3.18 Combustion appliances – protection from combustion products

Mandatory Standard

Standard 3.18

Every building must be designed and constructed in such a way that any component part of each fixed combustion appliance installation used for the removal of combustion gases will withstand heat generated as a result of its operation without any structural change that would impair the stability or performance of the installation.

3.18.0 Introduction

The fire service attends many calls to chimney fires and other fires where a chimney defect has allowed fire spread into a building. Whilst the guidance in this standard cannot prevent fires, the structural precautions recommended help to limit the damage to flues and thus prevent fire from spreading into the building.

It is essential that flues continue to function effectively when in use without allowing the products of combustion to enter the building. Chimneys and flue-pipes are now tested to harmonised European standards to establish their characteristics relative to safe operation.

Very low flue-gas temperatures are achieved by modern, high efficiency appliances, particularly during night conditions, thus causing condensation. Materials need to withstand these aggressive situations.

Explanation of terms

The following terms are included to provide clarity to their meaning in this Technical Handbook.

Chimney – a structure enclosing 1 or more flues, but not a flue-pipe, and including any openings for the accommodation of a combustion appliance, but does not include a chimney terminal

Custom-built chimneychimney that is installed or built on-site using a combination of compatible chimney components that may be from 1 or different sources

Double-walled chimneychimney consisting of a flue liner and an outer wall

Factory-made chimney – see system chimneys

Flue – passage for conveying the products of combustion to the outside atmosphere

Flue-block – factory-made chimney components with 1 or more flues

Flue liner – wall of a chimney consisting of components the surface of which is in contact with products of combustion

Flue-pipe – (correctly termed ‘connecting flue-pipe’) is a pipe, either single walled (insulated or non-insulated) or double-walled, that connects a combustion appliance to a flue in a chimney

Single-walled chimneychimney where the flue liner is the chimney

System chimneys – (factory-made chimney) chimney that is installed using a combination of compatible chimney components, obtained or specified from one manufacturing source with product responsibility for the whole chimney.

Some of these terms are explained in greater depth later in the guidance to this standard.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

3.18.1 Chimneys generally

Combustion appliances are very often changed after the original installation. Unless an appliance is supplied to be used with a specified system chimney or with an integral duct assembly, e.g. balanced flue, it is desirable, and sometimes more economical, to cater initially for the most severe conditions as regards the danger of fire, generally a traditional open fire, and to ensure that all components are compatible.

Combustion appliances, other than flueless appliances such as gas cookers, should incorporate, or be connected to, a flue-pipe and/or a chimney that will withstand the heat generated by the normal operation of the appliance. A chimney of a higher specification than the designation strings given (see clause 3.18.2) may be used if required, such as a chimney generally suitable for use with an open-flued solid fuel appliance may be used with an open flued gas-fired appliance.

The National Association of Chimney Engineers (NACE) (http://www.nace.org.uk/) was set up to ensure the safety of all fuel users who depend upon a chimney or flue for the operation of a heating appliance. They provide a register of competent and qualified chimney engineers for all types of chimney work. Advice is also available from the British Flue and Chimney Manufacturers’ Association (BFCMA) ( http://www.feta.co.uk/). These organisations do not have a mandatory status.

Sweeping chimneys - the process of burning will naturally cause deposits of soot in the flue. Chimneys and flue-pipes therefore should be swept at least annually if smokeless solid fuel is burnt and more often if wood, peat and/or other high volatile solid fuel such as bituminous coal is burnt. Mechanical sweeping with a brush is the recommended method of cleaning.

Every chimney should have such capacity, be of a height and location and with an outlet so located that the products of combustion are discharged freely and will not present a fire hazard.

A flue should be free from obstructions. The surface of the flue should be essentially uniform, gas-tight and resistant to corrosion from combustion products. Chimneys should be constructed in accordance with:

  1. the recommendations of BS 6461: Part 1: 1984 for masonry chimneys, or

  2. the recommendations of BS 7566: Parts 1 - 4: 1992 for metal system chimneys, or

  3. BS 5410: Part 1: 1997 and OFTEC Technical Book 3, where serving an oil-firing appliance, or

  4. BS 5440: Part 1: 2000, where serving a gas-fired appliance.

3.18.2 Chimney designations

Designations for chimneys, according to BS EN 1443: 2003, are dependant on the fuel to be used, the type of appliance and the operating conditions. The designation string prescribes limiting values or categories for temperature, pressure, condensate resistance, corrosion resistance, soot fire resistance and distance to combustibles. Values for which the chimney is suitable are specified by the system chimney manufacturer or the designer of a custom built or re-lined chimney. For a new chimney installation the chimney designation should be chosen to suit the intended appliance installation. For an existing chimney the appliance performance should be chosen to match the designation of the chimney. Advice on the appropriate chimney specification should be sought from the appliance manufacturer.

