CMC · California Mechanical Code
When are engineered combustion air installations required and what do they require?
If the prescriptive opening rules aren’t used, the California Mechanical Code requires an engineered combustion‑air design that demonstrates adequate combustion, ventilation, and dilution air; mechanical supplies must meet at least 0.35 cfm per 1,000 Btu/h, provide makeup for exhaust, and include proof/interlocks and compliant ducting and louvers (see **§ 701.8**, **§ 701.9**, **§ 701.10**, **§ 701.11**, **§ 701.12**).
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
An engineered combustion air installation is required whenever the ordinary prescriptive openings or standard indoor/outdoor opening calculations are not used and an engineered approach is chosen to provide air for appliances. The California Mechanical Code states that engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods — see § 701.8.
The single most important rule: engineered combustion‑air systems must be sized and detailed by engineering methods to ensure adequate combustion, ventilation, and dilution air for the appliances served (see § 701.8).
Requirements in detail
Below are the decision‑relevant elements you must address when using an engineered combustion‑air approach. All items below are required either directly by § 701.8 or by nearby CMC provisions that govern how engineered or mechanical systems must perform and be constructed.
Purpose & scope (what engineered must cover)
- Provide adequate combustion air for safe burner operation, plus any required ventilation air and dilution air for vented appliance combustion products, using engineering calculations and methods. See § 701.8.
Minimum mechanical supply rates (when all air is supplied mechanically)
If you supply all combustion air by a mechanical air supply, the minimum rate is 0.35 cubic feet per minute per 1,000 Btu/h of total appliance input (0.034 m3/min per kW). This is the baseline capacity an engineered mechanical supply must meet or exceed. See § 701.9.
If exhaust fans are present, the engineered design must provide additional make‑up air to replace exhausted air. See § 701.9.1.
Appliances served by a mechanical supply must be interlocked so the main burner cannot operate unless the mechanical combustion‑air system is running (proof of operation). See § 701.9.2.
Components, termination, and construction details
When the engineered solution uses ducts, the combuston‑air ducts must meet construction and termination requirements in the code (materials, termination in unobstructed space, single‑space service, slopes, and screening rules). See § 701.11 and the list of duct requirements.
Louvers/grilles/screens — size the opening based on net free area (use the louver/grille free area if known; otherwise assume wood louvers 25% free area and metal louvers/grilles 75% free area). Screens must be at least 1/4‑inch mesh unless otherwise allowed. Motorized louvers must be interlocked & proven open before burner ignition. See § 701.10, § 701.10.1, and § 701.10.2.
Limitations on dampers and fire‑penetration issues
- Manually operated dampers are prohibited in combustion‑air openings; combustion‑air ducts shall not require openings where fire dampers are required. Power‑actuated louvers require interlocks and proof of full open position. See § 701.12.
Decision table — quick reference of key values and code refs
| Decision / value | Typical value or requirement | Code Reference |
|---|---|---|
| Engineered approach required text | Provide adequate combustion, ventilation, dilution air by engineering methods | § 701.8 |
| Minimum mechanical combustion‑air rate | 0.35 cfm per 1,000 Btu/h (0.034 m3/min per kW) | § 701.9 |
| Exhaust fan compensation | Additional makeup air required to replace exhausted air | § 701.9.1 |
| Interlock requirement | Appliances must be interlocked to mechanical supply | § 701.9.2 |
| Net free area calculation | Use actual free area; if unknown, assume wood 25%, metal 75% | § 701.10 |
| Minimum screen mesh size | 1/4‑inch mesh minimum | § 701.10.1 |
| Minimum outdoor opening dimension (when using outdoor openings) | 3 inches (minimum dimension for air openings) | § 701.7.3 |
| Combustion‑air duct construction | Galvanized steel or equivalent corrosion‑resistant material; terminate in unobstructed space; serve a single space; duct specific rules apply | § 701.11 and list of duct rules |
| Dampers | Manually operated dampers in combustion openings prohibited; power louvers allowed only with interlock and proof of open | § 701.12 |
Exceptions & special cases
The code text for § 701.8 itself is brief and does not enumerate precise situations that force an engineered installation; it states the engineered solution must be used when such an approach is chosen and requires engineering methods to determine adequacy. See § 701.8.
When all combustion air is provided mechanically, follow the mechanical minimum supply rate and interlock requirements in § 701.9 (this effectively defines one common engineered approach).
Some appliances or installations may be covered by other prescriptive rules (indoor openings, outdoor openings, combination methods) — an engineered design is typically used when those prescriptive options cannot be met or when a mechanical or special solution is proposed. See the combination/opening rules referenced in § 701.7 and § 701.6 for prescriptive alternatives.
The code includes an exception allowing certain direct‑vent appliances installed in solid‑fuel fireplaces to use manufacturer instructions for combustion air — consult the specific text in the applicable section for that exception. See § 701.11 exceptions.
Note: The text of § 701.8 does not prescribe a single calculation method — it requires engineering methods be used. Where the mechanical supply option or the prescriptive opening‑sizing methods are chosen, the adjacent sections supply specific numeric requirements.
Common mistakes
Treating § 701.8 as if it contains numeric sizing rules. It does not — it requires an engineering determination. Use § 701.9 and the prescriptive opening rules for numeric baselines, or attach engineering calculations to justify different values.
Forgetting interlocks for mechanically supplied combustion air. Any appliance served by a mechanical supply must be interlocked so the burner cannot run without the supply in operation (§ 701.9.2).
Sizing louvers/grilles by gross area instead of net free area or failing to account for louver free‑area percentages (wood 25% / metal 75%) when the manufacturer’s free area is unknown — see § 701.10.
Installing manually operated dampers in combustion openings (prohibited) or failing to provide proof that motorized louvers are open (required). See § 701.12 and § 701.10.2.
