A. Be of galvanized steel, in commercial buildings. In residential structures, unobstructed stud and joist spaces may be used, provided not more than one (1) required fire stop is removed. (NFPA 54, 184.108.40.206).
B. Have a minimum cross-sectional dimension of three (3) inches.
C. Terminate in a space at least three (3) inches in depth, open to the front or firebox side of the appliance enclosure.
D. Have the same cross-sectional areas as the free area of the openings to which they connect.
E. Serve a single appliance enclosure. (NFPA 54, 220.127.116.11).
F. Serve only upper or lower combustion air openings; the separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. (NFPA 54, 18.104.22.168).
G. Combustion air ducts shall not be installed so as to pass through construction where fire dampers are required. Manually operated dampers shall not be installed in combustion air ducts.
H. Neither end of ducts that terminate in an attic shall be screened. (NFPA 54, 22.214.171.124).
I. When using outside air, the air must pass through continuous ducts of the required cross-sectional area extending from the appliance enclosure to the outside of the building. The required upper combustion air duct shall extend horizontally or upwards to the outside of the building. Where not otherwise prohibited, combustion air may be obtained from a properly ventilated attic area.