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The BFCMA guide

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Choosing and using flues and chimneys for domestic solid fuel and wood burning appliances

This guide is intended to give useful guidelines on the choice and installation of factory made systems for building and also relining chimneys, that can be used with solid fuel burning appliances and fires. It is an updated version of the last guide issued in November 1988.

Recommendations are also given on the use and maintenance of chimneys. that are important to ensure that the installation operates safely and efficiently. Useful points of reference and contacts are also included.

The installation of both chimneys and heating appliances is governed by Building Regulations and British Standards. It is therefore the responsibility of the specifier, installer and user to ensure tile products used and installation as a whole meets these requirements. This guide is not a substitute for these requirements and the BFCMA cannot accept any liability for actions taken as a result of the information given in this guide.

The BFCMA is the only Trade Association that represents British based manufacturers and sole national distributors who sell factory made chimney and flue products in the UK. The Association and ~s members actively promote the safe and efficient use of chimneys and also encourage continual improvements of both British and European standards.

All members of the BFCMA offer a free information service on request.

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An efficient chimney is a practical asset for any house. It allows the freedom to choose a wide range

of heating systems from stoves, cookers to the warming enjoyment of a real open fire. A working chimney creates natural ventilation which helps to provide a healthy atmosphere and reduces the risk of condensation.

A chimney or rather the flue (which is the working part) operates on the principle of drawing the products of combustion (the fumes and gases created by the fuel burning process) from the appliance or fire and discharging them safely into the outside atmosphere.

A successful “chimney draw” is dependant on maintaining hot flue gases of 15000 - 50000 which creates an up draught. The correct sizing and installation of a well insulated flue together with the correct operation of the appliance are important factors to ensure a good draw. A suitable air supply is also needed for the appliance of fire to operate safely and efficiently.

To help understand more about chimneys they are a few terms that need to be mentioned. Examples of two common types of chimney and appliance arrangement are also shown.


There is a wide choice of factory made chimney and flue products available from members of the BFCMA. These are listed in the separate BFCMA Directory, available on request.

It is recommended that you get professional advice, together with the full product and installation details before committing yourself to a particular system. If you require a guarantee or warranty on a product be sure to get full printed details of what is covered and what the conditions are. Most manufacturers warranties are very specific on the use and maintenance requirements for their products.

Beware of any products that do not have independent proof of testing approvals and are not available with full installation and user instructions. Verbal warranties that are not backed up in writing with all the small print should be carefully checked to make sure you do in fact have the cover you think you are getting.

If you are thinking of using your chimney with a gas fired appliance or Decorative Fuel Effect gas fire that imitates a solid fuel burning open fire, there are different requirements that must be followed. A separate leaflet on chimneys and flues for gas fired appliances is published by the BFCMA. It is important to remember that by law the fitting, alteration and connection of all gas fires can only be done by a CORGI registered installer.


There are three common types of product available, which offer different advantages and a choice of appearances.

These are made from lightweight precast concrete and have an integral or separate liner of clay or concrete. Designed for quick installation the blocks can accept traditional render, plaster or brick cladding finishes.

Fittings such as cappings. offsets and precast fireplace components are available with this type of system.

There are also Flue Blocks which are specifically designed for gas fired appliances only.

These are made from clay, ceramic material or precast concrete using High Alumina and fire resistant aggregate such as pumice. For easy location and to meet British Standards these liners must have spigot and socket joints.

To form a chimney these liners have to be surrounded with at least 100mm thick brick, block or stonework with the gap in between filled with insulating concrete or lean mix mortar. Some of these liners can also be used for relining old chimneys

These are factory made in easy to handle sections that simply lock together. The liner is usually made of 316 grade stainless steel surrounded by insulation and then a metal outer casing. Some systems have a ceramic or refractory concrete liner, which can offer a longer life.

A choice of fittings such as bends, brackets and terminals are available with these systems. There are also metal chimneys specifically designed for use with gas fired appliances, which must not be used with solid fuel appliances.


