A cowl is a usually hood-shaped covering used to increase the draft of 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 ...

and prevent backflow. The cowl, usually made of

galvanized iron Galvanization or galvanizing ( also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged ...

, is fitted to the chimney pot to prevent wind blowing the

smoke Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product ...

back down into the room below. Undoubtedly named after the resemblance of many designs to the

cowl A cowl is an item of clothing consisting of a long, hooded garment with wide sleeves, often worn by monks. Originally it may have referred simply to the hooded portion of a cloak. In contemporary usage, however, it is distinguished from a clo ...

garment worn by monks, they have been in use for centuries. When using an open fire to heat a room the smoke rises through a

flue 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 al ...

to a chimney pot on the roof. Under normal conditions the warm air from the fire will rise up the chimney emitting the smoke with it and dispersing it at rooftop level where it is less of a nuisance. In strong winds the pressure of the wind may overwhelm the updraft and push the airflow in reverse down the flue. Smoke will then fill the room it is intended to heat posing a health and fire risk, causing discomfort and dirtying furnishings in its path. When raw

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...

rather than smokeless fuel is burnt, the amount of smoke may be considerable and measures to prevent backflow occurring are a necessity. A secondary function is to prevent birds and squirrels from nesting in the chimney. Cowls often also act as a rain guard to keep rain from going down the chimney. A metal wire mesh is sometimes added as a spark arrestor. Wooden

cowls A cowl is an item of clothing consisting of a long, hooded garment with wide sleeves, often worn by monks. Originally it may have referred simply to the hooded portion of a cloak. In contemporary usage, however, it is distinguished from a clo ...

were used on

oast An oast, oast house or hop kiln is a building designed for kilning (drying) hops as part of the brewing process. They can be found in most hop-growing (and former hop-growing) areas and are often good examples of vernacular architecture. Many ...

s to prevent the ingress of rain into kilns, and create a flow of air through the kiln. An H-style cap (cowl) is a chimney top constructed from chimney pipes shaped like the letter H. It is an age-old method to regulate draft in situations where prevailing winds or turbulence cause downdraft and backpuffing. Although the H-cap has a distinctive advantage over most other downdraft caps, it fell out of favor because of its bulky looks. It is found mainly in marine use but has been gaining popularity again for its energy saving functionality. The H-cap stabilizes the draft rather than increasing it. Other downdraft caps are based on the

Venturi effect The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian physicist, Giovanni Battista V ...

, solving downdraft problems by increasing the updraft constantly resulting in much higher fuel consumption.

Examples

File:Common chimney cowl Oregon.JPG, Wind directional cowl found on many homes along the windy

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

coast, USA. File:Oast cowl.jpg, An

cowl File:H-style cap.jpg, A H-style cowl File:Cowlchimney.jpg, Wind-rolling home cowl File:Manche a air.JPG,

Boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats. Small boats are typically found on inl ...

ventilator A ventilator is a piece of medical technology that provides mechanical ventilation by moving breathable air into and out of the lungs, to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. Ventilators ...

cowl Cowls are also used where flues are no longer in use and therefore need to be capped off for protection against ingress of

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

and nesting birds. Such cowls are designed to afford protection to the flue while still allowing a degree of ventilation.

References

{{commonscat, Chimney cowls

Wolverhampton History and Heritage Society. Accessed April 2007
Chimney Cowl
at English Heritage thesaurus. Accessed April 2007
Chimney sweep houston
Smoke Building technology Chimneys

Examples

See also

Further reading

References