Using combustion‑air ducts that terminate behind screening or into spaces prohibited by the duct rules (e.g., the space remaining around a chimney liner) — follow § 701.11 requirements.
Worked example — concrete scenario
Scenario: A mechanical room contains three appliances with combined input of 100,000 Btu/h. The designer proposes a mechanical combustion‑air supply for the room.
Baseline mechanical supply rate (minimum):
- Use 0.35 cfm per 1,000 Btu/h from § 701.9.
- Required flow = 0.35 × (100,000 / 1,000) = 35 cfm minimum.
Check for exhaust fans:
- If the room has exhaust fans that remove, for example, 100 cfm, the engineered design must supply that makeup air in addition to the 35 cfm minimum (see § 701.9.1). The total supply would therefore be at least 135 cfm, and appliances must be interlocked with the supply fan per § 701.9.2.
Ducts & terminations:
- If the system uses a combustion‑air duct, select materials and terminations to comply with the duct construction and termination rules in § 701.11 and the associated list (galvanized steel or equivalent, terminate to unobstructed space, do not serve multiple upper/lower openings incorrectly).
Documentation:
- Because the designer used an engineered approach, include the calculations and assumptions (airflow, interlocks, pressure relationships, louvers/net free area, makeup for exhaust) as part of the submittal to demonstrate adequacy as required by § 701.8.
(If instead the designer used prescriptive openings or the standard volume method, follow the combination/indoor/outdoor opening rules — see § 701.6 and § 701.7 for guidance and Appendix examples for standard‑method calculations.) file
Related provisions (CMC sections)
- § 701.6 — Outdoor combustion air (location and general prescriptive openings).
- § 701.7 — Combination indoor and outdoor combustion air (reduction factors, minimum dimensions).
- § 701.9 — Mechanical combustion air supply (minimum cfm rate, exhaust, interlock, specified combustion air).
- § 701.10 — Louvers, grilles, and screens (net free area, free‑area assumptions, screens, motorized louvers).
- § 701.11 — Combustion air ducts (construction, termination, prohibited uses).
- § 701.12 — Dampers prohibited (limitations on dampers and requirements for motorized louvers).
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 108.1 High 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 § 701.6.1 High relevance — show source text
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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 § 505.1 High 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
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 § 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 § 1.859 Medium relevance — show source text
78|–|1.859|0.463| |35.31|–|2.247|0.559| |38.84|–|2.667|0.663| |42.37|–|3.121|0.775| |45.90|–|3.607|0.895| |49.43|–|4.125|1.022| |52.97|–|–|1.157| |56.50|–|–|1.299| |60.03|–|–|1.449| |63.56|–|–|1.607| |67.09|–|–|1.772| |70.62|–|–|1.944| |81.21|–|–|2.503| |91.81|–|–|3.127| |102.40|–|–|3.813|
For SI units: 1 standard cubic foot per minute = 28.32 SLPM, 1 inch = 25 mm, 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
Notes: 1 Based on pressure of 14.7 psig (101 kPa) at 68°F (20°C). 2 Based on pressure of 55 psig (379 kPa) at 68°F (20 °C).
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HEALTH CARE FACILITIES AND MEDICAL GAS AND MEDICAL VACUUM SYSTEMS
TABLE 1323.1.4(5) PRESSURE LOSS FOR VACUUM (continued)
FLOW
RATE
(SCFM)1VACUUM LOSS (inch of mercury)
PER 100 FEET FOR COPPER TUBE2Col3 Col4 Col5 Col6 FLOW
RATE
(SCFM)13⁄4 INCH
TUBE1 INCH
TUBE11⁄4 INCH
TUBE11⁄2 INCH
TUBE2 INCH
TUBE49.43 – – – 3.645 0.956 52.97 – – – 4.122 1.081 56.50 – – – 4.626 1.212 63.56 – – – – 1.495 70.62 – – – – 1.803 77.68 – – – – 2.138 84.74 – – – – 2.497 91.81 – – – – 2.882 98.87 – – – – 3.291 105.93 – – – – 3.724 112.99 – – – – 4.181 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 § 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 § 9.3.8.7 Medium 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 § 101.0 Medium relevance — show source text
2025 CALIFORNIA PLUMBING CODE 487
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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 § 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
Frequently asked questions
When does an installation *have* to be engineered (not prescriptive)?
If the designer elects to provide combustion air by engineering methods rather than using the prescriptive indoor/outdoor opening methods or standard mechanical‑supply baseline, § 701.8 requires the engineered design to demonstrate adequate combustion, ventilation, and dilution air. The code doesn’t list a single trigger; engineered designs are used when prescriptive options are infeasible or when mechanical/complex systems are proposed.
What is the minimum mechanical supply rate I must meet if I use fans?
Minimum is 0.35 cfm per 1,000 Btu/h of total appliance input (0.034 m3/min per kW), per § 701.9.
Do appliances served by a mechanical combustion‑air supply need interlocks?
Yes. Each appliance served must be interlocked to prevent main burner operation where the mechanical supply is not operating. See § 701.9.2.
Can I use motorized louvers for combustion air?
Yes, but power‑actuated louvers must be electrically interlocked and proven in the full‑open position prior to burner ignition and during operation; means must prevent burner ignition if louvers fail to open. Manually operated dampers, however, are not allowed in combustion‑air openings. See § 701.10.2 and § 701.12. file
What documentation should I submit for an engineered combustion‑air design?
Submit the engineering calculations and assumptions that show adequate combustion, ventilation, and dilution air (flow rates, makeup for exhaust, duct sizing and terminations, interlocks, proof‑of‑open for louvers, net free area calculations). § 701.8 makes the requirement for engineering methods; adjacent sections provide numerical requirements that must be satisfied.
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