CMC · California Mechanical Code

When is combustion air or makeup air required for appliances and how to size it?

If exhaust fans or kitchen dryer vents disturb gas appliance burners, the California Mechanical Code requires makeup air; otherwise verify the appliance room meets combustion‑air volume rules (50 ft³ per 1000 Btu or the ACH method). If adjacent rooms are used, openings must be sized (1 in²/1000 Btu, min 100 in² and 3‑inch minimum) and located near the top and bottom; if outdoor openings or mechanical supply are used the code gives specific sizing formulas and minimum rates.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

In plain English: makeup air must be provided whenever exhaust equipment (exhaust fans, clothes dryers, kitchen ventilation) interferes with appliance operation — see § 701.3 . If using indoor air to supply appliances, the room(s) must have a required volume calculated by either the Standard Method or the Known Air Infiltration Rate Method in § 701.4 . Openings that allow adjacent interior spaces to count toward that volume must be sized/located per § 701.5 .

The single most important rule: if exhaust equipment causes negative pressures that affect burners, provide makeup air; otherwise confirm the appliance space volume meets the required cubic‑foot-per‑Btu rules in § 701.4.

Requirements in detail

When is makeup air required?

  • Makeup air: required where exhaust fans, clothes dryers, or kitchen ventilation systems interfere with appliance operation — § 701.3.

How to determine if indoor air is acceptable (how to size required volume)

You determine the required volume of the indoor space using one of two methods in § 701.4:

  • Standard Method (use unless you have a measured infiltration < 0.40 ACH): 50 cubic feet per 1000 Btu/h of combined appliance input — § 701.4.1.
  • Known Air Infiltration Rate Method (used when infiltration is known — mandatory if < 0.40 ACH): use the equations in § 701.4.2. For appliances that are not fan‑assisted the constant is 21 ft³ (in the numerator); for fan‑assisted appliances the constant is 15 ft³ — § 701.4.2.
    • Note: do not use an infiltration rate greater than 0.60 ACH in those equations.

How to size indoor openings that let adjacent rooms count

Per § 701.5 the openings that connect the appliance space to other interior spaces must meet these sizing/location rules:

  • For spaces on the same story:
    • Each required opening: minimum free area = 1 in² per 1000 Btu/h of total appliance input, but not less than 100 in².
    • One opening must begin within 12 inches of the top of the enclosure and one within 12 inches of the bottom.
    • Minimum opening dimension = 3 inches.
  • For spaces in different stories: the connecting openings (doors/floor openings) must have 2 in² per 1000 Btu/h total free area.

(If indoor openings are provided, the volumes of communicating rooms are treated as part of the required volume for § 701.4 calculations.)

Quick reference table (decision-relevant values)

Decision / value needed Value / threshold When to apply Code reference
Makeup air required? When exhaust fans, dryers, or kitchen vents interfere Any installation where exhaust affects appliance operation § 701.3
Standard required volume 50 ft³ per 1000 Btu/h Default sizing for indoor combustion air § 701.4.1
Known‑infiltration constants 21 ft³ (non‑fan), 15 ft³ (fan‑assisted) in the equations Use when ACH is measured / known; mandatory if ACH < 0.40 § 701.4.2
Max infiltration used in equations 0.60 ACH Do not use greater than this in Eqns § 701.4.2(3)
Indoor opening free area (same story) 1 in² per 1000 Btu/h, minimum 100 in² When using adjacent indoor spaces as supply § 701.5(1)
Indoor opening min dimension 3 inches All indoor/outdoor openings § 701.5 / § 701.6
Vertical duct outdoor openings (two‑opening method) 1 in² per 4000 Btu/h (each opening) Outdoor inlet/outlet via vertical ducts § 701.6.1(1)
Horizontal duct outdoor openings 1 in² per 2000 Btu/h (each opening) Outdoor inlet/outlet via horizontal ducts § 701.6.1(2)
Mechanical combustion air supply rate 0.35 cfm per 1000 Btu/h When all combustion air is supplied mechanically § 701.9

Table notes: the code bases opening sizes on net free area (account for louver/grille free area) and requires the minimum opening dimension of 3 in; use the published free‑area percentages or the actual free area for calculations — see § 701.10.

Exceptions & special cases

  • Direct‑vent appliances are exempt from the Chapter 7 combustion‑air provisions — see § 701.1.
  • Type I clothes dryers have makeup‑air rules in Chapter 5 (see cross‑reference in § 701.1).
  • Appliances that are not natural‑draft / not Category I or have power burners must follow the manufacturer’s instructions for combustion air — § 701.1.1.
  • If you provide all combustion air mechanically, the system must supply at least 0.35 cfm per 1000 Btu/h, be from outdoors, provide replacement air for exhaust, and appliances must be interlocked so they cannot fire when the mechanical supply is off — § 701.9, § 701.9.1, § 701.9.2.