To keep options open, it is recommended to have a minimum internal flue size of 185mm square of 200mm round, as this size is suitable for most solid fuel burning appliances and also gas fires.
These are the minimum sizes for a flue above a fireplace recess or for an open fire with an opening up to 500mm wide by 550mm high. For large open fires, such as inglenooks. dog grate installations or special appliances I stoves designed to operate with a fire opening greater than 500mm x 550mm. the flue size should be between 14% and 16% of the free unobstructed area of the fire opening
(including sides if open). For free-standing boilers, cookers or stoves that are not in a fireplace recess the minimum flue size is 150mm round or square, depending on the appliance manufacturers instructions.

The appliance manufacturers chimney sizing recommendations should always be followed. The flue size should never be less than the outlet size on the appliance.

Many Decorative Fuel Effect gas fires (DFEs) that imitate a coal or log burning open fire require the same chimney arrangement as for solid fuel open fires and must be installed in accordance with

British Standard BS5871 : Part3.


The ideal chimney position is inside the building so it can benefit from being kept warm. Chimneys situated outside the building can be affected by cold weather causing poor up-draught and condensation, particularly if they are un-insulated. It is therefore important that a cavity wall is continued around a lined masonry chimney or a factory made insulated chimney system is used for external applications.


British Standards recommend that bends in the chimney are avoided, as a straight chimney gives better performance. If bends are absolutely necessary there must be no more than two in the length of the chimney (i.e. one offset). The angle of the bend should be no greater than 30o from
the vertical. However, in some cases such as for chimney installations in flats it may be necessary to have more than two bends in each flue. In this situation advice must be sought from the manufacturer to ensure that the flue will create sufficient draw. Always use the standard bends or offset components which are available from the chimney manufacturer. For metal chimneys to BS 4543 the distance between bends must be no greater than 20% of the total chimney length.


Room heaters, stoves, cookers and boilers must be connected to tile chimney using correctly sized flue pipe which conforms to Building Regulations and British Standard 6461 Part 1: Which lists cast iron. stainless steel and vitreous enamel.

The flue pipe must be suitably sealed with spigot end facing down (into the socket of the pipe below). As excessive bends and horizontal runs can accumulate soot with the risk of blockage, keep the flue pipe as vertical as possible and the angle of bends no more than 450 from the vertical. The maximum horizontal length of flue pipe allowed from the back of an appliance into a chimney is 150mm.

Ensure all flue pipes are correctly installed and has easy access for cleaning the flueways, particularly at bends.

For open fires a suitable throated front lintel and gather must be provided above the fire opening, so that the front, back and sides slope up smoothly into the flue opening in the chimney at an angle no greater than 30 degrees from the vertical. Avoid flat surfaces or shelves which can cause turbulence and smoky fires. Most flue and chimney manufacturers provide standard gather and fireplace components, including precast fire chambers.

Some appliances such as free-standing and inset open fires have built in gathers


Building Regulations require that the flue inside the flue pipe and chimney are easily accessible for regular inspection and sweeping to prevent blockage. If this is not possible through the appliance or open fire, a purpose made access/sootdoor must be provided in the chimney. Most chimney manufacturers can provide components to suit.

Where the appliance connection is made with a flue pipe going into the side of the chimney a space must be provided with sootdoor access below this point to collect debris such as soot that may fall down the chimney.


The minimum chimney height recommended by British Standards is 4.5 m from the top of the
appliance to the top of the chimney. It is best to position the chimney, so that t goes straight up as near to the roof ridge as possible. In some cases, particularly when chimneys are towards the bottom of a sloping roof or at the eaves, it may be necessary to increase the chimney height above the minimum requirements. The reason for this, is to clear pressure zones created by wind hitting the roof and nearby structures. trees etc.. that may cause down draught.
The maximum free-standing stack height above the roof for a traditional masonry or block system chimney is 4.5 times the narrowest horizontal part of the chimney. This measurement is taken from the last point where the chimney stack passes the through or past the edge of the roof up to the chimney capping or termination.

