CMC · California Mechanical Code
How to size outdoor combustion air and combine indoor/outdoor openings?
If the appliance room doesn’t have enough volume, you must provide outdoor combustion air sized per §701.6 (two‑opening or one‑opening methods) and, if you credit indoor volume, reduce the outdoor area by the factor 1 − (available indoor volume ÷ required volume) per §701.7.3. Minimum opening dimensions and indoor‑opening rules still apply.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
Outdoor combustion air must be provided using the methods in § 701.6 (two‑opening or one‑opening methods) and a combination of indoor and outdoor openings is permitted only in accordance with § 701.7 and the sizing procedure in § 701.7.3. The combination method reduces the required outdoor opening area by a reduction factor based on the ratio of available indoor volume to the required volume. Minimum opening dimensions and placement requirements (top/bottom within 12 inches) still apply.
If the room volume is insufficient to supply all combustion air, provide outdoor openings sized per § 701.6 and then multiply that full outdoor area by the reduction factor 1 − (available indoor volume ÷ required volume) to get the minimum outdoor opening area (minimum dimension 3 in.).
Requirements in detail
Key defined terms (first mentions are bold)
- Available volume — the sum of volumes of all spaces that will communicate with the appliance room when combined (used in § 701.7.3).
- Required volume — the minimum room volume calculated by the standard or known‑infiltration method per § 701.4 (standard: 50 ft³ per 1,000 Btu/h).
- Reduction factor — equal to 1 − (available volume ÷ required volume); used to reduce outdoor opening area when indoor volume contributes. § 701.7.3.
How to size outdoor openings (methods in § 701.6)
- Two‑permanent‑openings method (one within 12 in. of top and one within 12 in. of bottom):
- If openings communicate directly to outdoors or via vertical ducts: 1 in² per 4,000 Btu/h for each opening. § 701.6.1.
- If openings communicate through horizontal ducts: 1 in² per 2,000 Btu/h for each opening. § 701.6.1(2).
- One‑permanent‑opening method (one opening within 12 in. of the top; appliance clearances required):
- 1 in² per 3,000 Btu/h of total appliance input, or not less than the sum of vent connector areas, whichever is greater. § 701.6.2.
How to combine indoor and outdoor openings (§ 701.7 / § 701.7.3)
- Follow § 701.7.1–701.7.3: indoor openings must meet § 701.5 sizing/location rules and outdoor openings must meet § 701.6 location requirements. Then calculate outdoor opening size using the three steps in § 701.7.3:
- Compute the ratio of interior spaces = (available indoor volume) ÷ (required volume).
- Compute outdoor size reduction factor = 1 − ratio.
- Minimum outdoor opening area = (full outdoor opening area per § 701.6) × (reduction factor). Minimum physical opening dimension must not be less than 3 in. § 701.7.3(3).
Decision‑relevant dimensions and values (table)
| Decision or value | Code value | Code Reference |
|---|---|---|
| Two‑opening method — vertical/direct openings | 1 in² per 4,000 Btu/h (each opening) | § 701.6.1 |
| Two‑opening method — horizontal duct | 1 in² per 2,000 Btu/h (each opening) | § 701.6.1(2) |
| One‑opening method — single opening | 1 in² per 3,000 Btu/h (or ≥ sum vent connector areas) | § 701.6.2 |
| Minimum opening physical dimension | 3 inches (76 mm) | § 701.6 and § 701.7.3 |
| Required indoor volume (standard method) | 50 ft³ per 1,000 Btu/h | § 701.4.1 |
| Reduction factor for outdoor area | 1 − (available indoor volume ÷ required volume) | § 701.7.3(1)–(3) |
| Indoor openings to combine same story | 1 in² per 1,000 Btu/h, but ≥ 100 in²; one near top and one near bottom within 12 in. | § 701.5(1) |
| Indoor openings between stories | 2 in² per 1,000 Btu/h (total) | § 701.5(2) |
Exceptions & special cases
- Direct‑vent appliances are exempt from these combustion‑air provisions (see § 701.1 exception (1)).
- If all combustion air is provided by a mechanical air supply, different rules apply (minimum 0.35 cfm per 1,000 Btu/h) and interlocks are required — see § 701.9 (mechanical supply). The combination reduction method in § 701.7.3 is not used for fully mechanical systems.
- When determining required volume, use the standard 50 ft³/1,000 Btu/h method or the known‑infiltration method in § 701.4.2 if infiltration is known. Do not assume infiltration greater than 0.60 ACH in the known‑infiltration equations.
Common mistakes
- Assuming the outdoor opening can be arbitrarily small because indoor volume exists. The outdoor opening must be reduced only by the factor in § 701.7.3, not eliminated.