Common mistakes

  • Using gross louver area instead of net free area (remember to apply the louver free‑area percentage or measured net free area) — see § 701.10.
  • Forgetting the minimum opening dimension of 3 in when sizing smaller openings — § 701.5 / § 701.6.
  • Relying on indoor volume without verifying the required volume (50 ft³/1000 Btu or using the 21/15 constants when ACH known) — § 701.4.1 / § 701.4.2.
  • Not accounting for multiple appliances (always sum inputs before sizing). This is required in § 701.4.
  • Installing motorized louvers without the required interlock that prevents burner ignition if the louver is closed — § 701.10.2.

Worked example — basement with a 100,000 Btu/h fan furnace + 40,000 Btu/h water heater

Given example (from the code commentary / Appendix): basement is 15 ft × 30 ft × 8 ft → 3600 ft³ available; combined appliance input = 140,000 Btu/h.

  1. Standard required volume = 50 ft³ / 1000 Btu/h → required = 50 × (140,000 / 1,000) = 7,000 ft³. Compare available 3600 ft³ → insufficient; outdoor combustion air required. § 701.4.1
  2. Compute full outdoor opening as if all air came from outdoors (example uses 1 in² per 3000 Btu/h for that calculation in Appendix): full size = 140,000 / 3,000 = 47 in².
    (Appendix uses the 3000 Btu/in² rule to size the single outdoor opening for the combined case.)
  3. Reduction factor from combining indoor volume: ratio = available / required = 3600 / 7000 = 0.51 → reduction factor = 1 − 0.51 = 0.49.
  4. Minimum outdoor opening = full size × reduction factor = 47 in² × 0.49 ≈ 23 in² (and minimum opening dimension must remain ≥ 3 in). § 701.7.3 / Appendix example.

This example and the step method are in the code commentary / appendices used by the CMC to demonstrate the combination indoor/outdoor approach.

Related provisions (quick links)

  • § 701.1 — Applicability (exceptions: direct‑vent; manufacturer instructions)
  • § 701.3 — Makeup air requirement (when exhaust interferes)
  • § 701.4 — Indoor combustion air methods (Standard & Known‑ACH)
  • § 701.5 — Indoor opening size & location (1 in² /1000 Btu, min 100 in², 3 in min dimension)
  • § 701.6 — Outdoor combustion air methods and opening sizing (two‑opening / single‑opening methods)
  • § 701.7 — Combination indoor/outdoor sizing (ratio and reduction factor)
  • § 701.9 — Mechanical combustion‑air supply (0.35 cfm / 1000 Btu/h and interlocks)
  • § 701.10 — Louvers, grilles, screens and net free area provisions
  • Appendix F / J — Examples and required‑volume tables used by the Code commentary (example calculations referenced above).

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 § 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 § 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

    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 § 1.11.0. High relevance — show source text
    Adopting Agency BSC BSC-
    CG    		 SFM 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-
    CG    		 SFM 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 X
    Adopt Entire Chapter as
    amended (amended sections
    listed below)
    Adopt only those sections
    that are listed below
    Chapter/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 MECHANICAL CODE 145

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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]

  • 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 § 608.9 High relevance — show source text

