The term 'fireplace duct' is often referred to as a FLUE, which is a four-letter word. A fireplace duct is a venting system that is used to safely remove smoke and other gases from a fireplace. It is typically made of metal and is connected to the chimney or other venting system. The purpose of the duct is to ensure that the smoke and other gases are safely removed from the home, preventing the buildup of dangerous levels of carbon monoxide.