Fireplace

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Modern open fireplace Open fireplace with icon.jpg
Modern open fireplace
An outdoor fireplace Outdoor Fireplace.JPG
An outdoor 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.

Contents

Historically, they were used for heating a dwelling, cooking, and heating water for laundry and domestic uses. A fire is contained in a firebox or fire pit; a chimney or other flue allows exhaust gas to escape. A fireplace may have the following: a foundation, a hearth, a firebox, a mantel, a chimney crane (used in kitchen and laundry fireplaces), a grate, a lintel, a lintel bar, an overmantel, a damper, a smoke chamber, a throat, a flue, and a chimney filter or afterburner. [1]

On the exterior, there is often a corbelled brick crown, in which the projecting courses of brick act as a drip course to keep rainwater from running down the exterior walls. A cap, hood, or shroud serves to keep rainwater out of the exterior of the chimney; rain in the chimney is a much greater problem in chimneys lined with impervious flue tiles or metal liners than with the traditional masonry chimney, which soaks up all but the most violent rain. Some chimneys have a spark arrestor incorporated into the crown or cap.

Organizations like the United States Environmental Protection Agency (EPA) and the Washington State Department of Ecology warn that, according to various studies, fireplaces can pose health risks. [2] [3] The EPA writes "Smoke may smell good, but it's not good for you." [2]

Types of fireplaces

Masonry and prefabricated fireplaces can be fueled by:

Ventless fireplaces (duct free/room-venting fireplaces) are fueled by either gel, liquid propane, bottled gas or natural gas.[ clarification needed ] In the United States, some states and local counties have laws restricting these types of fireplaces. They must be properly sized to the area to be heated. [4] There are also air quality control issues due to the amount of moisture they release into the room air, and an oxygen sensor and a carbon monoxide detector are safety essentials.

Direct vent fireplaces are fueled by either liquid propane or natural gas. They are completely sealed from the area that is heated, and vent all exhaust gasses to the exterior of the structure.

Chimney and flue types:

Newly constructed flues may feature a chase cover, a cap, and a spark arrestor at the top to keep small animals out and to prevent sparks from being broadcast into the atmosphere. All fireplaces require trained gas service members to carry out installations.

Accessories

A fender (colorized) is set in front of the fireplace to contain embers, soot and ash Fireplace fender (false colour).jpg
A fender (colorized) is set in front of the fireplace to contain embers, soot and ash

A wide range of accessories are used with fireplaces, which range between countries, regions, and historical periods. For the interior, common in recent Western cultures include grates, fireguards, log boxes, andirons and pellet baskets, all of which cradle fuel and accelerate combustion. A grate (or fire grate) is a frame, usually of iron bars, to retain fuel for a fire. Heavy metal firebacks are sometimes used to capture and re-radiate heat, to protect the back of the fireplace, and as decoration. Fenders are low metal frames set in front of the fireplace to contain embers, soot and ash. For fireplace tending, tools include pokers, bellows, tongs, shovels, brushes and tool stands. Other wider accessories can include log baskets, companion sets, coal buckets, cabinet accessories and more.

History

Cooking over a fireplace Kochen uber offenem Feuer.JPG
Cooking over a fireplace

Ancient fire pits were sometimes built in the ground, within caves, or in the center of a hut or dwelling. Evidence of prehistoric, man-made fires exists on all five inhabited continents. The disadvantage of early indoor fire pits was that they produced toxic and/or irritating smoke inside the dwelling.

Fire pits developed into raised hearths in buildings, but venting smoke depended on open windows or holes in roofs. The medieval great hall typically had a centrally located hearth, where an open fire burned with the smoke rising to the vent in the roof. Louvers were developed during the Middle Ages to allow the roof vents to be covered so rain and snow would not enter.

Also during the Middle Ages, smoke canopies were invented to prevent smoke from spreading through a room and vent it out through a wall or roof. These could be placed against stone walls, instead of taking up the middle of the room, and this allowed smaller rooms to be heated.