    608.9 Floor Registers . . . . . . . . . . . . . . . 143

    608.10 Exterior Wall and Interior Stud

    Partitions. . . . . . . . . . . . . . . . . . . . 143

    608.11 Wall Register . . . . . . . . . . . . . . . . 143

    608.12 Distance from Combustible . . . . . 143

    608.13 Vapor Barrier . . . . . . . . . . . . . . . . 143

    608.14 Prohibited . . . . . . . . . . . . . . . . . . . 143

    609.0 Automatic Shutoffs . . . . . . . . . . . . 143

    609.1 Air-Moving Systems and Smoke Detectors . . . . . . . . . . . . . . . . . . . 143

    609.2 Air-Moving Systems and Smoke Detectors in Group I-2 Occupancies . . . . . . . . . . . . . . . . 144

    CHAPTER 7 COMBUSTION AIR . . . . . . . . . . . 145

    701.0 General. . . . . . . . . . . . . . . . . . . . . 147

    701.1 Applicability. . . . . . . . . . . . . . . . . . 147

    701.2 Pressure Difference . . . . . . . . . . . 147

    701.3 Makeup Air . . . . . . . . . . . . . . . . . . 147

    701.4 Indoor Combustion Air . . . . . . . . . 147

    701.5 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . 147

    701.6 Outdoor Combustion Air . . . . . . . . 147

    701.7 Combination Indoor and Outdoor

    Combustion Air . . . . . . . . . . . . . . . 148

    701.8 Engineered Installations . . . . . . . . 149

    701.9 Mechanical Combustion Air

    Supply. . . . . . . . . . . . . . . . . . . . . . 149

    701.10 Louvers, Grilles, and Screens . . . 149

    701.11 Combustion Air Ducts. . . . . . . . . . 150

    701.12 Dampers Prohibited . . . . . . . . . . . 150

    702.0 Extra Device or Attachment . . . . . 150

    702.1 General. . . . . . . . . . . . . . . . . . . . . 150

    CHAPTER 8 CHIMNEYS AND VENTS. . . . . . . 151

    801.0 General. . . . . . . . . . . . . . . . . . . . . 153

    801.1 Applicability. . . . . . . . . . . . . . . . . . 153

    801.2 Venting of Gas Appliances . . . . . . 153

    801.3 Appliances Fueled by Other Fuels. 153

    802.0 Venting of Appliances. . . . . . . . . . 153

    xxxiv

  • CMC § 506.4.2 High relevance — show source text

    The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 506.4.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.

    (2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2]

    506.5 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 506.5.1 through Section 506.5.3. [NFPA 54:9.3.4] (See Appendix J for example calculations) 506.5.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 506.3. [NFPA 54:9.3.4(1)] 506.5.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 506.4. [NFPA 54:9.3.4(2)] 506.5.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:

    (1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.

    (2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.

    (3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air

    openings shall not be less than 3 inches (76 mm).

    [NFPA 54:9.3.4(3)]

    506.6 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5]

    506.7 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]

    506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]

    506.7.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]

    506.7.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]

  • CMC § 2025 High relevance — show source text

    Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.

    Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.

    Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.

    Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.

    Chapter 9 Installation of Specific Appliances. Chapter 9 regulates the minimum requirements for the design, construction and installation of specific appliances. The provisions address the minimum requirements for gas-fired appliances, oil-fired appliances, wood-fired appliances, and electric-type appliances. In addition to the requirements of this chapter, appliances are also required to comply with the general requirements of Chapter 3.

    Chapter 10 Boilers and Pressure Vessels. Chapter 10 regulates the construction, installation, operation, repair, and alteration of boilers and pressure vessels. The safety provisions within this chapter address controls and limit devices for automatic boilers, methods of determining expansion tank capacities, discharge piping, relief valves, shutoff valves, gas-pressure and combustion regulators, and inspections and tests. Potable water heaters are free from the requirements of Chapter 10 as they are within the scope of the Uniform Plumbing Code (UPC).

    Pressure vessels store large amounts of energy and must comply with ASME Boiler and Pressure Vessel Code (BPVC) Section VIII.1. The stored energy must be contained to prevent disastrous failures. Boilers must comply with ASME BPVC Section I, ASME BPVC Section IV, or NFPA 85. Installing a safety relief valve and expansion tank prevents pressures in the tank from exceeding the design threshold.

    Chapter 11 Refrigeration. Chapter 11 regulates the design, installation, and construction requirements of refrigeration systems and the installation and construction of cooling towers. Refrigeration is a method used for achieving heat transfer to cool spaces. Refrigerants are the most common medium used to transfer the heat energy from the low-temperature level to the high-temperature level. Table

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    FORMAT OF THE UNIFORM MECHANICAL CODE

  • CMC § 2025 High relevance — show source text

    Systems that carry nonabrasive exhaust, such as smoke, moderate abrasives such as sawdust, and high abrasives such as manganese or acid vapors use product-conveying ducts. Part II provides the minimum fire safety requirements related to the design, installation, inspection, and maintenance of grease-type operations, such as cooking, for both fuel-gas and solid fuel. Cooking produces a significant amount of smoke, fumes, vapors, heat, and other pollutants. Therefore, acceptable kitchen ventilation is necessary to prevent fires from the build-up of grease and to provide the occupants protection from smoke, unpleasant odors, pollutants, and dangerous gases. There are two types of exhaust hoods (Type I and Type II) used in commercial kitchen applications.

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    FORMAT OF THE UNIFORM MECHANICAL CODE

    Type I hoods are intended to be installed above equipment or appliances that generate grease or smoke. Type II hoods are intended to be installed above equipment or appliances that generate steam, vapor, heat, or odors and do not generate grease or smoke.

