CMC · California Mechanical Code
How to size indoor combustion air (standard and known infiltration methods)?
Short answer for a homeowner: The code requires enough indoor air volume for fuel appliances — usually **50 cubic feet per 1,000 Btu/h** of appliance input. If you know the house’s air‑change rate (ACH), the code lets you use a formula that uses **21 ft³** per 1,000 Btu/h for non‑fan appliances and **15 ft³** per 1,000 Btu/h for fan‑assisted appliances divided by the ACH (but you cannot use ACH above **0.60**, and if ACH is under **0.40** you must use the known‑infiltration method). If your available indoor volume is too small, you must provide outdoor combustion air openings or other approved measures.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
The California Mechanical Code gives two ways to size indoor combustion air: the Standard Method and the Known Air Infiltration Rate Method. Use the Standard Method unless the building’s air change rate (ACH) is known to be less than 0.40 ACH, in which case the Known Air Infiltration Rate Method must be used — see § 701.4. § 701.4.1 sets the Standard Method rate as 50 ft³ per 1000 Btu/h and § 701.4.2 gives the equations and limits for the Known Infiltration method.
The single most important rule: if the structure’s infiltration is known to be less than 0.40 ACH, you must size required indoor combustion air using the Known Air Infiltration Rate Method (not the Standard Method). § 701.4
Requirements in detail
Key defined terms (first mention bolded)
- ACH — air changes per hour (fraction of the space volume exchanged per hour, expressed as a decimal). § 701.4.2 explains use of ACH in the sizing equations.
- Standard Method — the default sizing method using 50 ft³ per 1000 Btu/h. § 701.4.1.
- Known Air Infiltration Rate Method — use when ACH is known (or required when ACH < 0.40). § 701.4 and § 701.4.2.
The rules (plain-English + code pointers)
- Standard rule: Provide at least 50 ft³ of indoor volume per 1000 Btu/h of combined appliance input. See § 701.4.1.
- If the building’s infiltration rate is known (or is less than 0.40 ACH), calculate required volume using the Known Infiltration equations in § 701.4.2. The method uses different multipliers for fan‑assisted and other (non fan‑assisted) appliances and limits the ACH used (cannot use a value greater than 0.60 ACH).
- Rooms that communicate directly with the appliance room through permanent openings (not doors) or through properly sized indoor combustion-air openings are counted as part of the available volume. § 701.4.
Decision-relevant dimensions and values
| Item | Value / Expression | When to use | Code Reference |
|---|---|---|---|
| Standard required volume factor | 50 ft³ per 1000 Btu/h | Default sizing for indoor combustion air | § 701.4.1 |
| Known-infiltration — non fan-assisted factor | 21 ft³ per 1000 Btu/h (used in equation with ACH) | Use in § 701.4.2 equation for appliances other than fan‑assisted | § 701.4.2 |
| Known-infiltration — fan-assisted factor | 15 ft³ per 1000 Btu/h (used in equation with ACH) | Use in § 701.4.2 equation for fan‑assisted appliances | § 701.4.2 |
| ACH trigger to force Known Method | 0.40 ACH — if known infiltration < 0.40, use Known Method | § 701.4 | |
| ACH maximum to be used in equations | 0.60 ACH — do not use an infiltration rate greater than this in § 701.4.2 equations | § 701.4.2(3) | |
| Units conversion | 1000 Btu/h = 0.293 kW; 1 ft³ = 0.0283 m³ | For SI conversions | § text (Appendix notes) |
How the Known Air Infiltration equations are applied (code‑based phrasing)
The code provides two required-volume calculations in § 701.4.2:
For appliances other than fan‑assisted (non fan‑assisted): required indoor-volume is determined using the 21 ft³ per 1000 Btu/h factor together with the building’s ACH. See § 701.4.2(1).
For fan‑assisted appliances: required indoor-volume is determined using the 15 ft³ per 1000 Btu/h factor together with the building’s ACH. See § 701.4.2(2).
(Presented mathematically in the code — use the code equations in § 701.4.2 directly when doing calculations.)
Exceptions & special cases
- Direct-vent appliances are exempt from these combustion-air sizing provisions (see applicability exceptions under § 701.1).
- If the infiltration rate is known to be less than 0.40 ACH, the Known Infiltration Rate Method in § 701.4.2 MUST be used (you cannot rely on the Standard Method). § 701.4.
- When applying the Known Method, do not use an infiltration rate greater than 0.60 ACH — the code caps the ACH value used in the equations at 0.60. § 701.4.2(3).
