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Chimney sweep Watford
Chimney sweeps - Solid fuel fires - Hertfordshire and Bedfordshire

All About Chimneys

What is a chimney and what does it do?

A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside of a building. Chimneys are usually vertical to ensure that the gases flow smoothly, drawing air into what is known as the stack, or chimney. The space inside a chimney is called a flue. Chimneys may be found on the roof of buildings and on the top of steam locomotives and ships. In the United States, the term smokestack, these are colloquially known as a stack, is also used when referring to locomotive chimneys or ship chimneys, and the term funnel can also be used.

The height of a chimney plays a role in their ability to transfer flue gases using stack effect, the dispersion of pollutants at higher a altitude helps to lower its influence on the surroundings. In the case of chemically aggressive output, a tall chimney allows partial or complete self neutralization of chemicals in the air before they can reach the ground. The dispersion of pollutants over a greater area reduces their concentrations in compliance with various regulatory limits.

The history of the chimney

The Romans used tubes fitted inside the walls of their properties to draw smoke out but actual chimneys appeared only in northern Europe around the 12th century. Industrial chimneys became far more common in the late 18th century. The earliest surviving example of an English chimney is at Conisbrough Castle in Yorkshire, which dates from the year 1185 AD.

Chimneys have traditionally been built from bricks, both in small and large buildings. Early chimneys were a simple brick construction. Later chimneys were constructed by placing the bricks around tile liners. To control downdrafts, venting caps, these are now called chimney pots, with many designs are sometimes placed on the top of chimneys.

In the eighteenth and nineteenth centuries, the methods used to extract lead from its ore produced large amounts of toxic fumes. In the north of England, long horizontal chimneys were built, often more than two miles long, which usually 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 occasionally workers would be sent along the chimneys to scrape off these valuable deposits.

The construction of chimneys

Due to a brick's rather limited ability to handle transverse loads, chimneys in houses were usually 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. Central heating systems today have made chimney placement far less critical, and the use of non structural gas vent pipe allows a flue gas conduit to be installed around obstructions and through the walls.

Many modern high efficiency heating appliances do not need a chimney. These appliances are installed near to an outside wall, and a noncombustible wall thimble allows the vent pipe to be run directly through the outside wall.

Industrial chimneys are usually referred to as flue gas stacks and tend to be external structures, as opposed to being built into the wall of a building. They are generally located opposite a steam generating boiler or an 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 component in the construction of industrial chimneys. Refractory bricks are often used as a lining for these chimneys, 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.

Chimney pots

A chimney pot is placed on top of the chimney to cheaply extend the length of the chimney, and to improve the chimney's draft. A chimney with more than one pot on it indicates that there is more than one fireplace on different floors within the property. All the pots share the same chimney.

A chimney cowl is placed on top of the chimney to prevent birds and other small animals from nesting in the chimney. They often feature a rain guard to keep rain from going down the chimney. A metal wire mesh is often used as a spark arrestor to cut down the risk of burning debris coming out of the chimney and landing on 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 any collected water.

A wind directional cap is a helmet shaped chimney cap that rotates to align with the wind and prevent a backdraft of smoke and wind back down the chimney.

An H style cap or cowl is a chimney top constructed from chimney pipes shaped like the letter H. It is a very old method to regulate draft in situations where prevailing winds or turbulences cause down draft and backpuffing. Although the H cap has a distinctive advantage over most other downdraft caps, it is not as popular today as it tends to be rather bulky. It is found mainly in marine use but has been gaining popularity again due to its energy saving functionality. The H-cap stabilizes the draft rather than increasing it. Other down draft caps are based on the Venturi effect, solving downdraft problems by increasing the up draft constantly resulting in much higher fuel consumption.

A chimney damper is a metal spring like door that is placed at the top of the chimney with a long metal chain that allows one to open and close the chimney from the location of the fireplace.

In the latter part of the 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, keeps and the larger manor houses.

Chimney draught or draft

When coal, oil, natural gas, wood or any other fuel is burnt in a stove, oven, fireplace, hot water boiler or industrial furnace, the hot gases that are formed are referred to as flue gases. Those gases are generally expelled to the ambient outside air through chimneys or industrial flue gas stacks.

The flue gases inside the chimneys or stacks are far 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 flow of air and flue gas is called natural draught or draft, natural ventilation, chimney or stack effect. The taller the chimney, the more draught or draft is created. There can be cases of diminishing returns: if a chimney is overly tall in relation to the heat being sent out of the chimney, the flue gases may cool before reaching the top of the chimney. This can result in rather poor drafting, and in the case of wood burning appliances, the cooling of the gases before emission can cause creosote to condense near to the top of the chimney.This creosote can restrict the exit of flue gases and may pose a serious fire hazard.

Maintenance and problems with the chimney

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

Chimneys that burn natural gas drastically reduce the amount of creosote buildup due to natural gas burning much cleaner and more efficiently than the 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 don't fall into disrepair. Damaged or loose chimney connections caused by rust and corrosion over a period of time can pose serious dangers for residents due to leakage of carbon monoxide into the home. It is recommended and in some jurisdictions even mandatory, that chimneys are inspected annually and cleaned on a regular basis to prevent these problems occurring. The people who perform this job are called chimney sweeps. This work used to be done mostly 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, this was to provide access to chimneys without the use of ladders.

Brick chimneys have also proved particularly susceptible to crumbling during seismic events such as earthquakes. Government housing authorities in earthquake prone places such as San Francisco and Los Angeles now recommend building new homes with stud framed chimneys around a metal flue. Bracing or strapping old brick chimneys has not proved to be very effective in preventing damage or injury from earthquakes. It is now possible to buy imitation brick facades to cover these modern chimneys.

Chimney liners have been standard in new building constructions for many years, but they are poorly lacking in older buildings whose masonry has not been restored and updated. Tile liners help keep flue gases where they should be. They isolate combustible building materials from high heat, and they prevent creosote and other by products of combustion from seeping through bricks and mortar.

Chimneys that have more than one purpose

Some very high chimneys are used for supporting antennas of mobile phone service providers and low power FM television transmitters. If these antennas are near the exhaust of the chimney then there is a real possibility of corrosion problems occuring.

In some instances the chimneys of power stations are used also as pylons. However this type of construction, which is used at several power stations in the formerly Russian Soviet Union, is not very common, because of corrosion problems of conductor cables.

The Dum Detí a Mládeže v Modranech in the city of Prague, Czech Republic is equipped with an observation deck.

The chimney of Pei Tou Incinerator in Taipei, Taiwan has a revolving restaurant.

Cooling tower used as an industrial chimney

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. These type of cooling towers can be seen in Germany at the Power Station Staudinger Grosskrotzenburg and at the Power Station Rostock. At power stations that are not equipped for removing sulfur dioxide, this usage of cooling towers could result in very serious corrosion problems.

Further information

If you would like to know more or are interested in a quote we would be happy to help. Phone us on 01923 661 614, email us at or fill in our free quote form and we will be in touch as soon as possible.

Ansell Chimneys

t: 01923 661 614

m: 07941 282 325


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