Tall chimneys may need bracing, always consult the manufacturer for advice.

Note: When a fan assisted solid fuel boiler is to be installed into a single storey boiler house and it is not possible to achieve the minimum freestanding chimney height, a vertical factory made chimney system operating on the balanced fluing principle may be used. However, the expert advice of the manufacturer must be sought.


There are many types of chimney pots and terminals, in different styles and shapes to suit almost
any taste and application. However, it is important to ensure the chosen pot or terminal does not restrict the exit of the products of combustion. The area of the outlet must therefore be the same as the flue area. All too often pots and terminals which have a reduced or tapered top are fitted, such as inserts, particularly the ˝ round or hood insert, which are primarily designed for ventilating a chimney where the fireplace has been closed off.

If down draught is being experienced (i.e. smoke being blown back into the room) it is best to check that there is nothing wrong with the chimney arrangement, before resorting to a special terminal. In many cases, the problem may be due to factors such as lack of ventilation, poor throating above an open fire or insufficient chimney height. Most of the time these problems can be easily cured with the help of an expert and sometimes by raising the height of the chimney or the fitting of a taller pot

If you want to stop rain from dropping down the chimney there are various pots and terminals available. The T shaped clay pot shown on the left is an example and commonly used with solid fuel and gas open fires.

Another alternative for traditional masonry chimneys is to build a Dovecote type termination, where a slab is placed on 4 masonry piers above the flue. It is important to ensure all four sides are left open. The height and width of each opening must not be less than the maximum horizontal cross section of the flue.


All heating appliances and particularly open fires need air to work efficiently and safely. Ensure that the appropriate permanent air supply as required by the appliance manufacturer and Building Regulations is provided into the room where the appliance or fire is situated. As a guideline a permanent air vent with an opening or openings giving a total free area of at least 550mm2 per kW of rated output above 5 kW must be
provided for open fires. Generally the same size of ventilation is used for room heaters or stoves, but always refer to the appliance manufacturers recommendations and Building Regulations.

This is important as air starvation will cause poor combustion and create problems similar to down draught and spillage of smoke and fumes back into the room, which can be unpleasant and dangerous.


Old chimneys are often very in-efficient and particularly if unlined can leak products of combustion and tars through the chimney walls. It is quite common to find that old chimneys are in a poor condition due to the flue surface and mortar joints being eaten away by corrosive condensates and soot deposits. The size of the flue may also be too large to suit the operating requirements of many modern heating appliances. It was only in 1965 that Building Regulations required all new chimneys had to be built with suitable flue liners of the correct flue size to protect the chimney structure.

It is therefore very wise to have any old chimney you are using checked for its condition and particularly if you are planning to reopen an old chimney after many years of not being used. The check is best done by a chimney specialist and would start with the chimney being swept to make sure it is clear of obstructions such as dislodged masonry or bird’s nests. A smoke test should then be carried out to establish whether there is any major fault that can cause products of combustion to leak through the chimney walls. A smoke test procedure is given later on in this guide. The general condition of the chimney in terms of structural stability should also be checked, as well as making sure it is wind and water tight.

Carrying out these checks should identify the repair work that is needed and more often than not, it is also necessary to have the chimney fitted with a new liner, which should improve both performance and safety. There are many factory made refining systems available and it is important to make sure that the correct flue size is fitted to suit the appliance or fire that will be used. Many of the same installation points as for new chimneys also apply.

Made from clay, ceramics or refractory concrete consisting of pumice or kiln burnt aggregate bonded with high alumina cement. These liners are simply lowered down the chimney on guide ropes with locating bands at the joints. The gap between the liners and chimney is then backfilled with an insulating lightweight concrete. Whilst this type of lining offers a long life, the existing flue opening has to be large enough for the liners to be installed.

Pre-made bends are available with most systems.

