The term 'flues' refers to the channels or ducts used to ventilate a building. These flues can come in the form of ducts (5 letters), stacks (6 letters), uptakes (7 letters) and airvents (8 letters). Ducts are typically made of metal and are used to transport air from one area to another. Stacks are vertical pipes that are used to exhaust smoke and other gases from a building. Uptakes are similar to stacks, but they are used to draw air into a building. Airvents are openings in walls or ceilings that allow air to enter or exit a building.