods (Type I and Type II) used in commercial kitchen applications.
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FORMAT OF THE UNIFORM MECHANICAL CODE
Type I hoods are intended to be installed above equipment or appliances that generate grease or smoke. Type II hoods are intended to be installed above equipment or appliances that generate steam, vapor, heat, or odors and do not generate grease or smoke.
Chapter 6 Duct Systems. Chapter 6 regulates requirements for ducts and plenums that are portions of a heating, cooling, ventilation, or exhaust system. This chapter contains material and installation requirements for metal, gypsum, factory-made, flexible, and plastic ducts. It also contains provisions for duct insulation, dampers, fire protection, and automatic shutoff for the building’s air distribution system. It is worth noting that exhaust ducts are regulated by Chapter 5, and combustion air ducts are regulated by Chapter 7.
Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.
Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.
Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.
Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.
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 § 2025 Medium relevance — show source text
Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.
Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.
Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.
Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.
Chapter 9 Installation of Specific Appliances. Chapter 9 regulates the minimum requirements for the design, construction and installation of specific appliances. The provisions address the minimum requirements for gas-fired appliances, oil-fired appliances, wood-fired appliances, and electric-type appliances. In addition to the requirements of this chapter, appliances are also required to comply with the general requirements of Chapter 3.
Chapter 10 Boilers and Pressure Vessels. Chapter 10 regulates the construction, installation, operation, repair, and alteration of boilers and pressure vessels. The safety provisions within this chapter address controls and limit devices for automatic boilers, methods of determining expansion tank capacities, discharge piping, relief valves, shutoff valves, gas-pressure and combustion regulators, and inspections and tests. Potable water heaters are free from the requirements of Chapter 10 as they are within the scope of the Uniform Plumbing Code (UPC).
Pressure vessels store large amounts of energy and must comply with ASME Boiler and Pressure Vessel Code (BPVC) Section VIII.1. The stored energy must be contained to prevent disastrous failures. Boilers must comply with ASME BPVC Section I, ASME BPVC Section IV, or NFPA 85. Installing a safety relief valve and expansion tank prevents pressures in the tank from exceeding the design threshold.
Chapter 11 Refrigeration. Chapter 11 regulates the design, installation, and construction requirements of refrigeration systems and the installation and construction of cooling towers. Refrigeration is a method used for achieving heat transfer to cool spaces. Refrigerants are the most common medium used to transfer the heat energy from the low-temperature level to the high-temperature level. Table
2025 CALIFORNIA MECHANICAL CODE xv
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FORMAT OF THE UNIFORM MECHANICAL CODE
CMC § 101.2 Medium relevance — show source text
Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.2
COMBUSTIBLE CONSTRUCTION, CLOSED
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0
COMBUSTIBLE CONSTRUCTION, OPEN
Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217.0
COMBUSTIBLE MATERIAL
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0
COMBUSTIBLE MATERIAL, LIMITED
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.0
COMBUSTION AIR
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0
Duct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 701.11, 701.12
General requirements . . . . . . . . . . . . . . . . . . Chapter 7
COMBUSTION AIR FOR
Boilers, low pressure,
installed in closets . . . . . . . . . . . . . . . . . . . . . . . 904.2
Engineered installations. . . . . . . . . . . . . . . . . . . . . 701.8
Fireplaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912.4
Fossil fuel heating equipment. . . . . . . . . . . . . E 502.6.1
Furnaces, central heating . . . . . . . . . . . . . . 904.2, 907.4
COMBUSTION AIR FROM
Bedroom or bathroom. . . . . . . . . . . . . . . . . . . . . . 902.2
Chimneys and vents . . . . . . . . . . . . . . . . . . . 802.5.11.1
Combination, indoor and outdoor. . . . . . . 701.7, F 103.0
Indoors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 701.4
Louvers, grilles and screens . . . . . . . . . . . . . . . . 701.10
Mechanical supply . . . . . . . . . . . . . . . . . . . . . . . . . 701.9
Outdoors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 701.6
Refrigeration machinery rooms . . . . . . . . . . . . . . 1106.5
COMMERCIAL FOOD
HEAT-PROCESSING EQUIPMENT
CMC § 8.13.6 Medium relevance — show source text
[ASHRAE 15:8.13.6]
»
(1) Operated, where occupied, to supply not less than 0.5 CFM/ft [2] [2.54 L/(s•m [2] )] of machinery room area or 20 cubic feet per minute (9.44 L/s) per person.
(2) Operable, where occupied at a volume required to not exceed the higher of a temperature rise of 18°F (10°C) above inlet air temperature or a maximum temperature of 122°F (50°C).
1106.4 Natural Ventilation. When a refrigerating system is located outdoors more than 20 feet (6096 mm) from building openings and is enclosed by a penthouse, lean-to, or other open structure, natural or mechanical ventilation shall be provided. The requirements for such natural ventilation shall be in accordance with the following:
(1) The free-aperture cross section for the ventilation of a machinery room shall be not less than as determined in accordance with Equation 1106.4.
F = √ G (Equation 1106.4)
Where:
F = The free opening area, ft [2] (m [2] ).
G = The mass of refrigerant in the largest system, any part of which is located in the machinery room, pounds.
For SI units: 1 square foot = 0.0929 m [2], 1 pound = 0.453 kg
(2) Locations of the gravity ventilation openings shall be based on the relative density of the refrigerant to air.
