CMC · California Mechanical Code
What are the requirements for extra devices or attachments to combustion air openings?
You cannot add any device (covers, manual dampers, screens that reduce area, etc.) to an appliance combustion‑air opening if it could impair combustion. Size openings by the required net free area, use the code’s free‑area assumptions when needed, and follow the specific limits on screens and dampers (see § 702.1, § 701.10, § 701.10.1, and § 701.12).
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas. This is the controlling rule in § 702.1.
In practice that means you may not add grills, dampers, screens, covers, or other devices to an appliance’s combustion air inlet if those items reduce the required net free area or otherwise prevent safe, reliable combustion. The brief statement in § 702.1 is the primary prohibition; related combustion-air rules that govern louvers, screens, dampers, and motorized devices are found nearby in Chapter 7 and must be used together with § 702.1 to determine compliance.
Most important rule
No extra device or attachment can be added to an appliance if it could impair combustion. § 702.1.
Requirements in detail
Core prohibition
- § 702.1 (General): No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas. This is a broad, performance‑oriented prohibition — the code does not list every banned device, it bans anything that could impair combustion. § 702.1.
How the prohibition is enforced through related combustion‑air rules
- When a device (louver, screen, grille, damper, etc.) is proposed for an opening used to supply combustion air, the opening must still provide the required net free area for combustion and ventilation as calculated under the combustion‑air provisions. The net free area must reflect the actual free area through any louver, grille, or screen installed. § 701.10.
- Manually operated dampers are specifically prohibited in combustion air openings; power‑actuated movable louvers are permitted only with prior approval and must be electrically interlocked with the main burner fuel‑supply valve so fuel cannot be delivered unless the louvers are fully open. § 701.12.
- Screens are permitted but must meet minimum mesh size and their effect on net free area must be included in sizing calculations. § 701.10 and § 701.10.1.
Decision‑relevant dimensions, values, and assumptions
| Requirement / value | Code reference |
|---|---|
| Broad prohibition on attachments that impair combustion: No device or attachment that could impair combustion § 702.1. | § 702.1. |
| Manually operated dampers in combustion air openings prohibited; motorized louvers allowed only with interlock and prior approval. | § 701.12. |
| Net free area: size openings by net free area of the installed louver/grille/screen (use known free area or assume if unknown). | § 701.10. |
| Default free‑area assumptions when louver/grille design unknown: wood louvers 25% free area, metal louvers/grilles 75% free area. | § 701.10. |
| Minimum screen mesh: 1/4‑inch (6.4 mm) mesh (screens shall not be smaller). | § 701.10.1. |
| Exterior combustion‑air opening minimum height above grade: 12 inches (305 mm) (location requirement for outdoor intake openings). | § 701.11 (and related text). |
Notes on the table: the controlling statement about attachments is in § 702.1, while the practical sizing, screening, and damper requirements that implement the rule appear in § 701.10, § 701.10.1, and § 701.12 (all in Chapter 7). Use § 702.1 as the policy constraint and the §701 provisions for design detail.
Exceptions & special cases
- The code text for § 702.1 is a blanket prohibition; where specific exceptions exist they are found in other, appliance‑specific or installation‑specific sections (for example, direct‑vent appliance installations may be addressed elsewhere). The code excerpt includes a specific allowance elsewhere for certain direct‑vent appliances installed in solid‑fuel fireplaces when installed per the manufacturer’s instructions — check the appliance and installation rules for that appliance type.
- Motorized louvers or power‑actuated movable openings are permitted only with prior approval and only when electrically interlocked so fuel cannot flow unless the louvers are proven fully open. § 701.12.
- The code allows use of known tested free area for louvers/grilles/screens; if the free area is unknown, the code provides conservative assumptions (wood 25%, metal 75%) to size openings. § 701.10.
If you have a proposed device not explicitly described in Chapter 7, evaluate it by the test: does it reduce required net free area or otherwise interfere with combustion? If yes, it is prohibited under § 702.1.
Common mistakes
- Installing a manually operated damper or cover at a combustion‑air opening (manually operated dampers are prohibited). § 701.12.
- Failing to calculate and use net free area (gross opening × free area factor) when adding louvers or screens — leading to undersized combustion air. § 701.10.
- Using screens finer than 1/4‑inch mesh or assuming a screen has no effect on free area; both are violations because screen mesh and actual free area must be accounted for. § 701.10.1 and § 701.10.
- Adding aftermarket attachments at the appliance outlet or inlet (guards, decorative covers, etc.) without confirming they do not “impair the combustion of gas” — the plain language of § 702.1 forbids this.
Worked example — sizing an exterior combustion‑air opening with a metal louver + bird screen
Scenario: You determine the appliances require a single outdoor combustion air opening with a required net free area of 23 in² (calculation from appliance inputs). You plan to use a factory metal louver (manufacturer does not supply net free‑area rating) and a bird screen.
Step 1 — use default free‑area assumption for a metal louver: 75% per § 701.10.
Gross opening needed if louver alone: 23 in² ÷ 0.75 = 30.7 in².
Step 2 — account for the bird screen. The code requires screens be no finer than 1/4‑inch mesh and the net free area used for sizing must reflect the assembled free area (louver + screen). § 701.10 and § 701.10.1.
- If the bird screen reduces the louver’s free area by, for example, 20% (manufacturer data or measured), the combined free area factor = 0.75 × 0.80 = 0.60.
- Gross opening required = 23 in² ÷ 0.60 = 38.3 in².
Conclusion: Do not install the screen or louver arrangement unless the assembled net free area is at least the required 23 in². And do not add any device (e.g., decorative cover) that would further reduce the net free area — that would violate § 702.1.
