Safety Concerns

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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.^[1] 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.

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, 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.

The term flue is also used to define certain pipe organ pipes, or rather, their construction or style.

Roman thermae constructed centuries ago had flues.

Another use of the term is for the internal flues of a flued boiler.

A flue is the passage within a chimney or within an appliance (appliance flue) that conveys products of combustion to the outdoor atmosphere.^[2] 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.^[2][3] 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.^[2] 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.^[3][4]

In U.S. model codes, a natural-draft venting system is a venting system that removes flue gases entirely by buoyancy (stack effect) under nonpositive static pressure, without mechanical fans.^[5] Natural-draft venting is typical of many Category I gas appliances (for example, draft-hood–equipped furnaces and atmospheric water heaters); appliance “Category” refers to expected condensate and vent pressure characteristics and governs permitted vent materials, but is not itself a “type of flue.”^[6]

Materials and systems. Natural-draft appliances are vented by listed systems such as lined masonry chimneys, Type B gas vents, or other materials allowed by the fuel gas code and the appliance listing.^[7][8] Vent connectors join the appliance outlet to the vent or chimney; they are part of the venting system and are distinct from HVAC ductwork.^[9]

Sizing. The fuel gas code provides prescriptive sizing for natural-draft venting systems serving one or more listed appliances (including draft-hood and fan-assisted Category I units listed for Type B vent). Correct sizing depends on total input, connector and vent height, lateral length, and other factors.^[10][11]

Installation basics. Vent connectors for natural-draft appliances must:

• Rise to the vent or chimney with a minimum upward slope of **1/4 inch per foot** (2%); avoid dips and sags.^[12]
• Observe maximum horizontal lengths and required clearances to combustibles per code and listing (e.g., single-wall connector max length typically 75% of chimney/vent height, unless engineered).^[13][14]
• Use listed/insulated materials where required (e.g., in unconditioned spaces).^[15]
• Not connect to any portion of a mechanical-draft system operating under positive pressure.^[16]

Combustion and dilution air. Natural-draft appliances depend on adequate combustion/dilution air. The fuel gas code sets methods for providing indoor or outdoor combustion air and addresses mechanical air supply when used.^[17] Because of the potential for spillage, placement in sleeping rooms and bathrooms is generally prohibited unless exceptions (such as direct-vent, sealed-combustion appliances) apply.^[18]

In U.S. model codes, a mechanical-draft venting system removes flue or vent gases by mechanical means and consists of either an induced-draft portion operating under nonpositive static pressure or a forced-draft portion operating under positive static pressure.^[19] Direct-vent appliances (sealed combustion) are defined separately; they take all combustion air from outdoors and discharge outdoors, and are installed per their listings and instructions.^[20]

Design and pressure. Portions of a venting system operating under positive pressure (forced-draft and any positive sections of induced-draft systems) must be designed and installed to prevent leakage of combustion products into the building. Vent connectors serving appliances vented by natural draft are not permitted to connect to any portion of a mechanical-draft system operating under positive pressure.^[21]

Termination and clearances. Through-the-wall direct-vent and non-direct-vent terminals must comply with the clearances in IFGC Table 503.8 and Figure 503.8 (e.g., mechanical-draft terminations at least 3 ft above any forced-air inlet within 10 ft, with listed exceptions).^[22]

Materials and listing. Mechanical-draft appliances commonly use listed special gas vents (including metallic systems listed to UL 1738 for positive-pressure/condensing categories) or other materials specifically identified in the appliance listing. Where plastic piping is used, the appliance must be listed for that venting material and the installation must follow the appliance and vent-system manufacturer’s instructions; plastic venting systems listed and labeled to UL 1738 must be installed per the vent manufacturer’s instructions.^[23][24] Trade guidance reflecting these code provisions emphasizes that (1) primer is required where specified and must be of contrasting color, (2) high-temperature polypropylene and stainless systems are often required for elevated flue-gas temperatures, and (3) components from different vent manufacturers must not be intermixed.^[25][26]

Sizing and engineering. Mechanical-draft chimney/vent sizing follows the code, listings, or engineering methods as applicable; where chimney venting uses mechanical draft, sizing by engineering methods is expressly required by adoptions based on the IFGC.^[27]

