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Each opening must have a minimum free area of at least 1 square inch per 1000 BTU per hour of total input rating of all appliances within the space but not less than 100 square inches. If the furnace is installed in a space within a building of tight construction and air must be supplied from outdoors. In this case, one opening shall be within 12" of the ceiling and the other within 12" of the floor. If vertical combustion ducts are run, each opening must have a free area of at least 1 square in