Chimneys were invented in northern Europe in the 11th or 12th century and largely fixed the problem of smoke, more reliably venting it outside. They made it possible to give the fireplace a draft, and also made it possible to put fireplaces in multiple rooms in buildings conveniently. They did not come into general use immediately, however, as they were expensive to build and maintain.

In 1678, Prince Rupert, nephew of Charles I, raised the grate of the fireplace, improving the airflow and venting system. The 18th century saw two important developments in the history of fireplaces. Benjamin Franklin developed a convection chamber for the fireplace that greatly improved the efficiency of fireplaces and wood-burning stoves. He also improved the airflow by pulling air from a basement and venting out a longer area at the top. In the later 18th century, Count Rumford designed a fireplace with a tall, shallow firebox that was better at drawing the smoke up and out of the building. The shallow design also improved greatly the amount of heat transfer projected into the room. Rumford's design is the foundation for modern fireplaces.

The Aesthetic movement of the 1870s and 1880s took on a more traditional spectra based on stone and deflected unnecessary ornamentation. Rather it relied on simple designs with little unnecessary ornamentation. In the 1890s, the Aesthetic movement gave way to the Arts and Crafts movement, where the emphasis was still placed on providing quality stone. Stone fireplaces at this time were a symbol of prosperity, which to some degree is still the notion today.

Evolution of fireplace design

Over time, the purpose of fireplaces has changed from one of necessity to one of visual interest. [5] Early ones were more fire pits than modern fireplaces. They were used for warmth on cold days and nights, as well as for cooking. They also served as a gathering place within the home. These fire pits were usually centered within a room, allowing more people to gather around it.

Many flaws were found in early fireplace designs. Along with the Industrial Revolution, came large-scale housing developments, necessitating a standardization of fireplaces. The most renowned fireplace designers of this time were the Adam Brothers: John Adam, Robert Adam, and James Adam. They perfected a style of fireplace design that was used for generations. It was smaller, more brightly lit, with an emphasis on the quality of the materials used in their construction, instead of their size.

By the 1800s, most new fireplaces were made up of two parts, the surround and the insert. The surround consisted of the mantelpiece and side supports, usually in wood, marble or granite. The insert was where the fire burned, and was constructed of cast iron often backed with decorative tiles. As well as providing heat, the fireplaces of the Victorian era were thought to add a cosy ambiance to homes. [6]

Heating efficiency

Fireplace with tubular grate heater, with a high surface area in its heat exchanger and a lift out ash tray to simplify cleanup 2 row grate heater with ash tray.jpg
Fireplace with tubular grate heater, with a high surface area in its heat exchanger and a lift out ash tray to simplify cleanup

Some fireplace units incorporate a blower, which transfers more of the fireplace's heat to the air via convection, resulting in a more evenly heated space and a lower heating load. Fireplace efficiency can also be increased with the use of a fireback, a piece of metal that sits behind the fire and reflects heat back into the room. Firebacks are traditionally made from cast iron, but are also made from stainless steel. [7]

Enclosed wood-fueled fireplace equipped with a blower fan which takes in air through the bottom vent, blows it around the firebox to heat it, and releases it via the top vent Lennox Ladera.jpg
Enclosed wood-fueled fireplace equipped with a blower fan which takes in air through the bottom vent, blows it around the firebox to heat it, and releases it via the top vent

Most older fireplaces have a relatively low efficiency rating. Standard, modern, wood-burning masonry fireplaces though have an efficiency rating of at least 80% (legal minimum requirement, for example, in Salzburg, Austria). [8] To improve efficiency, fireplaces can also be modified by inserting special heavy fireboxes designed to burn much cleaner and can reach efficiencies as high as 80% in heating the air. These modified fireplaces are often equipped with a large fire window, enabling an efficient heating process in two phases. During the first phase the initial heat is provided through a large glass window while the fire is burning. During this time the structure, built of refractory bricks, absorbs the heat. This heat is then evenly radiated for many hours during the second phase. Masonry fireplaces without a glass fire window only provide heat radiated from its surface. Depending on the outside temperature, 1 to 2 daily firings are sufficient to ensure a constant room temperature.

