Combustion, ventilation, and dilution air shall be permitted to be obtained entirely from the indoors in buildings that are not of unusually tight construction. In buildings of unusually tight construction, combustion air shall be obtained from the outdoors in accordance with Section 703, 705, 706 or 707.
Where the volume of the room in which the fuel-burning appliances are located does not comply with Section 702.2, additional inside combustion, ventilation, and dilution air shall be obtained by opening the room to adjacent spaces so that the combined volume of all communicating spaces meets the volumetric requirement of Section 702.2. Openings connecting the spaces shall comply with Sections 702.3.1, 702.3.2 and Figure A-1 of Appendix A of this code.
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.
The net free area of each opening, calculated in accordance with Section 708, shall be a minimum of 1 square inch per 1,000 Btu/h (2201 mm2/kW) of input rating of the fuel-burning appliances drawing combustion, ventilation, and dilution air from the communicating spaces and shall be not less than 100 square inches (64 516 mm2).