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506.0 Air for Combustion and Ventilation

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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]
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 m3/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)]
  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)]
    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)
    For SI units: 1 cubic foot = 0.0283 m3, 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 the equations in Section 506.2.2(1) and Section 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 m2/kW) of the total input rating of all appliances in the space but not less than 100 square inches (0.065 m2). 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 m2/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 (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 m2/kW) of total input rating of all appliances in the enclosure. [See Figure 506.4.1(1) and Figure 506.4.1(2)]
  2. Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 square inch per 2000 Btu/h (0.001 m2/kW) of total input rating of all appliances in the enclosure. [NFPA 54:9.3.3.1] [See Figure 506.4.1(3)]
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
ALL COMBUSTION AIR FROM OUTDOORS THROUGH SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
506.4.2 One Permanent Opening Method
One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided. The appliance shall have clearances of at least 1 inch (25.4 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 506.4.2) and shall have a minimum free area of the following:
  1. One square inch per 3000 Btu/h (0.0007 m2/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 combustion 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 and shall be approved by the Authority Having Jurisdiction. [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 (m3/min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
506.7.1 Exhaust Fans
Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
506.7.2 Interlock
Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]
506.7.3 Specified Combustion Air
Where combustion air is provided by the building's mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]
506.8 Louvers, Grilles, and Screens
The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
506.8.1 Minimum Screen Mesh Size
Screens shall not be smaller than 1/4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
506.8.2 Motorized Louvers
Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation. [NFPA 54:9.3.7.3]
506.9 Combustion Air Ducts
Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
  1. Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
    Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fire-block is removed. [NFPA 54:9.3.8.1]
  2. Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances. [NFPA 54:9.3.8.2]
  3. Ducts shall serve a single space. [NFPA 54:9.3.8.3]
  4. Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
  5. Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
  6. Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level. [NFPA 54:9.3.8.8]
  7. Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
  8. The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.
    Exception: Direct-vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer's installation instructions. [NFPA 54:9.3.8.7]

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