Health effects

Wood

A literature review published in the Journal of Toxicology and Environmental Health concludes that there are a wide variety of health risks posed by residential wood combustion. It states:

With regard to adults, studies show that prolonged inhalation of wood smoke contributed to chronic bronchitis, chronic interstitial lung disease, pulmonary arterial hypertension and corpulmonale (Pulmonary heart disease), and altered pulmonary immune defense mechanisms. While adverse effects on adults are notable, children appear to be at greatest risk. Many studies that focused specifically on RWC [Residential Wood Combustion] have concluded that young children living in homes heated by a wood-burning stove had a greater occurrence of moderate and severe chronic respiratory symptoms than children of the same age and sex who did not live in homes heated with a wood burning stove. Exposure of preschool children living in homes heated with wood burning stoves or in houses with open fireplaces yielded these effects: decreased pulmonary lung function in young asthmatics; increased incidence of acute bronchitis and severity/frequency of wheezing and coughing; and increased incidence, duration, and possibly severity of acute respiratory infections. Residential wood combustion emissions also contain sulfur oxides, nitrogen oxides, carbon monoxide and potentially carcinogenic compounds including polycyclic aromatic hydrocarbons, benzene, formaldehyde and dioxins. Some of these pollutants are known to cause cancer but their effects on human health via exposure to wood smoke have not been extensively studied. [9]

The Washington State Department of Ecology also published a booklet explaining why wood smoke can be dangerous. It explains that human lung and respiratory systems are unable to filter particulates emitted by wood combustion, which penetrate deeply into the lungs. For months, carcinogens can continue to cause changes and structural damage within the respiratory system. Young children, seniors, pregnant women, smokers and individuals with respiratory diseases are most vulnerable. Wood smoke can cause disease and even death in children, because it is associated with lower respiratory tract infections. [3] Home fireplaces have caused fatal carbon monoxide poisoning. [10]

Gases and ethanol

Propane, butane, and methane are all flammable gases used in fireplaces (natural gas is mostly methane, liquefied petroleum gas mostly propane). If they are allowed to accumulate unburned, gases can cause asphyxia by displacing air, [13] and gas explosions.[ citation needed ] Ethanol (a liquid, also sold in gels) fires can also cause severe burns. [14]

Burning hydrocarbons can decrease indoor air quality. Emissions include airborne particulate matter (such as black carbon) and gases like nitrogen oxide. These harm health: they weaken the immune system, and increase infections, blood pressure, cardiovascular diseases, and insulin resistance. Some forms of fuel are more harmful than others. [15]

Burning hydrocarbon fuels incompletely can produce carbon monoxide, which is highly poisonous and can cause death and long-term neurological disorders. [16]

Environmental effects

Burning any hydrocarbon fuel releases carbon dioxide and water vapor. Other emissions, such as nitrogen oxides and sulfur oxides, can be harmful to the environment.

Glossary

Several of these terms may be compounded with chimney or fireplace such as chimney-back.

See also

Related Research Articles

<span class="mw-page-title-main">Kiln</span> Oven that generates high temperatures

A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay into pottery, tiles and bricks. Various industries use rotary kilns for pyroprocessing—to calcinate ores, to calcinate limestone to lime for cement, and to transform many other materials.

<span class="mw-page-title-main">Hearth</span> Place for a fire to heat the home and to cook food, usually of masonry

A hearth is the place in a home where a fire is or was traditionally kept for home heating and for cooking, usually constituted by at least a horizontal hearthstone and often enclosed to varying degrees by any combination of reredos, fireplace, oven, smoke hood, or chimney. Hearths are usually composed of masonry such as brick or stone. For centuries, the hearth was such an integral part of a home, usually its central and most important feature, that the concept has been generalized to refer to a homeplace or household, as in the terms "hearth and home" and "keep the home fires burning". In the modern era, since the advent of central heating, hearths are usually less central to most people's daily life because the heating of the home is instead done by a furnace or a heating stove, and cooking is instead done with a kitchen stove/range alongside other home appliances; thus many homes built in the 20th and 21st centuries do not have hearths. Nonetheless, many homes still have hearths, which still help serve the purposes of warmth, cooking, and comfort.