    Chapter 6 Duct Systems. Chapter 6 regulates requirements for ducts and plenums that are portions of a heating, cooling, ventilation, or exhaust system. This chapter contains material and installation requirements for metal, gypsum, factory-made, flexible, and plastic ducts. It also contains provisions for duct insulation, dampers, fire protection, and automatic shutoff for the building’s air distribution system. It is worth noting that exhaust ducts are regulated by Chapter 5, and combustion air ducts are regulated by Chapter 7.

    Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.

    Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.

    Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.

    Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.

  • CMC § 8.13.6 High relevance — show source text

    [ASHRAE 15:8.13.6]

    »

    (1) Operated, where occupied, to supply not less than 0.5 CFM/ft [2] [2.54 L/(s•m [2] )] of machinery room area or 20 cubic feet per minute (9.44 L/s) per person.

    (2) Operable, where occupied at a volume required to not exceed the higher of a temperature rise of 18°F (10°C) above inlet air temperature or a maximum temperature of 122°F (50°C).

    1106.4 Natural Ventilation. When a refrigerating system is located outdoors more than 20 feet (6096 mm) from building openings and is enclosed by a penthouse, lean-to, or other open structure, natural or mechanical ventilation shall be provided. The requirements for such natural ventilation shall be in accordance with the following:

    (1) The free-aperture cross section for the ventilation of a machinery room shall be not less than as determined in accordance with Equation 1106.4.

    F = √ G (Equation 1106.4)

    Where:

    F = The free opening area, ft [2] (m [2] ).

    G = The mass of refrigerant in the largest system, any part of which is located in the machinery room, pounds.

    For SI units: 1 square foot = 0.0929 m [2], 1 pound = 0.453 kg

    (2) Locations of the gravity ventilation openings shall be based on the relative density of the refrigerant to air.

    [ASHRAE 15:8.14]

    1106.5 Combustion Air. No open flames that use combustion air from the machinery room shall be installed where refrigerant is used. Combustion equipment shall not be installed in the same machinery room with refrigerant-containing equipment except under one of the following conditions:

    (1) Combustion air shall be ducted from outside the machinery room and sealed in such a manner as to prevent refrigerant leakage from entering the combustion chamber.

    (2) A refrigerant detector, that is in accordance with Section 1106.2.5, shall be installed to automatically shut down the combustion process in the event of refrigerant leak age.

    Exception: Machinery rooms where carbon dioxide (R-744) or water (R-718) is the refrigerant.

    1106.6 Ventilation Intake. Makeup air intakes to replace the exhaust air shall be provided to the refrigeration machinery room directly from outside the building. Intakes shall be located as required by other sections of the code and fitted with backdraft dampers or other approved flow-control means to prevent reverse flow. Distribution of makeup air shall be arranged to provide thorough mixing within the refrigeration machinery room to prevent short circuiting of the makeup air directly to the exhaust.

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    REFRIGERATION

    1106.11.6.1 Mechanical Ventilation. The machinery room shall have a mechanical ventilation system in accordance with Section 1106.11.11. The mechanical ventilation system shall:

    (1) Run continuously, and failure of the mechanical ventilation system actuates an alarm, or

  • CMC § 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 § 9.3.4 Medium relevance — show source text

    [NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]

    701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]

    701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]

    701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]

    701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.

    [NFPA 54:9.3.7.3]

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    COMBUSTION AIR

    701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:

    (1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.

  • CMC § 0.002 Medium 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 § 701.4.2 Medium 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 § 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:

Frequently asked questions

Do I always have to add outdoor makeup air when I install a new exhaust fan?

Not always. First check whether the appliance room volume already meets the required volume per § 701.4 (50 ft³ per 1000 Btu/h or the ACH method). If the exhaust causes negative pressure that interferes with appliance operation, § 701.3 requires makeup air.

How do I size openings if I use adjoining rooms to supply combustion air?

Size two permanent openings (one near the top, one near the bottom) at 1 in² per 1000 Btu/h of total appliance input, but each opening’s free area must be at least 100 in²; openings must begin within 12 in of top and bottom — § 701.5.

Can I use motorized louvers for outdoor combustion‑air openings?

Yes, but motorized louvers must be interlocked with the appliance so the main burner cannot operate unless the louvers are proven open; see § 701.10.2.

If I supply combustion air mechanically, how much airflow is required?

When all combustion air is supplied mechanically it must be at least 0.35 cfm per 1000 Btu/h (from outdoors), and the appliances must be interlocked so they will not run if the mechanical supply stops — § 701.9.

What if my building is very leaky (high ACH)?

If you know the ACH, use the Known Air Infiltration Rate Method in § 701.4.2 (with the 21 ft³ / 15 ft³ constants). Do not use an infiltration rate greater than 0.60 ACH in those calculations.

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