- Rooms that open directly into the appliance space through permanent openings (not doors) and through indoor combustion-air openings sized per § 701.5 are considered part of the available volume; include them in your volume total. § 701.4.
Common mistakes
- Treating ACH as a percentage instead of a decimal (e.g., using 40 instead of 0.40) — ACH is expressed as a decimal in the equations. § 701.4.2.
- Forgetting to sum the input ratings of all appliances in the space before applying the 50 ft³/1000 Btu/h standard factor. § 701.4 / § 701.4.1.
- Applying Known‑Method multipliers (15 or 21) without dividing by ACH (remember the result is inversely related to ACH). See § 701.4.2 — read the equations carefully.
- Using an ACH > 0.60 in the Known Method (the code forbids that). § 701.4.2(3).
- Omitting communicating rooms or miscounting openings (check § 701.5 for opening sizing/location rules when you rely on adjacent indoor spaces). § 701.4 and § 701.5.
Worked example — concrete scenario (numbers)
Scenario: Basement with combined appliance input of 140,000 Btu/h. Basement volume = 3,600 ft³ (15' × 30' × 8'). One appliance is fan‑assisted (100,000 Btu/h); another is non fan‑assisted (40,000 Btu/h). This example follows the code appendix illustration and applies both methods for comparison.
Standard Method (use § 701.4.1):
- Required volume = (140,000 / 1000) × 50 ft³ = 140 × 50 = 7,000 ft³.
- Available = 3,600 ft³ → insufficient using Standard Method. § 701.4.1.
Known Infiltration Method (assume measured ACH = 0.50) — use § 701.4.2:
- Fan‑assisted portion (100,000 Btu/h) uses 15 ft³ per 1000 Btu/h: V_fan ≥ (15 × 100,000) / (1000 × ACH) = (15 × 100) / 0.50 = 1,500 / 0.50 = 3,000 ft³.
- Non fan‑assisted portion (40,000 Btu/h) uses 21 ft³ per 1000 Btu/h: V_other ≥ (21 × 40,000) / (1000 × ACH) = (21 × 40) / 0.50 = 840 / 0.50 = 1,680 ft³.
- Total required using Known Method = 3,000 + 1,680 = 4,680 ft³. Available = 3,600 ft³ → still insufficient. Note: you may not use ACH > 0.60 in these equations. § 701.4.2(3).
Conclusion from example: whether by Standard Method (7,000 ft³) or Known Method at 0.50 ACH (4,680 ft³), the available 3,600 ft³ is insufficient and outdoor combustion air or additional indoor connecting spaces must be provided per the Code. § 701.4 / § 701.4.2.
Related provisions (pull these for implementation details)
- § 701.5 — Indoor opening size and location for connecting indoor spaces (how to count communicating rooms).
- § 701.1 — Applicability and exceptions (direct‑vent appliances etc.).
- § 701.9 — Mechanical combustion-air supply rates and interlock requirements (when mechanical supply is used).
- § 701.10 — Louvers, grilles, screens and net free area considerations (use net free area when sizing openings).
- Appendix examples (F / J in the code text) show step‑by‑step calculations using these methods (useful for practice).
If you need, I can: (a) run the exact numeric calculation for your building (tell me room volume, appliance inputs, and known ACH if any), or (b) size required indoor openings (per § 701.5) or the corresponding outdoor opening if indoor volume is insufficient. Note: all requirements above are grounded in § 701.4, § 701.4.1, and § 701.4.2 of the CMC.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 701.3 High relevance — show source text
701.3 Makeup Air. Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided. [NFPA 54:9.3.1.5]
701.4 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method
in Section 701.4.1 or Section 701.4.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 701.4.2 shall be used. The total required volume shall be the sum of the required volume calculated for appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section 701.5, are considered a part of the required volume. [NFPA 54:9.3.2]
701.4.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 British thermal units per hour (Btu/h) (4.83 m [3] /kW). [NFPA 54:9.3.2.1]
701.4.2 Known Air Infiltration Rate Method. Where
the air infiltration rate of a structure is known, the minimum required volume shall be determined as follows
[NFPA 54:9.3.2.2]:
(1) For appliances other than fan-assisted, calculate using the following Equation 701.4.2(1). [NFPA 54:9.3.2.2(1)]
[Equation 701.4.2(1)]
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 701.4.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 701.4.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
I other = All appliances other than fan-assisted input (Btu/h) (kW) I = Fan-assisted appliance input (Btu/h) (kW) fan ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h
( )
For SI units: 1 cubic foot = 0.0283 m [3], 1000 British thermal units per
hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 701.4.2(1) and Equation 701.4.2(2). [NFPA 54:9.3.2.2(3)]