- Using the wrong full‑area basis for the combination calculation — you must first compute the full outdoor opening area per § 701.6 (1/3,000; 1/4,000; or 1/2,000 in² per Btu/h as applicable), then apply the reduction factor.
- Forgetting minimum 3‑inch dimension requirement for openings — a tiny area is not acceptable if any physical dimension would be under 3 in. § 701.6 / § 701.7.3.
- Overlooking that indoor connecting openings must meet § 701.5 (size and placement) before they can be credited as “available volume.”
Worked example — concrete scenario
Scenario: A basement contains a 100,000 Btu/h fan‑assisted furnace and a 40,000 Btu/h draft‑hood water heater (total 140,000 Btu/h). Basement dimensions: 15 ft × 30 ft × 8 ft = 3,600 ft³. No other indoor spaces are available.
Step 1 — Required volume:
- From the standard table (50 ft³ per 1,000 Btu/h): 140,000 Btu/h → 7,000 ft³ required (see Appendix F / § 701.4).
Step 2 — Ratio of interior spaces:
- Ratio = available ÷ required = 3,600 ÷ 7,000 = 0.51.
Step 3 — Reduction factor:
- Reduction factor = 1 − 0.51 = 0.49. § 701.7.3(2).
Step 4 — Full outdoor opening area (one‑opening method assumption):
- Full outdoor area (one opening per § 701.6.2) = 140,000 ÷ 3,000 = 47 in².
Step 5 — Minimum outdoor opening when combining:
- Minimum outdoor area = 47 in² × 0.49 = 23 in². Ensure opening dimension ≥ 3 in. § 701.7.3(3).
This worked example is the same approach shown in Appendix F (example calculation).
Related provisions
- § 701.4 — Required indoor volume (standard and known‑infiltration methods).
- § 701.5 — Indoor opening size and location (rules for combining spaces).
- § 701.9 — Mechanical combustion air supply (when all combustion air is mechanically supplied).
- § 701.10 — Louvers, grilles, and screens; net free area basis.
- Appendix F — Example calculations and required‑volume table used in the standard method.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 701.6.2 High relevance — show source text
The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 701.6.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2] 701.7 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 701.7.1 through Section 701.7.3. [NFPA 54:9.3.4] (see Appendix F for example calculations)
FIGURE 701.6.1(1) ALL COMBUSTION AIR FROM OUTDOORS – INLET
AIR FROM VENTILATED CRAWL SPACE AND OUTLET AIR TO
VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(a)]
FIGURE 701.6.1(2)
ALL COMBUSTION AIR FROM OUTDOORS
THROUGH VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(b)]
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COMBUSTION AIR
FIGURE 701.6.1(3) ALL COMBUSTION AIR FROM OUTDOORS
THROUGH HORIZONTAL DUCTS
[NFPA 54: FIGURE A.9.3.3.1(2)]
FIGURE 701.6.2
ALL COMBUSTION AIR FROM OUTDOORS THROUGH
SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
701.7.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 701.5. [NFPA 54:9.3.4(1)]
701.7.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 701.6. [NFPA 54:9.3.4(2)]
701.7.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 701.6, multiplied by the reduction factor. The minimum dimension of air openings shall not be less than 3 inches (76 mm).
CMC § 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 § 101.0 High 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 § 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 § 506.4.2 High relevance — show source text
FIGURE 506.4.2
ALL COMBUSTION AIR FROM OUTDOORS THROUGH
SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
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WATER HEATERS
(305 mm) of the top of the enclosure and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors, as follows:
(1) Where directly communicating with the outdoors or where communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4000 Btu/h (0.0005 m [2] /kW) of total input rating of all appliances in the enclosure. [See Figure 506.4.1(1) and Figure 506.4.1(2)]
(2) Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 square inch per 2000 Btu/h (0.001 m [2] /kW) of total input rating of all appliances in the enclosure. [NFPA 54:9.3.3.1] [See Figure 506.4.1(3)] 506.4.2 One Permanent Opening Method. One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25.4 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 506.4.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2]
506.5 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 506.5.1 through Section 506.5.3. [NFPA 54:9.3.4] (See Appendix J for example calculations) 506.5.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 506.3. [NFPA 54:9.3.4(1)] 506.5.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 506.4. [NFPA 54:9.3.4(2)] 506.5.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
CMC § 0.03 High 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.