In U.S. model codes, a direct-vent appliance is constructed and installed so that all combustion air is taken directly from outdoors and all flue gases are discharged outdoors; the combustion system is sealed from the room. Listed direct-vent appliances are installed in accordance with the manufacturer’s instructions and the fuel gas code. ^[28][29]

Locations. Because they do not draw combustion air from the room, direct-vent gas appliances are typically permitted as exceptions to the general prohibition on locating fuel-fired appliances in sleeping rooms and bathrooms, when installed per their listing. ^[30]

Termination clearances. Through-the-wall terminals for direct-vent and non-direct-vent systems must meet the clearances in IFGC §503.8 (table/figure), such as required separation from doors, windows, and air inlets; local adoptions often specify a minimum of 12 in. above finished grade for the vent terminal and air intake. ^[31][32]

Materials and listing. Direct-vent appliances commonly fall under Categories II/III/IV for venting and use listed special gas vents (metallic or polymeric). Where plastic piping is used, the appliance must be listed for that venting material; plastic vent systems either follow the appliance-specified product standards or are listed and labeled to UL 1738 (USA) and installed per the vent manufacturer’s instructions (including requirements such as contrasting-color primer where applicable). Mixing components from different vent manufacturers is not permitted in UL-1738 systems. ^[33][34][35]

Practice notes (trade/education). RMGA’s code-driven guidance aligns with the model codes: (1) both pipes (combustion air and exhaust) must be installed and terminate outdoors to qualify as direct-vent; (2) manufacturer instructions/listings govern materials (e.g., UL-1738-listed polypropylene or stainless systems, or manufacturer-specified CPVC/PVC systems); and (3) direct-vent appliances are excluded from room-volume combustion-air calculations because they do not rely on indoor air. ^{[36][37][38][39]}

• Flue gas stack
• Combustion
• Masonry heater

Look up flue in Wiktionary, the free dictionary.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Chimney" – news · newspapers · books · scholar · JSTOR (March 2013) (Learn how and when to remove this message)

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 typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney effect. The space inside a chimney is called the flue. Chimneys are adjacent to large industrial refineries, fossil fuel combustion facilities or part of buildings, steam locomotives and ships.

In the United States, the term smokestack industry refers to the environmental impacts of burning fossil fuels by industrial society, including the electric industry during its earliest history. The term smokestack (colloquially, stack) is also used when referring to locomotive chimneys or ship chimneys, and the term funnel can also be used.^[1][2]

The height of a chimney influences its ability to transfer flue gases to the external environment via stack effect. Additionally, the dispersion of pollutants at higher altitudes can reduce their impact on the immediate surroundings. The dispersion of pollutants over a greater area can reduce their concentrations and facilitate compliance with regulatory limits.

Industrial chimney use dates to the Romans, who drew smoke from their bakeries with tubes embedded in the walls. However, domestic chimneys first appeared in large dwellings in northern Europe in the 12th century. The earliest surviving example of an English chimney is at the keep of Conisbrough Castle in Yorkshire, which dates from 1185 AD,^[3] but they did not become common in houses until the 16th and 17th centuries.^[4] Smoke hoods were an early method of collecting the smoke into a chimney. These were typically much wider than modern chimneys and started relatively high above the fire, meaning more heat could escape into the room. Because the air going up the shaft was cooler, these could be made of less fireproof materials. Another step in the development of chimneys was the use of built-in ovens which allowed the household to bake at home. Industrial chimneys became common in the late 18th century.

Chimneys in ordinary dwellings were first built of wood and plaster or mud. Since then chimneys have traditionally been built of brick or stone, both in small and large buildings. Early chimneys were of simple brick construction. Later chimneys were constructed by placing the bricks around tile liners. To control downdrafts, venting caps (often called chimney pots) with a variety of designs are sometimes placed on the top of chimneys.