Not to be confused with single skin liners for gas only. These multifuel flexible liners are factory made from two overlapping strips of high grade stainless steel to give a smooth sealed flueway. They are simply lowered down or pulled up the chimney and can go around most bends. Whilst their slim profile enables quick installation into chimneys where other systems might not fit. their life can be reduced if abnormally high corrosive soot or condensate deposits are created and allowed to accumulate in the flue.


All chimneys operate on the principle of having a natural up draught created by maintaining warm flue gases of between 150 oC to 500oC. It is therefore very important to use the appliance correctly and maintain a bright, warm fire so that under normal operating conditions the flue gas temperatures created are kept between 1500oC and 500oC.

Burning solid fuel slowly with insufficient air supply, particularly on stoves or closed appliances must be avoided. Low flue gas temperatures will cause condensation and greatly increases the risk of producing excessive tar and corrosive soot deposits. This is a common problem, particularly when burning wet wood or coal and must be avoided. If soot and condensate deposits are allowed to accumulate in a flue, the deposits can ignite causing a chimney fire which is likely to cause damage to the chimney and appliance. These deposits can also be very corrosive and if they are not regularly removed can cause corrosion of metal parts of both the chimney and appliances

Always use suitable solid fuels. There are some processed fuels such as raw petrocoke which can cause spitting when being bumt and also produce abnormally high temperatures together with very corrosive deposits. It is therefore recommended to get advice from an Approved Coal Merchants.

Never burn household waste, plastics or chemicals which can produce harmful fumes and corrosive vapours, as this can be dangerous to health and cause premature corrosion of metals.

If wood is used as a fuel it is important to ensure that it is dry and well seasoned. This means timber that has been cut into logs, then split and stacked out in the open under a waterproof cover which is open on all sides to allow free flow of air for natural drying. Most timber needs one or two years drying Out depending on moisture content. Taking logs into the house a few days before burning will help to reduce the moisture content and improve performance.


To start with all flueways should be checked monthly to assess the amount of deposits being created. The frequency of sweeping needed to prevent build up of deposits can then be determined. As a guide all flues should be swept before and during the heating season, and ideally at the end of the heating season to prevent tar and soot deposits having a corrosive effect on the chimney and appliance during the dormant period.

It is not recommended that the appliance is overi red or chimney fires are started in an attempt to dean the chimney. Deposits of soot and tar will be greatly increased if unseasoned wood is burnt, which can lead to chimney fires. Should a chimney fire occur, it is advisable to have your chimney and appliance arrangement checked for damage before reusing the fire or heating appliance.

It is also good practise to check at least every year or two the exposed parts of a chimney, flashings and terminals for signs of damage. Just like the outside of a house chimneys can suffer from the wear and tear of extreme weathering.

If at any time smoke or fumes are apparent or suspected from the appliance, chimney or liner, seek advice immediately from the installer or fuel authority in case there is a blockage or failure. Do not use the fire appliance or chimney until they have been thoroughly checked for safety and soundness. The escape of fumes can be dangerous.


Mechanical sweeping with brush and rods is the only method of cleaning recommended by British Standards, because materials other than soot can block flueways i.e. mortar, brickwork, birds nests etc.


For this reason cleaning a flue using just chemical chimney cleaners or vacuum cleaning cannot be recommended as an alternative for correct chimney cleaning.

Sweeping brushes should be made from suitable bristle and be of the same diameter or area as the flueway being swept, and be fitted with a ball or free running wheelboss on the end to prevent scraping the flue walls, particularly at bends. Many chimney manufacturers give recommendations of brush types to suit their systems. Use a qualified chimney sweep or member of NAGS (National Association of Chimney Sweeps) who gives and inspection and sweeping certificate.


If there is doubt about the condition of a chimney, or an old chimney is to be put in use after a long period of being un-used, his advisable to have it smoke tested. The purpose of the smoke test is to discover if there are any major defects which can cause a leak of fumes through the chimney walls during normal operation.

A smoke test should be carried out by a qualified person using the following procedure which is based on the test described in British Standard BS 6461 Part 1. There is a different procedure for smoke testing flues for gas fires. It is also recommended that this smoke test is carried out during the construction of traditional masonry chimneys and on completion of all chimney installations.

