All Air from Outdoors: If all combustion air is taken from the outdoor air, then one opening should be within 12 inches of the top and one within 12 inches of the bottom of the space. The openings are permitted to connect to spaces directly communicating with the outdoor air, such as a ventilated crawlspace or ventilated attic space. Each opening should have a free area of at least 1 square inch per 4,000 BTU per hour of total input rating of all appliances in the space when using ......ducts, or 2,000 BTU per hour if using .......ducts.