In the 18th and 19th centuries, the methods used to extract lead from its ore produced large amounts of toxic fumes. In the north of England, long near-horizontal chimneys were built, often more than 3 km (2 mi) long, which typically terminated in a short vertical chimney in a remote location where the fumes would cause less harm. Lead and silver deposits formed on the inside of these long chimneys, and periodically workers would be sent along the chimneys to scrape off these valuable deposits.^[5]

As a result of the limited ability to handle transverse loads with brick, chimneys in houses were often built in a "stack", with a fireplace on each floor of the house sharing a single chimney, often with such a stack at the front and back of the house. Today's central heating systems have made chimney placement less critical, and the use of non-structural gas vent pipe allows a flue gas conduit to be installed around obstructions and through walls.

Most modern high-efficiency heating appliances do not require a chimney. Such appliances are generally installed near an external wall, and a noncombustible wall thimble^{[clarification needed]} allows a vent pipe to run directly through the external wall.

On a pitched roof where a chimney penetrates a roof, flashing is used to seal up the joints. The down-slope piece is called an apron, the sides receive step flashing and a cricket is used to divert water around the upper side of the chimney underneath the flashing.^[6]

Industrial chimneys are commonly referred to as flue-gas stacks and are generally external structures, as opposed to those built into the wall of a building. They are generally located adjacent to a steam-generating boiler or industrial furnace and the gases are carried to them with ductwork. Today the use of reinforced concrete has almost entirely replaced brick as a structural element in the construction of industrial chimneys. Refractory bricks are often used as a lining, particularly if the type of fuel being burned generates flue gases containing acids. Modern industrial chimneys sometimes consist of a concrete windshield with a number of flues on the inside.

The 300 m (980 ft) high steam plant chimney at the Secunda CTL's synthetic fuel plant in Secunda, South Africa consists of a 26 m (85 ft) diameter windshield with four 4.6 metre diameter concrete flues which are lined with refractory bricks built on rings of corbels spaced at 10 metre intervals. The reinforced concrete can be cast by conventional formwork or sliding formwork. The height is to ensure the pollutants are dispersed over a wider area to meet legal or other safety requirements.

A flue liner is a secondary barrier in a chimney that protects the masonry from the acidic products of combustion, helps prevent flue gas from entering the house, and reduces the size of an oversized flue. Since the 1950s, building codes in many locations require newly built chimneys to have a flue liner. Chimneys built without a liner can usually have a liner added, but the type of liner needs to match the type of appliance it services. Flue liners may be clay or concrete tile, metal, or poured in place concrete.

Clay tile flue liners are very common in the United States, although it is the only liner that does not meet Underwriters Laboratories 1777 approval and frequently they have problems such as cracked tiles and improper installation.^[7] Clay tiles are usually about 2 feet (0.61 m) long, available in various sizes and shapes, and are installed in new construction as the chimney is built. A refractory cement is used between each tile.

Metal liners may be stainless steel, aluminum, or galvanized iron and may be flexible or rigid pipes. Stainless steel is made in several types and thicknesses. Type 304 is used with firewood, wood pellet fuel, and non-condensing oil appliances, types 316 and 321 with coal, and type AL 29-4C is used with high efficiency condensing gas appliances. Stainless steel liners must have a cap and be insulated if they service solid fuel appliances, but following the manufacturer's instructions carefully.^[7] Aluminum and galvanized steel chimneys are known as class A and class B chimneys. Class A are either an insulated, double wall stainless steel pipe or triple wall, air-insulated pipe often known by its genericized trade name Metalbestos. Class B are uninsulated double wall pipes often called B-vent, and are only used to vent non-condensing gas appliances. These may have an aluminum inside layer and galvanized steel outside layer.

Concrete flue liners are like clay liners but are made of a refractory cement and are more durable than the clay liners.

Poured in place concrete liners are made by pouring special concrete into the existing chimney with a form. These liners are highly durable, work with any heating appliance, and can reinforce a weak chimney, but they are irreversible.

A chimney pot is placed on top of the chimney to expand the length of the chimney inexpensively, and to improve the chimney's draft. A chimney with more than one pot on it indicates that multiple fireplaces on different floors share the chimney.