[ASHRAE 15:8.14]
1106.5 Combustion Air. No open flames that use combustion air from the machinery room shall be installed where refrigerant is used. Combustion equipment shall not be installed in the same machinery room with refrigerant-containing equipment except under one of the following conditions:
(1) Combustion air shall be ducted from outside the machinery room and sealed in such a manner as to prevent refrigerant leakage from entering the combustion chamber.
(2) A refrigerant detector, that is in accordance with Section 1106.2.5, shall be installed to automatically shut down the combustion process in the event of refrigerant leak age.
Exception: Machinery rooms where carbon dioxide (R-744) or water (R-718) is the refrigerant.
1106.6 Ventilation Intake. Makeup air intakes to replace the exhaust air shall be provided to the refrigeration machinery room directly from outside the building. Intakes shall be located as required by other sections of the code and fitted with backdraft dampers or other approved flow-control means to prevent reverse flow. Distribution of makeup air shall be arranged to provide thorough mixing within the refrigeration machinery room to prevent short circuiting of the makeup air directly to the exhaust.
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REFRIGERATION
1106.11.6.1 Mechanical Ventilation. The machinery room shall have a mechanical ventilation system in accordance with Section 1106.11.11. The mechanical ventilation system shall:
(1) Run continuously, and failure of the mechanical ventilation system actuates an alarm, or
CMC § 314.2 Medium 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.
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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 § 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.
CMC § 608.9 Medium 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 § 11.7 Medium relevance — show source text
(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:
140 000 Btu/h = 47 square inches (0.03 m [2] ) 3000 British thermal units per square inch (Btu/in [2] )
(6) Determine the minimum outdoor combustion air opening area:
Outdoor opening area = 0.49 (from Step 4) x 47 square inches (0.03 m [2] ) = 23 square inches (0.01 m [2] )
Section 506.5.3(3) requires the minimum dimension of the air opening should not be less than 3 inches (76 mm).
[NFPA 54:I.1]
140 000 Btu/h = 47 square inches (0.03 m [2] ) 3000 British thermal units per square inch (Btu/in [2] )
2025 CALIFORNIA PLUMBING CODE 489
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APPENDIX J
TABLE J 101.2 STANDARD METHOD: REQUIRED VOLUME, ALL APPLIANCES*
[NFPA 54: TABLE A.9.3.2.1]
CMC § 0.03 Medium relevance — show source text
140 000 Btu/h = 47 in [2] (0.03 m [2] ) 3000 Btu/in [2]
(6) Determine the minimum outdoor combustion air opening area:
Outdoor opening area = 0.49 (from Step 4) x 47 in [2] (0.03 m [2] ) = 23 in [2 ] (0.01 m [2] )
Section 701.7.3(3) requires the minimum dimension of the air opening should not be less than 3 inches (76 mm). [NFPA 54:I.1]
3600 ft [3]
= 0.51 7000 ft [3]
»
and is equal to the draft hood outlet diameter. A 4 inch (102 mm) vent connector is selected.
Furnace Vent Connector Diameter. Using Table 803.2(1), Vent Connector Capacity, read down the total Vent Height (H) column to 30 feet (9144 mm), and read across the 3 feet (914 mm) Connector Rise (R) row to the first Btu/h rating in the FAN MAX column that is equal to or greater than the furnace input rating. The 100 000 Btu/h (29 kW) furnace in this example falls within this range, so a 4 inch (102 mm) connector is adequate.
Chimney Liner Diameter. The total input to the common vent is 135 000 Btu/h (40 kW). Using the Common Vent Capacity portion of Table 803.2(1), read down the total Vent Height (H) column to 30 feet (9144 mm) and across this row to find the smallest vent diameter in the FAN + NAT column
that has a Btu/h rating greater than 135 000 Btu/h (40 kW). The 4 inch (102 mm) common vent has a capacity of 138 000 Btu/h (40.4 kW). Reducing the maximum capacity by 20 percent results in a maximum capacity for a 4 inch (102 mm) corrugated liner of 110 000 Btu/h (32 kW), less than the total input of 135 000 Btu/h (40 kW). So a larger liner is needed. The 5 inch (127 mm) common vent capacity listed in Table 803.2(1) is 210 000 Btu/h (62 kW), and after reducing by 20 percent is 168 000 Btu/h (49.2 kW). Therefore, a 5 inch (127 mm) corrugated metal liner should be used in this example.
Single Wall Connectors. Once it has been established that relining the chimney is necessary, Type B double-wall vent connectors are not specifically required. This example could be redone using Table 803.2(2) for single-wall vent connectors. For this case, the vent connector and liner diameters would be the same as found for Type B double-wall connectors. [NFPA 54:F.2.4]
F 103.0 Example of Combination Indoor and Out- door 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 draft hoodequipped 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.
Frequently asked questions
When can I use an engineered combustion‑air installation?
Engineered installations are permitted where the installer/design professional demonstrates an adequate supply of combustion, ventilation, and dilution air using engineering methods; see §701.8 for the engineered‑installation requirement and approach .
What is the minimum mechanical combustion‑air supply rate?
When all combustion air is supplied mechanically from outdoors, the CMC requires a minimum of 0.35 cubic feet per minute per 1,000 Btu/h (0.034 m3/min per kW) for the appliances in the space; see §701.9 for this requirement and related interlock/exhaust provisions .
Are dampers allowed in combustion‑air ducts or openings?
No. Combustion‑air ducts or plenums must not be installed where fire dampers are required, and manually operated dampers are prohibited in combustion‑air openings; motorized louvers are allowed only if interlocked and proven open during burner operation, per §701.12 .
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