<span class="mw-page-title-main">Chimney</span> Ventilation for hot gases or smoke

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.

<span class="mw-page-title-main">Barbecue grill</span> Device for barbecueing or grilling

A barbecue grill or barbeque grill is a device that cooks food by applying heat from below. There are several varieties of grills, with most falling into one of three categories: gas-fueled, charcoal, or electric. There is debate over which method yields superior results.

<span class="mw-page-title-main">Franklin stove</span> Type of fireplace

The Franklin stove is a metal-lined fireplace named after Benjamin Franklin, who invented it in 1742. It had a hollow baffle near the rear and relied on an "inverted siphon" to draw the fire's hot fumes around the baffle. It was intended to produce more heat and less smoke than an ordinary open fireplace, but it achieved few sales until it was improved by David Rittenhouse. It is also known as a "circulating stove" or the "Pennsylvania fireplace".

<span class="mw-page-title-main">Stove</span> Device used to generate heat or to cook

A stove or range is a device that burns fuel or uses electricity to generate heat inside or on top of the apparatus, to be used for general warming or cooking. It has evolved highly over time, with cast-iron and induction versions being developed. Stoves can be powered with many fuels, such as electricity, gasoline, wood, and coal.

<span class="mw-page-title-main">Wood fuel</span> Wood used as fuel for combustion

Wood fuel is a fuel such as firewood, charcoal, chips, sheets, pellets, and sawdust. The particular form used depends upon factors such as source, quantity, quality and application. In many areas, wood is the most easily available form of fuel, requiring no tools in the case of picking up dead wood, or few tools, although as in any industry, specialized tools, such as skidders and hydraulic wood splitters, have been developed to mechanize production. Sawmill waste and construction industry by-products also include various forms of lumber tailings.

<span class="mw-page-title-main">Kitchen stove</span> Kitchen appliance designed for the purpose of cooking food

A kitchen stove, often called simply a stove or a cooker, is a kitchen appliance designed for the purpose of cooking food. Kitchen stoves rely on the application of direct heat for the cooking process and may also contain an oven, used for baking. "Cookstoves" are heated by burning wood or charcoal; "gas stoves" are heated by gas; and "electric stoves" by electricity. A stove with a built-in cooktop is also called a range.

<span class="mw-page-title-main">Fire-tube boiler</span> Type of boiler

A fire-tube boiler is a type of boiler in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

<span class="mw-page-title-main">Rumford fireplace</span>

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 its streamlined throat minimizes turbulence, thereby carrying away smoke with little loss of heated room air.

<span class="mw-page-title-main">Ondol</span> Traditional Korean underfloor heating system

Ondol or gudeul in Korean traditional architecture, is underfloor heating that uses direct heat transfer from wood smoke to heat the underside of a thick masonry floor. In modern usage it refers to any type of underfloor heating, or to a hotel or a sleeping room in Korean style.

<span class="mw-page-title-main">Gas heater</span>

A gas heater is a space heater used to heat a room or outdoor area by burning natural gas, liquefied petroleum gas, propane or butane.

<span class="mw-page-title-main">Flue</span> Exhaust for a fireplace, furnace etc

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.

<span class="mw-page-title-main">Pellet stove</span>

A pellet stove is a stove that burns compressed wood or biomass pellets to create a source of heat for residential and sometimes industrial spaces. By steadily feeding fuel from a storage container (hopper) into a burn pot area, it produces a constant flame that requires little to no physical adjustments. Today's central heating systems operated with wood pellets as a renewable energy source can reach an efficiency factor of more than 90%.

