page_title: Chimney
section_title: Chimney pots, caps and tops
hierarchical_section_title: Chimney / Chimney pots, caps and tops
caption_attribution_description: Deutsch: Schornstein in Clovelly, Devon, England
context_page_description: 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 is also used when referring to locomotive chimneys or ship chimneys, and the term funnel can also be used.
  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.
context_section_description: 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 backdraft 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 backpuffing. 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 crow-stepped gables arose to allow maintenance access to the chimney top, especially for tall structures such as castles and great manor houses.
caption: Rows of chimney pots in an English town, 2013.
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