The flue pipe is the galvanized steel or ceramic pipe that runs up the length of your chimney. It is used to help keep the smoke and debris from your furnace from getting to the block or brick of your chimney and causing damage or even a fire. The most common diameters of flue pipes run from three inches to five inches. Anything above five inches is unusual. The most standard diameter used on a chimney flue pipe is four inches. The length of the flue pipe will vary depending on the height of your chimney and the distance the pipe needs to cover from your furnace or fireplace to the base of the chimney.