A flue is a duct or pipe that allows smoke and gases from a fireplace, furnace, or stove to escape safely outside a building. It helps to vent harmful byproducts of combustion, such as carbon monoxide, ensuring that indoor air remains safe and breathable. Flues are commonly found in residential and commercial heating systems. Flues can be made from various materials, including metal, clay, or masonry. They are often lined to improve efficiency and prevent the buildup of creosote, a flammable substance that can lead to chimney fires. Proper maintenance of chimneys and flues is essential for safety and performance.