(Notes: the 20% screen reduction above is a hypothetical illustration. The code requires you to use actual tested free‑area values when available or conservative assumptions when not. Always document the net free area used for sizing.)
Related provisions
- § 702.1 — Extra Device or Attachment (General prohibition).
- § 701.10 — Louvers, Grilles, and Screens; net free area and default free‑area assumptions.
- § 701.10.1 — Minimum screen mesh size (1/4‑inch).
- § 701.11 — Combustion air ducts and some location/termination requirements (e.g., minimum 12 in. above grade for exterior intakes).
- § 701.12 — Dampers prohibited in combustion air openings; requirements for motorized louvers and interlock.
If you need to permit a novel device or are unsure whether a proposed attachment “impairs combustion,” review the appliance manufacturer’s installation instructions, consult the AHJ, and document net free area and interlocks as required by the cited sections.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 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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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 CMC § 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]
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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.)
CMC § 101.0 Medium relevance — show source text
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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 § 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] )
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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 § 9.3.4 Medium 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 § 108.1 Medium relevance — show source text
The tag stating the date of the test and the name of the tester shall be
attached to the appliance at the main valve.
(b) Oil and liquid fuel burners of 1 000 000 Btu/h (293 kW) input or more require a supervised startup in accordance with Section C 108.1(10)(a).
(c) Installation of oxygen trim systems, modulating dampers, or other draft control or combustion devices require a supervised startup in accordance with Section C 108.1(10)(a).
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APPENDIX C
(d) Direct-fired heaters shall require a supervised startup in accordance with Section C 108.1(10)(a).
(11)The complete control diagram of the installation and operating instructions shall be supplied and posted by the installer of the appliance.
C 109.0 Special Requirements Based on Btu/h Input.
C 109.1 General.
(1) Zero to 400 000 Btu/h (0 kW to 117 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel oil filter, installed on the supply piping.
(c) Approved automatic safety shutoff valve to provide 100 percent shutoff of all oil.
(d) A flame safeguard control capable of providing 100 percent shutoff in the event of flame failure. Flame failure response timing shall not exceed the control manufacturer’s instructions.
(e) Two controls, one operating and one high limit, activated by temperature or pressure, as appropriate.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(h) Vent dampers and flue dampers shall be properly interlocked to prevent burner ignition unless safely
open.
(2) Four hundred thousand and one to 999 999 Btu/h (117.2 kW to 292.9 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, maximum five seconds closing time.
(d) One electronic flame safeguard pilot control providing a separately supervised and proven pilot, 100 percent shutoff manual reset. Flame failure response time shall not exceed the control manufacturer’s
instructions.
Direct-spark ignition shall be allowed where approved by the Authority Having Jurisdiction and where used on number 2 or lighter oil.
(e) Two controls, one operating and one high limit, activated by temperature or pressure, as appropriate.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(k) Where hot water or steam, one low water cutoff.
(l) An atomizing medium proving switch.
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 § 701.4 Medium relevance — show source text
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
802.1 Listing . . . . . . . . . . . . . . . . . . . . . . 153
802.2 Connection to Venting Systems . . 153
802.3 Minimum Safe Performance . . . . . 153
802.4 Type of Venting System to be Used . . . . . . . . . . . . . . . . . . . . . . . 154
802.5 Masonry, Metal, and Factory-Built Chimneys . . . . . . . . . . . . . . . . . . . 154
Table 802.4 Type of Venting System to be Used . . . . . . . . . . . . . . . . . . . . . . . 155
802.6 Gas Vents. . . . . . . . . . . . . . . . . . . 157
Table 802.6.1 Roof Slope Heights. . . . . . . . . . . . 158
802.7 Single-Wall Metal Pipe . . . . . . . . . 159
Table 802.7.3.3 Clearance for Connectors. . . . . . . 160
802.8 Through-the-Wall Vent Termination . . . . . . . . . . . . . . . . . . 160
802.9 Condensation Drain . . . . . . . . . . . 160
802.10 Vent Connectors for Category I Appliances . . . . . . . . . . . . . . . . . . 160
Table 802.10.1.3 Minimum Thickness for Galvanized
Steel Vent Connectors for Low Heat Appliances . . . . . . . . . . . . . . 161
CMC § 1.11.0. Medium relevance — show source text
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 Adopt Entire Chapter as
amended (amended
sections listed below)Adopt only those sections
that are listed belowChapter/Section This state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.0.
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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:
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 § 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 § 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
Can I add a bird screen to my outdoor combustion‑air opening?
Yes — but only if the assembled net free area (louver + screen) still meets the required combustion‑air net free area. Screens must be at least 1/4‑inch mesh and their effect on net free area must be included in sizing calculations. § 701.10 and § 701.10.1.
Are manually operated dampers allowed at combustion‑air openings?
No. Manually operated dampers are prohibited in combustion air openings. Motorized louvers may be permitted with prior approval and must be electrically interlocked to prevent fuel flow unless proven fully open. § 701.12.
The manufacturer’s louver data is missing — what free‑area should I use in calculations?
If the louver/grille design and tested free area are unknown, the code tells you to assume 25% free area for wood louvers and 75% free area for metal louvers/grilles when sizing openings. § 701.10.
My contractor wants to add a decorative cover to the appliance inlet — is that allowed?
Not if the cover could “impair the combustion of gas.” The plain prohibition in § 702.1 forbids any device or attachment that could interfere with combustion. Evaluate whether the cover reduces net free area or blocks airflow; if it does, it is not allowed. § 702.1.
Where is the single controlling statement about attachments?
The controlling, plain‑English prohibition is in § 702.1: “No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas.”
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