For any fire to work successfully it must be connected to a sound chimney and a correctly sized flue. A chimney works because hot air rises. Factors such as running the appliance at a very slow rate or cold air leaking into the flue, will cools the gases down and affect the performance of the chimney. An efficient wood burning stove or boiler produces flue gases that are cooler than an open fire. For this reason it is necessary to install an insulated liner within an existing chimney when a wood burning stove is installed to ensure that there is a sufficient draw of air. If there is no existing chimney it is possible to install a wood burning stove or boiler by constructing a twin walled flue using one of the proprietary systems.
It is sometimes confusing when the words 'chimney' and 'flue' are used almost interchangeably. 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 manufacturers 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 high grade stainless steel).
The Building Regulations
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
our the requirements in relation to flues and chimneys. The latest
edition (2002) of ADJ came into force on the 1st April 02. Responsibility
for compliance with ADJ rests with the person carrying out the work,
although it is the householder who will be served with an enforcement
notice if the work does not comply (a good reason for choosing a
properly qualified installer). The new Building Regulations 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.
Chimneys need to be swept at least once a year in order to avoid the risk of a chimney fire and carbon monoxide poisoning. As with the burning of any fuel for the production of heat, wood can produce this poisonous gas if the flue or chimney is blocked, the equipment is faulty or there is a lack of ventilation. A list of chimney sweeps is available from the National Association of Chimney Sweeps.
Flue outlets must be above the roof line to effect clear, unhindered dispersal of combustion products without creating a fire hazard. The direction of the flue should be should be straight and vertical where possible. Horizontal runs should be avoided and in any event must not exceed 150 cm in length. Bends should not exceed 45º to the vertical to maintain natural draft and ease of cleaning. There should be no more than two bends in a flue. 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.
Houses built since 1965 have 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 may 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 appliances which include:
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.
Pumped refractory concrete around an inflatable former by a specialist contractor.
Rigid metal liners made from high grade stainless steel pipes lowered from the top.
Flexible metal liner in continuous length either lowered down or pulled up the flue.
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.