It must be remembered that the purpose of the smoke test is to identify and deal with any faults which would cause fumes to escape during the normal operation of the appliance and chimney. By closing the bottom and top of the chimney during the test, the smoke generated by the pellet together with normal barometric conditions will generate a positive pressure that would not be created during the normal use of the installation, which operates under negative pressure i.e. sucking in air and drawing the products of combustion up the flue.

The pressure created during the procedure is therefore more than capable of creating minor leakage of smoke from either joints in a traditionally masonry chimney, or connections between pre-fabricated metal chimneys and flue pipes. Minor leakage detected during this test, would therefore not be a major risk when the installation is used during normal operating conditions, providing the point of leakage does not indicate a fault that could get worse. However, if significant or heavy smoke leakage is seen, the cause must be investigated and rectified. Broken components, incorrect fitting and incomplete jointing of flue liners are the most common problems which cause major leakage and require remedial action.

1. If there is an appliance fitted at the base of the bottom of the chimney, burn some newspaper in the fire/grate for 2 to 3 minutes to establish a flue draw. A longer warming up time may be needed with wet or cold flues. If there is no grate or fire box fitted use a blow lamp for 10 minutes or more to establish a flue draw. It should be realised that neither of these methods create the same temperatures or volume of hot gases that would normally be created whilst the appliance is in use. They therefore are intended to assist the testing and not simulate real conditions.

2. Place two flue testing smoke pellets on a brick or similar in the opening at the base of the flue or in the appliance if it is fitted. Follow the pellet manufacturers safety instructions.

3. Ignite the pellet and when it starts smoking seal the opening with a board or plate sealed at the edges or if an appliance is fitted, close all doors, ashpit cover and vents.

4. When smoke begins rising out of the top of the chimney, seal the top of the flue, terminal or pot. (i.e. use an inflated football bladder or plastic bag sealed in position with tape).

5. Examine the full length of the chimney for any leakage of smoke. If possible check for signs of
smoke leakage from wall cavities at the sides and back of the chimney, if it is built into a wall. Also check for smoke leakage at openings around windows near to the chimney and the roof space area.

6. If any smoke is seen establish the point of leakage and carry out remedial work to correct any

7. After completing the remedial work repeat the testing procedure until no major leakage or fault is
apparent. Remove the closures at the top and bottom of the chimney.


All chimney systems or liners both for new build and relining must conform to Building Regulations, Britsh Standards or have a British Board of Agreement approval certificate or similar test approval certificate acceptable to the local building control officer. Building control approval is necessary for building new chimneys and in some cases for relining old chimneys particularly if some alteration or change of the heating appliance occurs.


Approved Document J of the Building Regulations. Copies can be obtained from HMSO. Advice is also available from Local Authority Building Control Departments.

British Standards. Copies of British Standards can be obtained from British Standards Institution, 389 Chiswick High Road, Chiswick, London W4 4AL. Tel: 0181 996 9000.

BS 4543 - Specification for factory made chimneys.

BS6461 Installation of chimneys and flues for domestic appliances burning solid fuel (including wood and peat). Part 1 - Code of practice for masonry chimneys and flue pipes.

BS8303- Installation of domestic heating and cooking appliances burning solid fuel mineral fuels.


HETAS- Heating Equipment of Solid Fuel Products and Services. P0 Box 37, Bishops Cleeve,Glos GL5 24TE. They publish annually The official guide to approved solid fuel products and services, which has a comprehensive list of factory made chimney systems and flues from members of the BFCMA.

NACS - National Association of Chimney Sweeps, St. Mary’s Chambers, 19, Station Road, Stoke on Trent, ST15 8JP Tel: 07185 -81 1732. They can provide a list of their members throughout Britain.

NFA - National Fireplace Association, 8th Floor, Bridge House, Smallbrook, Queensway, Birmingham B5 4JP, Tel:0121-643 1133. They publish a series of guides on fireplaces and associated work including chimneys.

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