A flue is a duct, pipe, or opening in a chimney that conveys exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. It can also refer to a pipe organ pipe or the internal flues of a flued boiler. Flues are essential for proper ventilation and the safe disposal of combustion byproducts. They are regulated by building codes and standards to ensure safety and efficiency