CMC § 0.002 High relevance — show source text
Each opening shall have a minimum free area of 1 square inch per 1000 Btu/h (0.002 m [2] /kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.065 m [2] ). One permanent opening shall commence within 12 inches (305 mm) of the top of the enclosure and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 701.5). The minimum dimension of air openings shall not be less than 3 inches (76 mm). (2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more permanent openings in doors or floors having a total minimum free area of 2 square inches per 1000 Btu/h (0.004 m [2] /kw) of total input rating of all appliances. [NFPA 54:9.3.2.3] 701.6 Outdoor Combustion Air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accor
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COMBUSTION AIR
dance with the methods in Section 701.6.1 or Section 701.6.2. The minimum dimension of air openings shall not be less than
FIGURE 701.5
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
THROUGH INDOOR COMBUSTION AIR OPENINGS
[NFPA 54: FIGURE A.9.3.2.3(1)]
701.6.1 Two Permanent Openings Method. Two permanent openings, one commencing within 12 inches (305 mm) of the top of the enclosure and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors as follows:
(1) Where directly communicating with the outdoors or where communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4000 Btu/h (0.0005 m [2] /kW) of total input rating of all appliances in the enclosure.
[See Figure 701.6.1(1) and Figure 701.6.1(2)] (2) Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of not less than 1 square inch per 2000 Btu/h (0.001 m [2] /kW) of total input rating of all appliances in the enclosure. [See Figure 701.6.1(3)] [NFPA 54:9.3.3.1] 701.6.2 One Permanent Opening Method. One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25.4 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance.
CMC § 103.0 High relevance — show source text
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.
Fan-Assisted Furnace Input: 100 000 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 ft [3] (101.94 m [3] )
140 000 Btu/h = 47 in [2] (0.03 m [2] ) 3000 Btu/in [2]
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APPENDIX F
» TABLE F 103.0 STANDARD METHOD: REQUIRED VOLUME, ALL APPLIANCES
[NFPA 54: TABLE A.9.3.2.1]
APPLIANCE INPUT
(Btu/h)REQUIRED VOLUME
(cubic feet)5000 250 10 000 500 15 000 750 20 000 1000 25 000 1250 30 000 1500 35 000 1750 40 000 2000 45 000 2250 50 000 2500 55 000 2750 60 000 3000 65 000 3250 70 000 3500 75 000 3750 80 000 4000 85 000 4250 90 000 4500 95 000 4750 100 000 5000 105 000 5250 110 000 5500 115 000 5750 120 000 6000 125 000 6250 130 000 6500 135 000 6750 140 000 7000 145 000 7250 150 000 7500 160 000 8000 170 000 8500 180 000 9000 190 000 9500 200 000 10 000 210 000 10 500 220 000 11 000 230 000 11 500 240 000 12 000 250 000 12 500 260 000 13 000 270 000 13 500 280 000 14 000 290 000 14 500 300 000 15 000 For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot = 0.0283 m [3]
CMC § 9.3.2.3 High relevance — show source text
THROUGH INDOOR COMBUSTION AIR OPENINGS
[NFPA 54: FIGURE A.9.3.2.3(1)]
701.6.1 Two Permanent Openings Method. Two permanent openings, one commencing within 12 inches (305 mm) of the top of the enclosure and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors as follows:
(1) Where directly communicating with the outdoors or where communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4000 Btu/h (0.0005 m [2] /kW) of total input rating of all appliances in the enclosure.
[See Figure 701.6.1(1) and Figure 701.6.1(2)] (2) Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of not less than 1 square inch per 2000 Btu/h (0.001 m [2] /kW) of total input rating of all appliances in the enclosure. [See Figure 701.6.1(3)] [NFPA 54:9.3.3.1] 701.6.2 One Permanent Opening Method. One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25.4 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 701.6.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2] 701.7 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 701.7.1 through Section 701.7.3. [NFPA 54:9.3.4] (see Appendix F for example calculations)
FIGURE 701.6.1(1) ALL COMBUSTION AIR FROM OUTDOORS – INLET
AIR FROM VENTILATED CRAWL SPACE AND OUTLET AIR TO
VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(a)]
FIGURE 701.6.1(2)
ALL COMBUSTION AIR FROM OUTDOORS
THROUGH VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(b)]
148 2025 CALIFORNIA MECHANICAL CODE
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COMBUSTION AIR
CMC § 1.11.0. High 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.
2025 CALIFORNIA PLUMBING CODE 487
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488 2025 CALIFORNIA PLUMBING CODE
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APPENDIX J
COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND
VENTILATION OPENING DESIGN
The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.
J 101.0 General.
