M-1004.1 All air from the outdoors: Where all combustion and dilution air is to be provided by outdoor air, the required combustion and dilution air shall be obtained by opening the room to the outdoors. Openings connecting the room to the outdoor air shall comply with Sections M-1004.1.1 through M-1004.1.4. For the purposes of this chapter, an opening to a naturally ventilated crawl space or attic space shall be considered as equivalent to an opening to the outdoors.

M-1004.1.1 Number and location of openings: Two openings shall be provided, one within 1 foot (305 mm) of the ceiling of the room and one within 1 foot (305 mm) of the floor.

M-1004.1.2 Size of direct openings: The net free area of each direct opening to the outdoors, calculated in accordance with Section M-1008.0, shall be a minimum of 1 square inch per 4,000 Btuh (550 mm2/kW) of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room.

M-1004.1.3 Size of horizontal openings: The net free area of each opening, calculated in accordance with Section M-1008.0 and connected to the outdoors through a horizontal duct, shall be a minimum of 1 square inch per 2,000 Btuh (1100 mm2/kW) of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room. The cross-sectional area of the duct shall be equal to or greater than the required size of the opening.

M-1004.1.4 Size of vertical openings: The net free area of each opening, calculated in accordance with Section M-1908.0 and connected to the outdoors through a vertical duct, shall be a minimum of 1 square inch per 4,000 Btuh (550 mm2/kW) of combined input rating of the fuel-burning appliances drawing combustion and dilution air from the room. The cross-sectional area of the duct shall be equal to or greater than the required size of the opening.