M-1005.1 Combination of air from inside and from outdoors: Where the communicating interior spaces containing the fuel-burning appliances comply with all of the requirements of Section M-1003.0, except the volumetric requirement of Section M-1003.1, required combustion and dilution air shall be obtained by opening the room to the outdoors utilizing a combination of inside and outdoor air, prorated in accordance with Section M-1005.1.6. Openings connecting the interior spaces shall comply with Sections M-1003.2.1 and M-1003.2.2. The ratio of interior spaces shall comply with Section M-1005.1.5. The number, location and ratios of openings connecting the space with the outdoor air shall comply with Sections M-1005.1.1 through M-1005.1.4.

M-1005.1.1 Number and location of openings: At least 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-1005.1.2 Ratio of direct openings: Where direct openings to the outdoors are provided in accordance with Section M-1004.1, the ratio of direct openings shall be the sum of the net free areas of both direct openings to the outdoors, divided by the sum of the required areas for both such openings as determined in accordance with Section M-1004.1.2.

M-1005.1.3 Ratio of horizontal openings: Where openings connected to the outdoors through horizontal ducts are provided in accordance with Section M-1004.1, the ratio of horizontal openings shall be the sum of the net free areas of both such openings, divided by the sum of the required areas for both such openings as determined in accordance with Section M-1004.1.3.

M-1005.1.4 Ratio of vertical openings: Where openings connected to the outdoors through vertical ducts are provided in accordance with Section M-1004.1, the ratio of vertical openings shall be the sum of the net free areas of both such openings, divided by the sum of the required areas for both such openings as determined in accordance with Section M-1004.1.4.

M-1005.1.5 Ratio of interior spaces: The ratio of interior spaces shall be the available volume of all communication spaces, divided by the required volume as determined in accordance with Section M-1003.1.

M-1005.1.6 Prorating of inside and outdoor air: In spaces that utilize a combination of inside and outdoor air, the sum of the ratios of all direct openings, horizontal openings, vertical openings, and interior spaces, shall equal or exceed one.