A cowl is placed on top of the chimney to prevent birds and other animals from nesting in the chimney. They often feature a rain guard to prevent rain or snow from going down the chimney. A metal wire mesh is often used as a spark arrestor to minimize burning debris from rising out of the chimney and making it onto the roof. Although the masonry inside the chimney can absorb a large amount of moisture which later evaporates, rainwater can collect at the base of the chimney. Sometimes weep holes are placed at the bottom of the chimney to drain out collected water.

A chimney cowl or wind directional cap is a helmet-shaped chimney cap that rotates to align with the wind and prevent a downdraft of smoke and wind down the chimney.

An H-style cap is a chimney top constructed from chimney pipes shaped like the letter H. It is an age-old method of regulating draft in situations where prevailing winds or turbulences cause downdraft and back-puffing. Although the H cap has a distinct advantage over most other downdraft caps, it fell out of favor because of its bulky design. It is found mostly in marine use but has been regaining popularity due to its energy-saving functionality. The H-cap stabilizes the draft rather than increasing it. Other downdraft caps are based on the Venturi effect, solving downdraft problems by increasing the updraft constantly resulting in much higher fuel consumption.

A chimney damper is a metal plate that can be positioned to close off the chimney when not in use and prevent outside air from entering the interior space, and can be opened to permit hot gases to exhaust when a fire is burning. A top damper or cap damper is a metal spring door placed at the top of the chimney with a long metal chain that allows one to open and close the damper from the fireplace. A throat damper is a metal plate at the base of the chimney, just above the firebox, that can be opened and closed by a lever, gear, or chain to seal off the fireplace from the chimney. The advantage of a top damper is the tight weatherproof seal that it provides when closed, which prevents cold outside air from flowing down the chimney and into the living space—a feature that can rarely be matched by the metal-on-metal seal afforded by a throat damper. Additionally, because the throat damper is subjected to intense heat from the fire directly below, it is common for the metal to become warped over time, thus further degrading the ability of the throat damper to seal. However, the advantage of a throat damper is that it seals off the living space from the air mass in the chimney, which, especially for chimneys positioned on an outside of wall of the home, is generally very cold. It is possible in practice to use both a top damper and a throat damper to obtain the benefits of both. The two top damper designs currently on the market are the Lyemance (pivoting door) and the Lock Top (translating door).

In the late Middle Ages in Western Europe the design of stepped gables arose to allow maintenance access to the chimney top, especially for tall structures such as castles and great manor houses.

When coal, oil, natural gas, wood, or any other fuel is combusted in a stove, oven, fireplace, hot water boiler, or industrial furnace, the hot combustion product gases that are formed are called flue gases. Those gases are generally exhausted to the ambient outside air through chimneys or industrial flue-gas stacks (sometimes referred to as smokestacks).

The combustion flue gases inside the chimneys or stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draught/draft", "natural ventilation", "chimney effect", or "stack effect". The taller the stack, the more draught or draft is created. There can be cases of diminishing returns: if a stack is overly tall in relation to the heat being sent out of the stack, the flue gases may cool before reaching the top of the chimney. This condition can result in poor drafting, and in the case of wood burning appliances, the cooling of the gases before emission can cause creosote to condense near the top of the chimney. The creosote can restrict the exit of flue gases and may pose a fire hazard.

Designing chimneys and stacks to provide the correct amount of natural draft involves a number of design factors, many of which require iterative trial-and-error methods.

As a "first guess" approximation, the following equation can be used to estimate the natural draught/draft flow rate by assuming that the molecular mass (i.e., molecular weight) of the flue gas and the external air are equal and that the frictional pressure and heat losses are negligible: $$\displaystyle Q=C\,A\,\sqrt 2\,g\,H\,\frac T_i-T_eT_e$$ where:

• Q = chimney draught/draft flow rate, m³/s
• A = cross-sectional area of chimney, m² (assuming it has a constant cross-section)
• C = discharge coefficient (usually taken to be from 0.65 to 0.70)
• g = gravitational acceleration, 9.807 m/s²
• H = height of chimney, m
• T_i = average temperature inside the chimney, K
• T_e = external air temperature, K.

Combining two flows into chimney: A_t+A_f<A, where A_t=7.1 inch² is the minimum required flow area from water heater tank and A_f=19.6 inch² is the minimum flow area from a furnace of a central heating system.