**701.5 Indoor Opening Size and Location.
CMC § 701.0 High relevance — show source text
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CHAPTER 7
COMBUSTION AIR
701.0 General.
701.1 Applicability. Air for combustion, ventilation, and dilution of flue gases for appliances installed in buildings shall be obtained by application of one of the methods covered in Section 701.4 through Section 701.9.3. Where the requirements of Section 701.4 are not met, outdoor air shall be introduced in accordance with methods covered in Section
701.6 through Section 701.9.3.
Exceptions:
(1) This provision shall not apply to direct vent appliances.
(2) Type 1 clothes dryers that are provided with makeup air in accordance with Section 504.4.1. [NFPA 54:9.3.1.1]
701.1.1 Other Types of Appliances. Appliances of other than natural draft design, appliances not designated as Category I vented appliances, and appliances equipped with power burners shall be provided with combustion, ventilation, and dilution air in accordance with the appliance manufacturer’s instructions. [NFPA 54:9.3.1.2]
701.2 Pressure Difference. Where used, a draft hood or a barometric draft regulator shall be installed in the same room or enclosure as the appliance served so as to prevent any difference in pressure between the hood or regulator and the combustion air supply. [NFPA 54:9.3.1.4]
701.3 Makeup Air. Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided. [NFPA 54:9.3.1.5]
701.4 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method
in Section 701.4.1 or Section 701.4.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 701.4.2 shall be used. The total required volume shall be the sum of the required volume calculated for appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section 701.5, are considered a part of the required volume. [NFPA 54:9.3.2]
701.4.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 British thermal units per hour (Btu/h) (4.83 m [3] /kW). [NFPA 54:9.3.2.1]
701.4.2 Known Air Infiltration Rate Method. Where
the air infiltration rate of a structure is known, the minimum required volume shall be determined as follows
[NFPA 54:9.3.2.2]:
CMC § 701.4.2 High relevance — show source text
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 701.4.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 701.4.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
I other = All appliances other than fan-assisted input (Btu/h) (kW) I = Fan-assisted appliance input (Btu/h) (kW) fan ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h
( )
For SI units: 1 cubic foot = 0.0283 m [3], 1000 British thermal units per
hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 701.4.2(1) and Equation 701.4.2(2). [NFPA 54:9.3.2.2(3)]
701.5 Indoor Opening Size and Location. Openings used to connect indoor spaces shall be sized and located in accordance with the following: (1) Combining spaces on the same story. Each opening shall have a minimum free area of 1 square inch per 1000 Btu/h (0.002 m [2] /kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.065 m [2] ). One permanent opening shall commence within 12 inches (305 mm) of the top of the enclosure and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 701.5). The minimum dimension of air openings shall not be less than 3 inches (76 mm). (2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more permanent openings in doors or floors having a total minimum free area of 2 square inches per 1000 Btu/h (0.004 m [2] /kw) of total input rating of all appliances. [NFPA 54:9.3.2.3] 701.6 Outdoor Combustion Air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accor
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COMBUSTION AIR
dance with the methods in Section 701.6.1 or Section 701.6.2. The minimum dimension of air openings shall not be less than
FIGURE 701.5
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
CMC § 506.2.2 High relevance — show source text
(1) For appliances other than fan-assisted, calculate using the following Equation 506.2.2(1). [NFPA 54:9.3.2.2(1)]
[Equation 506.2.2(1)]
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 506.2.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 506.2.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
Iother = All appliances other than fan-assisted input in (Btu/h) Ifan = Fan-assisted appliance input in (Btu/h) ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h FIGURE 506.3
( )
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
THROUGH INDOOR COMBUSTION AIR OPENINGS
[NFPA 54: FIGURE A.9.3.2.3(1)]
For SI units: 1 cubic foot = 0.0283 m [3], 1000 British thermal units
per hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 506.2.2(1) and Equation 506.2.2(2).
[NFPA 54:9.3.2.2(3)] 506.3 Indoor Opening Size and Location. Openings used to connect indoor spaces shall be sized and located in accordance with the following:
(1) Combining spaces on the same story. Each opening shall have a minimum free area of 1 square inch per 1000 Btu/h (0.002 m [2] /kW) of the total input rating of all appliances in the space but not less than 100 square inches (0.065 m [2] ). One permanent opening shall commence within 12 inches (305 mm) of the top of the enclosure and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 506.3). The minimum dimension of air openings shall not be less than 3 inches (76 mm).
