A flue's a duct, pipe, or chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. In the U.S.A., for water heaters and modern furnaces they're also known as 'vents' and for boilers as 'breeching'. They usually operate by buoyancy, also known as the stack effect, or the combustion products may be 'induced' via a blower. As combustion products contain carbon monoxide and other dangerous compounds, proper 'draft', and admission of replaceme… (More on Flue)