Introduction
For any fire to successfully run it must be connected to a sound chimney and a correctly sized flue.
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Too great a chimney liner diameter can allow the gases to cool down too early; this will eventually result in tar and creosote deposits sticking to the wall of your liner slowly choking the flue and your stove, this can also lead to ill health. As a result of the larger flue your stove will be sluggish to run, more susceptible to down draughts, it will often need sweeping and there is a greater risk of chimney fires.
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Too small a chimney diameter will constrict your stove's ability to remove the dangerous gases; you will often see wisps of smoke seeping from the primary or secondary air inlets, you will almost certainly have a constant smell of soot.
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The correct chimney arrangement allows a clean flow of gases to stay at such temperature that the tar and creosote cannot condense until it reaches the atmosphere, by then the gases will be diluted and carried away.
A chimney works because hot air rises. Fill the bottom end of an upright tube with hot air and you will soon feel the effect at the top, the greater the diameter of the tube the more hot air you will move, the narrower the tube the faster the hot air will rise. However do not be fooled into thinking a narrow flue can give you more flue pull, a far greater consideration should be "how can I get a large volume of poisonous gas away quickly and safely".
Firing the appliance on a very low heat output or cold air leaking into the flue, will cool the gases down, these will condensate to form creosote and tar, this affects the performance of the chimney and often leads to chimney fires.
An efficient wood burning stove or boiler produces cooler flue gases than that of an open fire. For this reason, when installing a wood burning stove it is often necessary to fit an insulated liner within an existing chimney to ensure that there is a sufficient draw of air.
If there is no existing chimney it is still possible to install a wood burning stove or boiler by constructing a twin walled flue using one of the proprietary systems.
Terminology
It is sometimes confusing when the words 'chimney' and 'flue' are used almost as one. BSEN 1443 defines a chimney as "a structure consisting of a wall or walls enclosing a flue or flues" and a "flue" as "the passage for conveying the products of combustion to the outside atmosphere." A flue can, therefore, be made of not only brick or stone but other materials such as metal. A 'flue pipe' which is the generally accepted term for any flue made of metal or similar material. A flue pipe that passes through a roof space, partition, internal wall or floor must have sufficient clearance from combustible material to avoid any possibility of charring or fire. The clearance will be defined in the manufacturer's instructions. A 'flue liner' is the material used to form the flue within the chimney. Flue liners can be of fire clay, refractory quality concrete or metal (usually 316 grade stainless steel).
The Building Regulations and you
If you want to install, replace, repair or reuse a chimney or flue The Building Regulations apply. Approved Document J (ADJ) of the Building Regulations sets out the requirements in relation to flues and chimneys. The latest edition (2002) of ADJ came into force on the 1st April 02.
All responsibility for compliance with ADJ rests with the persons carrying out the work, although it is the householder who will be served with an enforcement notice if the work does not comply (a pretty good reason for choosing a properly qualified installer).
The new Building Regulations have also made it mandatory for the installer to fix a notice in a suitable position providing information on the hearth, fireplace, flue or chimney.
If you employ an installer registered with HETAS (Heating Equipment Testing and Approval Scheme) to carry out the work, there is no need to involve Building Control Services provided the building is no more than three stories high.
Chimney Maintenance
All chimneys need to be swept at regular intervals in order to avoid the risk of a chimney fire and carbon monoxide poisoning. We would recommend you treat your chimney to a minimum of these time scales
As with the burning of any fuel for the production of heat; wood can produce poisonous gases if the flue or chimney is blocked, the equipment is faulty or there is a lack of ventilation.
Flues
Flue outlets must be above the roof line to enable unhindered dispersal of combustion products without creating a fire hazard.
The direction of the flue should be straight and vertical where possible. Horizontal runs should be avoided and, in any event, must not exceed 300mm in length. Bends should not exceed 45º to the vertical to maintain natural draft and ease of cleaning. There should be no more than four bends in a flue (a 'Tee-piece' on the rear of the stove counts as two bends). Only one appliance may enter any flue and all flues should be insulated to prevent condensation of water/tar and to protect the building against the effects of chimney fires.
Flue Liners
Houses built after 1965 had chimneys built with liners during their construction. This was usually done with a clay liner which should last the life time of the building, although beware many do not due to poor installation standards. Prior to 1965 lining was less common and chimneys were instead rendered with a lime mortar.
Most old chimneys need to be relined before they can be re-used, particularly those that were built without a lining or have a flue that is too big and inefficient for modern stoves and fires. After years of use an old chimney can suffer attack from heat and soot deposits that erode mortar joints. This can cause many problems, such as staining and also leakage of dangerous fumes into the home. Installing a new liner that provides a new sealed and correctly sized flue can dramatically improve the efficiency and safety of an old chimney
There are several methods of doing this suitable for a solid fuel appliance which includes:
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Installing rigid sections of clay, refractory concrete, ceramic or pumice into the chimney by cutting into the flue wall or by lowering down from the top.
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Pumped refractory concrete around an inflatable former by a specialist contractor.
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Rigid metal liners made from high grade stainless steel pipes lowered from the top.
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Flexible metal liner in continuous length either lowered down or pulled up the flue.
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Spray on coating by a specialist contractor.
Generally, for solid fuel and wood burning, the space around the new lining should be filled with an insulating material such as Perlite, Vermiculite or Leca. Better still, the lining material itself should have a high insulation value i.e. liners made with pumice.