- For appliances other than fan-assisted, calculate using the following Equation 701.4.2(1). [NFPA 54:126.96.36.199(1)]
- For fan-assisted appliances, calculate using the following Equation 701.4.2(2). [NFPA 54: 188.8.131.52(2)]
- For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in the equations in Section 701.4.2(1) and Section 701.4.2(2). [NFPA 54:184.108.40.206(3)]
Required Volume other ≥ (21 ft3/ACH) × (Iother/1000 Btu/h)
Required Volume fan ≥ (15 ft3 /ACH) × (Ifan/1000 Btu/h)
|I other||=||All appliances other than fan-assisted input (Btu/h)|
|I fan||=||Fan-assisted appliance input (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
- Each opening shall have a free area of not less than 1 square inch per 1000 Btu/h (0.002 m2/kW) of the total input rating of appliances in the space, but not less than 100 square inches (0.065 m2). One opening shall commence within 12 inches (305 mm) of the top of the enclosure, and one opening shall commence within 12 inches (305 mm) of the bottom of the enclosure (see Figure 701.5). The dimension of air openings shall be not less than 3 inches (76 mm).
- The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more openings in doors or floors having a total free area of not less than 2 square inches per 1000 Btu/h (0.004 m2/kw) of total input rating of appliances. [NFPA 54:220.127.116.11]
- Where directly communicating with the outdoors or where communicating to the outdoors through vertical ducts, each opening shall have a free area of not less than 1 square inch per 4000 Btu/h (0.0005 m2/kW) of total input rating of appliances in the enclosure. [See Figure 701.6.1(1) and Figure 701.6.1(2)]
- Where communicating with the outdoors through horizontal ducts, each opening shall have a free area of not less than 1 square inch per 2000 Btu/h (0.001 m2/kW) of total input rating of appliances in the enclosure. [See Figure 701.6.1(3)] [NFPA 54:18.104.22.168]
- One square inch per 3000 Btu/h (0.0007 m2/kW) of the total input rating of all appliances located in the enclosure.
- Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:22.214.171.124]
- The ratio of interior spaces shall be the available volume of all communicating spaces divided by the required volume.
- The outdoor size reduction factor shall be one minus the ratio of interior spaces.
- 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)]
- Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
- Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances. [NFPA 54:126.96.36.199]
- Ducts shall serve a single space. [NFPA 54:188.8.131.52]
- 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 54:184.108.40.206]
- Ducts shall not be screened where terminating in an attic space. [NFPA 54:220.127.116.11]
- 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 54:18.104.22.168]
- Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:22.214.171.124]
- 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.