The combustion air opening shall be provided with a metal sleeve extending from the appliance enclosure to at least 6 inches (152 mm) above the top of the ceiling joists and ceiling insulation.
An inlet air duct within an outlet air duct shall be an acceptable means of supplying atticcombustion air to an appliance room provided that the inlet duct extends at least 12 inches (305 mm) above the top of the outlet duct in the attic space, as illustrated in Figure M1703.3.
The end of ducts that terminate in an attic shall not be screened.
For SI: 1 inch = 25.4 mm, 1 British thermal unit per hour = 0.2931 W, 1 square inch = 645.16 mm2.
NOTE: Each duct shall have a free area of at least 1 square inch per 4,000 Btu/h of the total input of all appliances in the space. The attic must be sufficiently ventilated to provide the required combustion air.
Where combustionair is obtained from an attic area, the attic ventilating openings shall not be subject to ice or snow blockage, and the attic shall ...
and lower combustionair openings shall be maintained to the source of combustionair. Ducts shall not be screened where terminating in an attic space ...