J 101.1 Applicability. This appendix provides general guidelines for the sizing of combination indoor and outdoor combustion and ventilation air openings. J 101.2 Example of Combination Indoor and Outdoor Combustion Air Opening Design. Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a drafthood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces can be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 British thermal units per hour (Btu/h) (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
(1) Determine the total available room volume. Appliance room volume:
CMC § 2025 High relevance — show source text
Appendix B Procedures to be Followed to Place Gas Equipment in Operation. Appendix B provides requirements for the procedures that apply after an appliance is installed in place, piped, and connected to its venting system. The requirements include adjusting the burner input, air adjustments, verifying operation of safety shutoffs, automatic ignition, and protective devices, checking draft for vent-connected appliances, and operating instructions.
Appendix C Installation and Testing of Oil (Liquid) Fuel-Fired Equipment. Appendix C governs the installation, testing, or repair of oil or liquid fuel-burning equipment used in buildings or structures and equipment.
Appendix D Fuel Supply: Manufactured/Mobile Home Parks and Recreational Vehicle Parks. The provisions of this appendix apply to the fuel gas piping systems of mobile home and recreational vehicle parks. These provisions also apply to the use, maintenance, and installation for supplying fuel gas for accessory buildings or structures, and building components.
2025 CALIFORNIA MECHANICAL CODE xvii
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FORMAT OF THE UNIFORM MECHANICAL CODE
Appendix E Sustainable Practices. This appendix provides a comprehensive set of technically sound provisions that encourage sustainable practices and works toward improving the design and construction of mechanical systems that result in a positive long-term environmental impact. Environmental sustainability is important because it involves natural resources that human beings need for economic or manufactured capital. Their sustainability is defined by their reliance on infinitely available resources that are naturally occurring, constant, and free to access.
Appendix F Sizing of Venting Systems and Outdoor Combustion and Ventilation Opening Design. Appendix F provides added information on the sizing of gas vents. This appendix is useful to the end user for the proper sizing of venting systems. A series of examples are given that show how to use the tables and other requirements of Chapter 8. In addition, Appendix F shows an example of how to determine the required combination of indoor and outdoor combustion air opening sizes for appliances under Chapter 7. The combustion air example also provides a table that contains the required volume of space per the appliance Btu/h input based on the standard method.
Appendix G Example Calculation of Outdoor Air Rate. Appendix G gives an example of how to calculate the required outdoor air rate under Chapter 4.
Appendix H Professional Qualifications. Appendix H provides a baseline of knowledge for installers, inspectors, or employers working with and around systems covered within the scope of this code to maintain health and safety.
Appendix I Indoor Horticultural Facilities. Appendix I addresses indoor spaces using environmental controls for horticultural cultivation and processing of plants for human ingestion, inhalation, and topical application. The appendix includes recommendations pertaining to fire protection and suppression systems, carbon dioxide detection systems, flammable solvents, ventilation and exhaust systems, particulate and odor control, and fumigation.
Appendix J Clean Air Delivery. Appendix J provides criteria for an increased level of protection for occupant health by delivering and monitoring clean air in occupied areas of certain buildings. This appendix addresses Demand Control Ventilation (DCV) for occupancy groups A, B, E, and I and recommends that each occupiable zone have CO 2 sensors which communicate with building mechanical systems to adjust airflow rates and maintain acceptable CO 2 levels. Additionally, this appendix contains provisions for mechanical system controls pertaining to sensor calibration, intervals for collected data, and automatic remediation actions to increase the amount of outdoor
CMC § 506.4 High relevance — show source text
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air
openings shall not be less than 3 inches (76 mm).
[NFPA 54:9.3.4(3)]
506.6 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5]
506.7 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
506.7.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]
506.7.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]
506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Frequently asked questions
Can I eliminate outdoor openings if my indoor volume is large enough?
Yes — if the indoor available volume ≥ required volume the ratio is ≥ 1 and the reduction factor becomes 0; however you must still meet indoor‑opening sizing and location requirements in § 701.5. See § 701.7.3.
Which full‑area basis do I use before applying the reduction factor?
Compute the full outdoor area exactly as required by § 701.6 (1/3,000, 1/4,000, or 1/2,000 in² per Btu/h depending on the method) and then multiply by the reduction factor from § 701.7.3.
What is the minimum physical opening size I can use?
No opening dimension can be less than 3 inches (76 mm), per § 701.6 and § 701.7.3.
If I use horizontal ducts to reach outdoors, how does sizing change?
When using horizontal ducts, size each opening at 1 in² per 2,000 Btu/h under the two‑opening method (see § 701.6.1(2)).
Do louvers/screens count at full size?
No — required areas are based on net free area. If free area of the grille/louver is known, use it; otherwise follow the assumptions in § 701.10.
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