Chimneys - A Guide to How they Work and it's Functions.


The history of the domestic chimney is unfortunately lost in the annuals of time. It is however possible to place pieces of the jigsaw into place from the development of the chimney from its origins of a hole in the roof of primitive round house dwelling to the more familiar architecture of the modern age and its immediate predecessors, the smoke bay and the smoke hood. Archaeological findings show the Romans used the embryonic forms of a chimney to draw smoke through the walls of their bakeries and outside. There isn’t really any evidence within northern Europe of the presence of chimneys in domestic dwellings until around 12th Century.For more information on the History of Fire, it's social, and political significance please click on the link) The earliest known chimney within the UK dates back to 1185 and is in Coinsbrough Castle Yorkshire.

Coinsbourgh castle

In the 16th and 17th Centuries chimneys became common place in domestic houses. The development of the chimney had a number of impacts upon the design and use of the home. No longer was the fire situated in a fire pit or fire hearth in the centre or corner of the room but moved to the walls of the room and not far from our hearts and minds today allowed the evolvement of elaborate and ornate fire surrounds to decorate and furnish our homes with oak, limestone, marble, grasnite and other stone fire surrounds in the architectural style we wish (for further imformation on the types of fire surrounds availiable please follow the link Your Fireplace Your Home). The birth of the chimney had a role to play in the development of  built in ovens and eventually the range, a not too distant cousin of the AGA  and Raeburn’s often found in country style kitchens today.   The chimneys’ major impact on today’s living is homes were no longer filled with dank heavy smoke making the upper regions of the home unsuitable to be used as living areas. The chimney is the biggest contributor to the development of upstairs living space, with floors and stairways being built making possible an increase in floor space and the separation of living and sleeping areas.

The earliest chimneys were built of wood, mud, plaster, or stone.  As chimney technology advanced bricks were used with the basic brick design being later modified with the addition of a tile liner. As chimney designs progressed there was a move from a single chimney stack for each fire to the multiple chimney stack where fires on separate floor and rooms are vented through a central stack.  Due to the inability of bricks to be able to carry heavy transverse loads and the effects of bends and curves on the evacuation of combustible and non-combustible waste products there is often a chimney stack situated back and front of most older homes. The arrival of modern central heating systems and fires, particularly those fuelled by gas, or electrical appliances for a brief period of time made the chimney redundant, with the former appliances being vented through non structural gas vent pipes ducted through an outside wall or the roof. Good news for those living in homes without a chimney, where either the home was built without chimneys, or the chimney stack was removed at some stage usually from the late 1960’s onwards, it is now  possible to install a real effect fire, although unfortunately not the real thing. There has in recent years been a huge revival in the opening up and installation of chimneys and flues partially in response to be more ecological in our lifestyle, the ever raising cost of oil and gas and as some would suggest satisfying a primeval need for a hearth in our homes.      

A chimney serves a multitude of functions within the domestic setting. First, it evacuates the non combustible and combustible waste products or as some would quite rightly argue in its oversimplified form the smoke from any form of combustion within an open fires, log or coal effect fires, boilers, log burner or multi fuel stoves and furnaces. The venting role of a chimney should not be underplayed. A chimney plays a critical role to our well being and health by keeping the immediate environment of the home safe for us to enjoy. Smoke contains some very nasty, hazardous and noxious chemicals, and particulate matter including sulphur oxides, nitrogen oxides, the silent killer carbon monoxide and potentially carcinogenic compounds including polycyclic aromatic hydrocarbons, benzene, formaldehyde and dioxins. This is why it is so important to ensure your chimney is suitable for the job it needs to do, is inspected, and cleaned at least annually. In the case of wood fires and stoves some would argue quarterly whilst the stove or fire hearth is in use, others advocate twice yearly. It is recommended and increasingly more and more home insurance companies are insisting that your chimneys are cleaned by a qualified chimney sweeps (National Association of Chimney Sweeps [NACS], Guild of Master Chimney Sweeps, The Association of Professional and Independent Chimney Sweeps [APICS]). As part of their service  a qualified sweep will inspect the chimney and identify any further maintenance that is required before presenting you with a certificate stating the chimney has been properly cleaned and inspected.  A chimney should also be free of any debris and blockages such as birds nests etc. It is also highly recommended that you install a Carbon Monoxide Alarm next to the site of your appliances, regardless of how they are fuelled, as many lives are lost each year through Carbon Monoxide poisoning.   

The second function of the chimney is closely linked to the venting of the waste products and although not solely this particularly applies to the combustion of solid fuels such as coal or wood within open fires and stoves.  A well maintained chimney not only vents combustion waste products but protects the combustible materials of the home such as floor joists from the excessive heat and stray sparks from the fire or flame. A chimney is much more than a simple venting device to evacuate the combustible and non-combustible waste from the site of the fire but provides the draw or draft to fuel the fire.   

Many of the issues and problems experienced with a fire or heating appliance be it solid fuel, biomass, or fossil fuelled lie in a poorly designed, poorly maintained, chimney or simply a mis- match between chimney and heating appliance. The air pressure at the bottom of the chimney is higher than the air pressure at the top of the chimney. This difference in air pressure between the bottom or hearth of the fire and the top of the chimney creates a continuous upward draft through the chimney. As a consequence chimney height is critical to a chimney’s performance the higher the chimney with all else being equal the greater its draw. The draw of the chimney is further assisted when the fire or appliance is lit as hot air rises as it less dense.  It is relatively easy to reduce the draw of a chimney where the draught is too strong but improving the draft on a short chimney is much more difficult. It is important that the chimney lining is in intact with no leaks. This crucially prevents smoke leakage and sparks entering into other areas of the house, bedrooms, loft etc with potentially catastrophic consequences and not overstating the possibilities may result in the loss of all your worldly good and even death! Any leaks within the chimney will adversely affect the performance of the chimney and therefore the heating appliance.  As a general rule of thumb the bigger the fire the bigger the chimney needed.  