<span class="mw-page-title-main">Masonry heater</span> Heating device

A masonry heater is a device for warming an interior space through radiant heating, by capturing the heat from periodic burning of fuel, and then radiating the heat at a fairly constant temperature for a long period. Masonry heaters covered in tile are called cocklestoves. The technology has existed in different forms, from back into the Neoglacial and Neolithic periods. Archaeological digs have revealed excavations of ancient inhabitants utilizing hot smoke from fires in their subterranean dwellings, to radiate into the living spaces. These early forms have evolved into modern systems.

<span class="mw-page-title-main">Chimney fire</span> Undesirable burning of residue deposits inside a chimney

A chimney fire is the combustion (burning) of residue deposits referred to as soot or creosote, on the inner surfaces of chimney tiles, flue liners, stove pipes, etc.

<span class="mw-page-title-main">Fireplace insert</span>

Invented in 1896 by Joab R. Donaldson of Oliphant Furnace, Pennsylvania, US, the fireplace insert is a device inserted into an existing masonry or prefabricated wood fireplace. Joab was a 59-year-old coal miner and father of 14 at the time of his patent. He came upon the idea as a means of using coke and incorporating the use of an electric blower to improve the efficiency. The selection of coke and coal tailings as a primary fuel enabled low-income families to heat their Appalachian homes with small-size coal that they could easily dig for themselves in their own backyards.

<span class="mw-page-title-main">Wood-burning stove</span> Type of stove

A wood-burning stove is a heating or cooking appliance capable of burning wood fuel and wood-derived biomass fuel, such as sawdust bricks. Generally the appliance consists of a solid metal closed firebox, often lined by fire brick, and one or more air controls. The first wood-burning stove was patented in Strasbourg in 1557, two centuries before the Industrial Revolution, which would make iron an inexpensive and common material, so such stoves were high end consumer items and only gradually spread in use.

<span class="mw-page-title-main">Rocket mass heater</span>

A rocket mass heater (RMH), also known as rocket stove mass heater, is a form of slow-release radiant heating system, designed to primarily heat people and secondarily to warm areas in line of sight around it. Variations of RMH can also be extended for the functions of cooking, heating water, and producing warm air for distribution.

A direct vent fireplace is a prefabricated metal fireplace that employs a direct-vent combustion system. "Direct vent" refers to a sealed-combustion system in which air for combustion is drawn from the outdoors, and waste combustion gasses are exhausted to the outdoors. "Direct vent" does not simply mean that all gasses from combustion are vented to the exterior of the structure in which it is installed.