(2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more permanent openings in doors or floors having a total minimum free area of 2 square inches per 1000 Btu/h (0.004 m [2] /kW) of total input rating of all appliances. [NFPA 54:9.3.2.3]
FIGURE 506.3
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
CMC § 0.293 High relevance — show source text
per hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 506.2.2(1) and Equation 506.2.2(2).
[NFPA 54:9.3.2.2(3)] 506.3 Indoor Opening Size and Location. Openings used to connect indoor spaces shall be sized and located in accordance with the following:
(1) Combining spaces on the same story. Each opening shall have a minimum free area of 1 square inch per 1000 Btu/h (0.002 m [2] /kW) of the total input rating of all appliances in the space but not less than 100 square inches (0.065 m [2] ). One permanent opening shall commence within 12 inches (305 mm) of the top of the enclosure and one permanent opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 506.3). The minimum dimension of air openings shall not be less than 3 inches (76 mm).
(2) Combining spaces in different stories. The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more permanent openings in doors or floors having a total minimum free area of 2 square inches per 1000 Btu/h (0.004 m [2] /kW) of total input rating of all appliances. [NFPA 54:9.3.2.3]
FIGURE 506.3
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
THROUGH INDOOR COMBUSTION AIR OPENINGS
[NFPA 54: FIGURE A.9.3.2.3(1)]
506.4 Outdoor Combustion Air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with the methods in Section 506.4.1 or Section 506.4.2. The minimum dimension of air openings shall not be less than 3 inches (76 mm). [NFPA 54:9.3.3] 506.4.1 Two Permanent Openings Method. Two permanent openings, one commencing within 12 inches
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WATER HEATERS
FIGURE 506.4.1(1)
ALL COMBUSTION AIR FROM OUTDOORS –
INLET AIR FROM VENTILATED CRAWL SPACE AND
OUTLET AIR TO VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(a)]
For SI units: 1 foot = 304.8 mm
FIGURE 506.4.1(2)
ALL COMBUSTION AIR FROM OUTDOORS
THROUGH VENTILATED ATTIC
[NFPA 54: FIGURE A.9.3.3.1(1)(b)]
FIGURE 506.4.1(3)
ALL COMBUSTION AIR FROM
OUTDOORS THROUGH HORIZONTAL DUCTS
[NFPA 54: FIGURE A.9.3.3.1(2)]
FIGURE 506.4.2
CMC § 11.7 High relevance — show source text
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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:
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
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 § 0.120 High relevance — show source text
(2) Bear a label with the word “Caution,” followed by the following statements:
(a) The heat-transfer medium shall be potable water or other nontoxic fluid recognized as safe by the FDA.
(b) The maximum operating pressure of the heat exchanger shall not exceed the maximum operating pressure of the potable water supply.
(3) The word “Caution” and the statements in letters shall have an uppercase height of not less than 0.120 of an inch (3.048 mm). The vertical spacing between lines of type shall be not less than 0.046 of an inch (1.168 mm). Lowercase letters shall be compatible with the uppercase letter size specification.
506.0 Air for Combustion and Ventilation.
506.1 General. Air for combustion, ventilation, and dilution of flue gases for appliances installed in buildings shall be obtained by application of one of the methods covered in Section 506.2 through Section 506.7.3. Where the requirements of Section 506.2 are not met, outdoor air shall be introduced in accordance with methods covered in Section 506.4 through Section 506.7.3.
Exception: This provision shall not apply to direct vent appliances. {NFPA 54:9.3.1.1} 506.1.1 Other Types of Appliances. Appliances of other than natural draft design, appliances not designated as Category I vented appliances, and appliances equipped with power burners shall be provided with combustion, ventilation, and dilution air in accordance with the appliance manufacturer’s instructions. [NFPA 54:9.3.1.2]
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WATER HEATERS
506.1.2 Draft Hood and Regulators. Where used, a draft hood or a barometric draft regulator shall be installed in the same room or enclosure as the appliance served so as to prevent any difference in pressure between the hood or regulator and the combustion air supply. [NFPA 54:9.3.1.4] 506.1.3 Makeup Air. Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.
