Flue

For other uses, see Flue (disambiguation).

Not to be confused with Flu.

A seven-flue chimney in a four-story Georgian house in London, showing alternative methods of sweeping

A flue is a 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.

A flue is the passage within a chimney or within an appliance (appliance flue) that conveys products of combustion to the outdoor atmosphere. In U.S. model codes, the regulated venting system is the continuous open passageway from the appliance’s flue collar or draft hood to outdoors, typically consisting of a vent or chimney and any vent connector; HVAC ductwork is not part of, and may not be used as, a venting system. Appliances generally discharge combustion products to the outdoors; venting may occur by natural draft (buoyancy) or by mechanical draft (fan-assisted), and direct-vent appliances are sealed-combustion units that take all combustion air from outdoors and discharge outdoors. Acceptable venting materials and terminations are prescribed by code and by the appliance/vent listing—for example, Type B gas vent for many Category I appliances, and listed special gas vent systems (e.g., systems listed to UL 1738) where positive pressure or condensate is expected—with installation following both the fuel-gas code and the manufacturer’s instructions.