References

  1. Skøtt, Torben (September 2007). "Chimney filters can save billions" (PDF). BioPress. pp. 10–11. Archived (PDF) from the original on February 16, 2021. Retrieved May 13, 2021.
  2. 1 2 "Wood Smoke and Your Health". United States Environmental Protection Agency . May 28, 2013. Archived from the original on May 14, 2021. Retrieved May 13, 2021.
  3. 1 2 "Smoke health effects & burning alternatives". Washington State Department of Ecology . Archived from the original on January 17, 2021. Retrieved May 13, 2021.
  4. Logan, Matt (November 6, 2015). "Best Ventless Fireplace Logs Review | Expert Guide | Updated |". Ventless Fireplace Review. Archived from the original on April 28, 2021. Retrieved May 13, 2021.
  5. American Institute of Architects (March 21, 2016). Hall, Dennis J. (ed.). Architectural Graphic Standards. Illustrated by The Magnum Group. Hoboken, New Jersey: Wiley. ISBN   9781118909508. OCLC   943373654. Archived from the original on May 14, 2021. Retrieved July 2, 2017.
  6. "History of Antique Fireplaces | Victorian, Georgian & Edwardian Fireplaces". Fireplace Antiques. Archived from the original on October 27, 2020. Retrieved May 13, 2021.
  7. Tom (January 16, 2012). "Three Steps to a More Efficient Fireplace". FireplaceMall. Archived from the original on January 15, 2013. Retrieved May 13, 2021.
  8. "RIS - Heizungsanlagen-Verordnung 2010 - Landesrecht konsolidiert Salzburg, Fassung vom 14.05.2021". www.ris.bka.gv.at. Archived from the original on 2018-02-15. Retrieved 2021-05-13.
  9. Zelikoff, J.T., 2002. "The Toxicology of Inhaled Woodsmoke Archived 2020-08-01 at the Wayback Machine ". Journal of Toxicology and Environmental Health, Part B, 5:269-282
  10. Partridge, Emma; Aubusson, Kate (June 9, 2015). "Derek Kehler and Helena Curic identified as young couple killed in Kurrajong cabin". The Sydney Morning Herald . Archived from the original on February 21, 2021. Retrieved May 14, 2021.
  11. Fukunaga, T; Yamamoto, H; Tanegashima, A; Yamamoto, Y; Nishi, K (25 October 1996). "Liquefied petroleum gas (LPG) poisoning: report of two cases and review of the literature". Forensic Science International. 82 (3): 193–200. doi:10.1016/s0379-0738(96)01995-0. PMID   8948127.
  12. Novosel, I; Kovačić, Z; Gusić, S; Batelja, L; Nestić, M; Seiwerth, S; Skavić, J (April 2011). "Immunohistochemical detection of early myocardial damage in two sudden deaths due to intentional butane inhalation. Two case reports with review of literature". Journal of Forensic and Legal Medicine. 18 (3): 125–31. doi:10.1016/j.jflm.2010.12.003. PMID   21420651.
  13. Suffocations may be caused by propane, [11] butane [12]
  14. Kraemer, Robert; Knobloch, Karsten; Lorenzen, Johan; Breuing, Karl H.; Koennecker, Soeren; Rennekampff, Hans-Oliver; Vogt, Peter M. (March–April 2001). "Severe Burn Injuries Caused by Bioethanol-Design Fireplaces—An Overview on Recreational Fire Threats". Journal of Burn Care & Research . American Burn Association. 32 (2): 173–177. doi:10.1097/BCR.0b013e31820aade7. eISSN   1559-0488. ISSN   1559-047X. PMID   21233730. S2CID   205483058.
  15. Apte, Komalkirti; Salvi, Sundeep (2016). "Household air pollution and its effects on health". F1000Research . 5: 2593. doi:10.12688/f1000research.7552.1. ISSN   2046-1402. PMC   5089137 . PMID   27853506. Burning of natural gas not only produces a variety of gases such as sulfur oxides, mercury compounds, and particulate matter but also leads to the production of nitrogen oxides, primarily nitrogen dioxide...The burning of biomass fuel or any other fossil fuel increases the concentration of black carbon in the air...[rest of material in linked fulltext]
  16. Casillas, Sebastian; Galindo, Antonio; Camarillo-Reyes, Luis A.; Varon, Joseph; Surani, Salim R. (October 15, 2019). "Effectiveness of Hyperbaric Oxygenation Versus Normobaric Oxygenation Therapy in Carbon Monoxide Poisoning: A Systematic Review". Cureus . 11 (10): e5916. doi:10.7759/cureus.5916. ISSN   2168-8184. PMC   6855999 . PMID   31788375.
  17. "Definition of Andiron". Merriam-Webster . Archived from the original on April 9, 2021. Retrieved May 14, 2021.
  18. 1 2 3 4 5 6 7 8 9 10 RSMeans (September 15, 2009). RSMeans Illustrated Construction Dictionary: The Complete Source of Construction Terms and Concept (4th ed.). Wiley. ISBN   9780876290927. OCLC   793656918. Archived from the original on May 14, 2021. Retrieved May 14, 2021.
  19. Perryman, Oliver (January 19, 2021). "Wood Burning Fireplace Insert (2021 Reviews Updated)". Dehumidifier Critic. Archived from the original on May 14, 2021. Retrieved May 13, 2021.
  20. "Definition of mantel". Merriam-Webster . Archived from the original on May 9, 2021. Retrieved May 14, 2021.
  21. "Chimney and Fireplace Configuration". Rockford Chimney Supply. Archived from the original on May 14, 2021. Retrieved May 14, 2021.

Further reading