The term AIR CHANNEL can be broken down into two crossword solutions: DUCT and FLUE. DUCT is a four-letter word meaning a tube or conduit, usually for conveying air, water, or other fluid. FLUE is also a four-letter word meaning a passage or DUCT for smoke and gases to escape from a fireplace, furnace, or boiler. Together, these two words refer to an air channel, which is a system of ducts and flues used to transport air from one place to another.