All Air from Inside the Dwelling: If all combustion air is taken from the inside of the dwelling, then two permanent openings should be installed. One opening should be within ........ inches of the top and one within ...... inches of the bottom of the space. Each opening shall have a free area equal to a minimum of 1 square inch per 1,000 BTU-per-hour input rating of all appliances installed within the space, but not less than 100 square inches.