![Seven-flue Stack 1834](https://upload.wikimedia.org/wikipedia/commons/5/5c/Seven-flue_Stack_1834.png)
A flue is a
duct,
pipe
Pipe(s), PIPE(S) or piping may refer to:
Objects
* Pipe (fluid conveyance), a hollow cylinder following certain dimension rules
** Piping, the use of pipes in industry
* Smoking pipe
** Tobacco pipe
* Half-pipe and quarter pipe, semi-circula ...
, or opening in a
chimney
A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typic ...
for conveying
exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
es from a
fireplace
A fireplace or hearth is a structure made of brick, stone or metal designed to contain a fire. Fireplaces are used for the relaxing ambiance they create and for heating a room. Modern fireplaces vary in heat efficiency, depending on the design.
...
,
furnace
A furnace is a structure in which heat is produced with the help of combustion.
Furnace may also refer to:
Appliances Buildings
* Furnace (central heating): a furnace , or a heater or boiler , used to generate heat for buildings
* Boiler, used t ...
,
water heater
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated t ...
,
boiler
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central h ...
, or
generator
Generator may refer to:
* Signal generator, electronic devices that generate repeating or non-repeating electronic signals
* Electric generator, a device that converts mechanical energy to electrical energy.
* Generator (circuit theory), an eleme ...
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
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
, also known as the
stack effect
The stack effect or chimney effect is the movement of air into and out of buildings through unsealed openings, chimneys, flue-gas stacks, or other containers, resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor a ...
, or the combustion products may be 'induced' via a
blower. As combustion products contain
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
and other dangerous compounds, proper 'draft', and admission of replacement air is imperative.
Building code
A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permission ...
s, and other standards, regulate their materials, design, and installation.
Heat retention
Flues are adjustable and are designed to release noxious gases to the atmosphere. They often have the disadvantageous effect of releasing useful household heat to the atmosphere when not properly set—the very opposite of why the fire was lit in the first place.
Fireplaces are one of the biggest energy wasters when the flue is not used properly. This occurs when the flue is left open too wide after the fire is started. Known as
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
, warm air from the house is pulled up the chimney, while cold air from outside is pulled into the house wherever it can enter, including around leaking windows and doors. Ideally, the flue should be open all the way when the fire is first started, and then adjusted toward closure as the fire burns until it is open just enough to slowly pull smoke from the fire up the chimney. After the flue heats up from the fire, they are easier to move, but also hotter. Hands should be protected when operating the flue lever; and if a new log is added to the fire, the flue must be adjusted again to ensure that smoke does not billow out into the house.
In some countries, wood fire flues are often built into a heat preserving construction within which the flue gases circulate over heat retaining bricks before release to the atmosphere. The heat retaining bricks are covered in a decorative material such as brick, tiles or stone. This flue gas circulation avoids the considerable heat loss to the chimney and outside air in conventional systems. The heat from the flue gases is absorbed quickly by the bricks and then released slowly to the house rather than the chimney. In a well insulated home, a single load fire burning for one and a half hours twice a day is enough to keep an entire home warm for a 24-hour period. In this way, less fuel is used, and noxious emissions are reduced. Sometimes, the flue incorporates a second combustion chamber where combustibles in the flue gas are burnt a second time, reducing soot, noxious emissions and increasing overall efficiency.
Other uses
Organs
The term flue is also used to define certain
pipe organ pipes, or rather, their construction or style.
Bath-houses
Roman
thermae
In ancient Rome, (from Greek , "hot") and (from Greek ) were facilities for bathing. usually refers to the large Roman Empire, imperial public bath, bath complexes, while were smaller-scale facilities, public or private, that existed i ...
constructed centuries ago had flues.
Boilers
Another use of the term is for the internal flues of a
flued boiler
A shell or flued boiler is an early and relatively simple form of boiler used to make steam, usually for the purpose of driving a steam engine. The design marked a transitional stage in boiler development, between the early haystack boilers and t ...
.
Flue types
Flue types include:
* Open flue – An open flued appliance draws combustion air from its surroundings; exhaust gases rise in a vortex up the open flue. The 19th century
Rumford fireplace
The Rumford fireplace is a tall, shallow fireplace designed by Sir Benjamin Thompson, Count Rumford, an Anglo-American physicist best known for his investigations of heat. Its shallow, angled sides are designed to reflect heat into the room, and it ...
is an appliance that elegantly refined this principle.
* Closed flue – When an air duct brings combustion air directly to an appliance, there is no air movement between the room containing the appliance. This makes Room Sealed appliances intrinsically safe, since products of combustion (POCs) cannot normally escape into the room. Two major types of closed flues are used with Room Sealed appliances:
** Balanced flue – Use the natural draught created by the hot exhaust, so the flue is as short as possible, usually sufficiently long to pass through an external wall against or near which the appliance is installed. The balanced flue terminal has both flue outlet and air inlet in close proximity, so any draughts or wind gusts pressure both equally and cancel out in the appliance. Thus the burner flame and even pilot lights are not affected.
** Power flue – Fanned flues use the pressure created by a fan to power the movement of outside air into the appliance and combustion products to external air. Thus air and flue ducts can be much longer, of smaller diameter, and include changes of direction. Most ducting is concentric; the air duct contains the smaller flue duct within it. Thus a leak from the inner flue duct will only leak the exhaust into the appliance rather than into a room. However longer runs are possible with two-pipe systems where the flue and air ducts run separately. Instead of using closed flues, some power flues use open flues, taking combustion air from the room housing the appliance. As with all open flues, the provision of adequate ventilation to supply combustion air to such appliances is vitally important.
* False flue – While two or three flues may be required for separate heat sources, several "false" flues may be added at the top of a chimney for aesthetic purpose.
*
Die flue – A flue in a die used in injection molding.
See also
*
Flue gas stack
A flue-gas stack, also known as a smoke stack, chimney stack or simply as a stack, is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue g ...
*
Combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
*
Masonry heater
A masonry heater (also called a masonry stove) is a device for warming an interior space through radiant heating, by capturing the heat from periodic burning of fuel (usually wood), and then radiating the heat at a fairly constant temperature ...
References
{{Authority control
Fireplaces
Heating, ventilation, and air conditioning