[NFPA 54:9.3.1.5] 506.2 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method in Section 506.2.1 or Section 506.2.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 506.2.2 shall be used. The total required volume shall be the sum of the required volume calculated for all appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section 506.3, are considered a part of the required volume. [NFPA 54:9.3.2] 506.2.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 British thermal units per hour (Btu/h) (4.83 m [3] /kW). [NFPA 54:9.3.2.1]
CMC § 11.7 High 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 § 506.1.2 High relevance — show source text
WATER HEATERS
506.1.2 Draft Hood and Regulators. Where used, a draft hood or a barometric draft regulator shall be installed in the same room or enclosure as the appliance served so as to prevent any difference in pressure between the hood or regulator and the combustion air supply. [NFPA 54:9.3.1.4] 506.1.3 Makeup Air. Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.
[NFPA 54:9.3.1.5] 506.2 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method in Section 506.2.1 or Section 506.2.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 506.2.2 shall be used. The total required volume shall be the sum of the required volume calculated for all appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section 506.3, are considered a part of the required volume. [NFPA 54:9.3.2] 506.2.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 British thermal units per hour (Btu/h) (4.83 m [3] /kW). [NFPA 54:9.3.2.1]
506.2.2 Known Air Infiltration Rate Method. Where the air infiltration rate of a structure is known, the minimum required volume shall be determined as follows [NFPA 54:9.3.2.2]:
(1) For appliances other than fan-assisted, calculate using the following Equation 506.2.2(1). [NFPA 54:9.3.2.2(1)]
[Equation 506.2.2(1)]
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 506.2.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 506.2.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
Iother = All appliances other than fan-assisted input in (Btu/h) Ifan = Fan-assisted appliance input in (Btu/h) ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h FIGURE 506.3
( )
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
THROUGH INDOOR COMBUSTION AIR OPENINGS
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]
2025 CALIFORNIA MECHANICAL CODE 149
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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 § 101.0 Medium relevance — show source text
2025 CALIFORNIA PLUMBING CODE 487
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488 2025 CALIFORNIA PLUMBING CODE
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APPENDIX J
COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND
VENTILATION OPENING DESIGN
The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.
J 101.0 General.
J 101.1 Applicability. This appendix provides general guidelines for the sizing of combination indoor and outdoor combustion and ventilation air openings. J 101.2 Example of Combination Indoor and Outdoor Combustion Air Opening Design. Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a drafthood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces can be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 British thermal units per hour (Btu/h) (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
(1) Determine the total available room volume. Appliance room volume:
15 feet by 30 feet (4572 mm by 9144 mm) with an 8 foot (2438 mm) ceiling = 3600 cubic feet (101.94 m [3] )
(2) Determine the total required volume. The standard method to determine combustion air is used to calculate the required volume. The combined input for the appliances located in the basement is calculated as follows:
100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
The standard method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m [3] /kW). Using Table J 101.2, the required volume for a 140 000 Btu/h (41 kW) combined input is 7000 cubic feet (198.22 m [3] ).
Conclusion: The indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m [3] ) does not meet the required volume of 7000 cubic feet (198.22 m [3] ). Therefore, additional combustion air shall be provided from the outdoors.
(3) Determine ratio of the available volume to the required
volume: Col2 3600 cubic feet = 0.51 7000 cubic feet 7000 cubic feet (4) Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on the ratio of indoor spaces:
1.00 – 0.51 (from Step 3) = 0.49
(5) Determine the single outdoor combustion air opening size as though all combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors:
Frequently asked questions
How do I convert the code factors to SI (metric) units?
Multiply Btu/h by 0.000293 to get kW (1,000 Btu/h = 0.293 kW) and multiply cubic feet by 0.0283 to get cubic meters. The code notes these conversions alongside § 701.4.2.
If I don't know ACH, which method do I use?
Use the Standard Method (50 ft³/1000 Btu/h) unless you have measured or otherwise known the infiltration rate; if you know ACH < 0.40, the Known Method is required. § 701.4 and § 701.4.1.
What if my calculated required volume is larger than the room but I have adjacent rooms?
Rooms that communicate by permanent openings sized per § 701.5 are counted as part of the available volume — include them when totaling available volume. § 701.4 and § 701.5.
Can I use an ACH value greater than 0.60 to reduce required volume?
No. The code expressly prohibits using an infiltration rate greater than 0.60 ACH in the Known Method equations. § 701.4.2(3).
Are direct‑vent appliances treated the same?
No. Direct‑vent appliances are excluded from these indoor combustion‑air sizing provisions (see applicability exceptions). § 701.1.
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