A
flue is a
duct,
pipe, or opening in a
chimney for conveying
exhaust gases from a
fireplace,
furnace,
water heater,
boiler, or
generator to the outdoors. Historically the term flue meant the chimney itself. In the United States, they are also known as
vents for boilers and as
breeching for water heaters and modern furnaces. 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 replacement air is imperative.
Building codes, and other standards, regulate their materials, design, and installation.