Gas fired appliances must have a draft hood to cool combustion products entering the chimney and prevent updrafts or downdrafts.^[8][9][10]

A characteristic problem of chimneys is they develop deposits of creosote on the walls of the structure when used with wood as a fuel. Deposits of this substance can interfere with the airflow and more importantly, they are combustible and can cause dangerous chimney fires if the deposits ignite in the chimney.

Heaters that burn natural gas drastically reduce the amount of creosote buildup due to natural gas burning much cleaner and more efficiently than traditional solid fuels. While in most cases there is no need to clean a gas chimney on an annual basis that does not mean that other parts of the chimney cannot fall into disrepair. Disconnected or loose chimney fittings caused by corrosion over time can pose serious dangers for residents due to leakage of carbon monoxide into the home.^[11] Thus, it is recommended—and in some countries even mandatory—that chimneys be inspected annually and cleaned on a regular basis to prevent these problems. The workers who perform this task are called chimney sweeps or steeplejacks. This work used to be done largely by child labour and, as such, features in Victorian literature. In the Middle Ages in some parts of Europe, a stepped gable design was developed, partly to provide access to chimneys without use of ladders.

Masonry (brick) chimneys have also proven to be particularly prone to crumbling during earthquakes. Government housing authorities in cities prone to earthquakes such as San Francisco, Los Angeles, and San Diego now recommend building new homes with stud-framed chimneys around a metal flue. Bracing or strapping old masonry chimneys has not proven to be very effective in preventing damage or injury from earthquakes. It is now possible to buy "faux-brick" facades to cover these modern chimney structures.

Other potential problems include:

• "spalling" brick, in which moisture seeps into the brick and then freezes, cracking and flaking the brick and loosening mortar seals.
• shifting foundations, which may degrade integrity of chimney masonry
• nesting or infestation by unwanted animals such as squirrels, racoons, or chimney swifts
• chimney leaks
• drafting issues, which may allow smoke inside building^[12]
• issues with fireplace or heating appliance may cause unwanted degradation or hazards to chimney

Several chimneys with observation decks were built. The following possibly incomplete list shows them.

At several thermal power stations at least one smokestack is used as electricity pylon. The following possibly incomplete list shows them.

Nearly all this structures exist in an area, which was once part of the Soviet Union. Although this use has the disadvantage that conductor ropes may corrode faster due to the exhaust gases, one can find such structures also sometimes in countries not influenced by the former Soviet Union. An example herefore is one chimney of Scholven Power Plant in Gelsenkirchen, which carries one circuit of an outgoing 220 kV-line.

Chimneys can also carry a water tank on their structure. This combination has the advantage that the warm smoke running through the chimney prevents the water in the tank from freezing. Before World War II such structures were not uncommon, especially in countries influenced by Germany.

Chimneys can carry antennas for radio relay services, cell phone transmissions, FM-radio and TV on their structure. Also long wire antennas for mediumwave transmissions can be fixed at chimneys. In all cases it had to be considered that these objects can easily corrode especially when placed near the exhaust. Sometimes chimneys were converted into radio towers and are not useable as ventilation structure any more.

As chimneys are often the tallest part of a factory, they offer the possibility as advertising billboard either by writing the name of the company to which they belong on the shaft or by installing advertisement boards on their structure.

At some power stations, which are equipped with plants for the removal of sulfur dioxide and nitrogen oxides, it is possible to use the cooling tower as a chimney. Such cooling towers can be seen in Germany at the Großkrotzenburg Power Station and at the Rostock Power Station. At power stations that are not equipped for removing sulfur dioxide, such usage of cooling towers could result in serious corrosion problems which are not easy to prevent.

Wikimedia Commons has media related to Chimneys.

Look up chimney in Wiktionary, the free dictionary.

• CICIND – International Committee on Industrial Chimneys
• Chimney Safety Institute of America
• Power Station Konakovskaya GRES, at which chimneys serve as electricity pylons
• Article about chimney breast removal
• Chimney Maintenance Information
• European Chimney Association ECA; to find further information on chimneys
• National Association of Chimney Engineers; UK trade association for the chimney engineering industry
• Collection of chimneys used as electricity pylons on Skyscraperpage.com