The flue or chimneys diameter needs to be of the correct size for the appliance. In the case of open fires, log or coal, so much heat disappears up the chimney and as we all know hot air rises even very large chimney will remain warm and evacuate the smoke and fumes efficiently. It is a different story with modern heating appliances with their increased efficiency most of the heat generated from the combustion of the fuel is circulated where it should be, back into the living space. As a result very little heat is sent or drawn up the chimney thus it is important that the flue is narrow, and well insulted enabling the waste products and fumes to retain their heat, and velocity, so as  not to condensate in the flue to complete their journey out of the top of the chimney. A chimney that is too wide for the appliance will allow the smoke and waste products to cool, loose their speed, and tar up the chimney creating a fire hazard and potentially spill fumes and smoke back into the home whilst hampering the efficiency of the appliance.  In extreme circumstances this may result in chimney reversal, where cold air being heavier will fall down the chimney to enter the house bringing with it the smoke, fumes and other waste combustion products. This situation can be further aggravated by the presence of extractor fans, clothes driers and other heating appliances which all draw air from the living space and evacuated outside. This lost air has to be replaced to equalise the air pressure between the living space and outside. A chimney with little or no draft makes a very convenient vent to the outside world where there is a ready supply of air available to be drawn into the home.  It is essential that there is a ready supply of air flow into the room and house to compensate for the draw of the chimney and other appliances when the fire is in use.  For a more in-depth discussion of problems associated with depressurisation to The Myth, The Science and the Solution to Open Fires.

The air flow into the fire is critical to the burning process and performance of the fire or appliance and why some fire hearths and appliance are installed with an external air inlet.  link to science of fire doc I will do this asap  The natural draw of  a chimney  is one of the reasons people install chimney balloons in to the chimneys servicing open fires in the summer months when the fire is not in use. Do not fall into the trap of assuming the chimney or flue should be the same diameter as the flue outlet collar of the stove or appliance. It is always best to consult the stove or the manufacturer of the appliance and the advice of the professionals to ascertain the correct flue cross section and conversely the correct fire size for the flue or chimney. Do keep in mind many that many coal and log effect gas fires and stoves require a flue or chimney which is suitable for a conventional open fire and that a conventional open fire log or fossil fuelled should not exceed the capacity of the cross section of the chimney or flue. For more advice concerning these issues and round and square cross sections see above link relating to chimney diameter

A chimney or flue should wherever possible run straight with no bends or turns. A 90 degree turn in a flue has the same effect on its draw as reducing the height of the chimney by 5 foot. If you have a 15 foot chimney with two 90 degree turns you effectively reduce the draw of the chimney to a chimney of 5 foot in height. Bends and turns should be kept as smooth and as shallow as possible and this places a greater emphasise upon the insulation qualities of the chimney to help retain as much heat as possible within the flue to counteract the reduced draft and therefore the flow rate of the chimney. Ideally a chimney should be located internal to the home this helps keep the chimney warm and as a result draw better with the added bonus of adding heat to the home from the warm chimney breast. A round flue or cross section due to the natural propensity of smoke to column in a circular fashion will draw better than a square or rectangular flue or cross section.



Since 1966 the chimneys built in new homes have all had to have separate linersbuilt into the chimney. The economic reality of this has frequently meant that clay liners have been installed and not necessarily installed correctly as unskilled labour was often used resulting in poorly installed liners that have not stood the test of time. Many of these liners consist of thirty or more pieces which need to be installed correctly to make sure there is no potential for fume or smoke spillage at each of the joints. It is a relatively common place to find these liners are in fact installed upside down resulting in leakage and condensation forming in the chimney. The situation has been further compounded as many of these liners prior to being installed needed to be stored and handled correctly the criteria of which was not necessarily adhered to on busy building site. In older homes fume tight flues were constructed by parging the internal surfaces of the chimney with lime mortar, prior to this materials such as cow dung were used.  In general these older style linings have stood the test of time and longevity although there are real issues relating to their suitability for the intense heat created by log burners, multi fuel stoves and other fossil fuelled appliance. It is highly advisable before installing a stove or modern fossil fuelled appliance that expert advice is sought and that the integrity of the liner is examined by a qualified chimney sweep or fire fitter. The consequences of a poorly installed, poorly maintained or inadequate liner are catastrophic with the potential of smoke and fume leakage into the home and the risk of fire.    

It has been said that a good chimney is more important than a good stove, furnace, or fire. We do understand, and after reading this article hope you as a reader now have a greater insight to, the necessity and importance of a good chimney.  We fully endorse that many of the problems related to smoke and fume spillage, poor fuel and heat efficiencies, and creosoted chimneys, can be traced back to poor chimney performances and whilst recognising a good chimney can compensate for a poor fire hearth or stove we would always advocate a good fire, stove or furnace to compliment a good chimney.  The real point being made is perhaps it is easier to replace a poor performing fire, stove or appliance than it is to correct a poor performing chimney that is inadequate for the stove, fire or appliance that is fitted. It is always and often a legal necessity to comply with Building Regulations when installing or renovating a chimney, or fitting a wood burner, multi fuel stove, or heating appliance. Whatever the choice of fuel or fire and stove do speak to the experts and an informal chat with your local showroom staff or chimney sweep is not a bad place to start. You never know they may be saving your life!

By Phil Cleaver


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#351 Maria on 2014-04-21

Long but very informative article. first i thought that its long to read full article but now very happy. Got many valuable information for chimney... :) thank

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