The recommended designation for chimneys and flue-pipes for use with natural draught, solid fuel appliances is T400 N2 D 3 Gxx.

The recommended designation for chimneys and flue-pipes for use with forced draught solid fuel appliances that have a positive pressure at the outlet of the appliance is T400 P2 D 3 Gxx.

The pressure designation P2 is regarded as the default specification. However the chimney can often generate an adequate natural draught, so that the appliance can be safely used with chimneys and flue-pipes with the negative pressure designation even if the appliance is fanned. The draught generated in a chimney may be calculated according to BS EN 13384-1: 2002. If there is any doubt, and/or unless the appliance manufacturer specifies N2, the designation P2 should apply.

Table 3.7. Recommended designation for chimneys and flue-pipes for use with oil-firing appliances with a flue gas temperature not more than 250ºC

Appliance type Fuel oil Designation
Boiler including combustion boiler - pressure jet Class C2 T250 N2 D 1 Oxx
Cooker - pressure jet burner Class C2 T250 N2 D 1 Oxx
Cooker and room heater - vaporising burner Class C2 T250 N2 D 1 Oxx
Cooker and room heater - vaporising burner Class D T250 N2 D 2 Oxx
Condensing pressure jet burner appliances Class C2 T160 N2 W 1 Oxx
Cooker - vapourising burner appliances Class D T160 N2 W 2 Oxx


Additional information:

The pressure designation N2 is regarded as the most likely specification to apply in the oil industry for both vaporising and pressure jet appliances. Most pressure jet appliances only generate adequate pressure to overcome flow resistance within the appliance so that the products of combustion entering the chimney will be at a negative pressure with respect to the atmosphere. Thus the appliance can be safely used with chimneys and flue-pipes with negative pressure designation. In the event that an appliance design produces a positive pressure at the outlet of the appliance, it is the manufacturer’s responsibility to inform the installer that a chimney with a positive designation should be used. If there is any doubt, the more onerous designation P2 should apply.

The appliance manufacturer’s instructions should always be checked. They may specify a higher designation.

Table 3.8. Recommended designation for chimneys and flue-pipes for use with gas appliances

Appliance Type Designation
Boiler - open - flued
  • natural draught

  • fanned draught

  • condensing

T250 N2 D 1 Oxx

T250 P2 D 1 Oxx[1]

T250 P2 W 1 Oxx[1]

Boiler - room - sealed
  • natural draught

  • fanned draught

T250 N2 D 1 Oxx

T250 P2 D 1 Oxx[1]

Gas fire
  • radient/convector

  • ILFE or DFE

T250 N2 D 1 Oxx
Air heater
  • natural draught

  • fanned draught

  • SE duct

T250 N2 D 1 Oxx

T200 P2 D 1 Oxx[1]

T450 N2 D 1 Oxx


Additional information:

The pressure designation P2 is regarded as the default specification. However the chimney can often generate an adequate natural draught, so that the appliance can be safely used with chimneys and flue-pipes with the negative pressure designation even for many fanned draught gas appliances, including condensing boilers that may otherwise have positive pressure at the outlet to the flue. The draught generated in a chimney may be calculated according to BS EN 13384-1: 2002. If there is any doubt, and/or unless the appliance manufacturer specifies N2, the designation P2 should apply.

3.18.3 Masonry chimneys

A new masonry chimney, usually custom-built on site, and normally with an outer wall of brick, block or stone, should be well constructed and incorporate a flue liner, or flue-blocks, of either clay material or precast concrete. A masonry chimney should be constructed in accordance with the recommendations in BS 6461: Part 1: 1984. If an outer wall is constructed of concrete it should be constructed in accordance with BS EN 12446: 2003.

It is a complex operation to upgrade the chimney at a later date to serve a new appliance that needs a higher classification of chimney to operate safely, thus a chimney designed for solid fuel will also serve for oil or gas. See clause 3.18.6 for guidance on flue liners.

Chimneys can also be constructed of prefabricated block components, designed for quick construction. Chimney components such as cappings, offsets and precast fireplace components are available with this type of system. Some flue-blocks are specially designed for gas-fired appliances only. Flue-blocks should be constructed and installed in accordance with recommendations in:

  1. BS EN 1858: 2003, for a precast concrete flue-block chimney

  2. BS EN 1806: 2006, for a clay flue-block chimney.

3.18.4 Metal chimneys

Metal chimneys may be either single-walled or double-walled. Each of these types is commonly factory-made by one manufacturer as sets of components for easy assembly on site (although they can be supplied as 1 unit) and is thus a system chimney. A choice of fittings such as bends, brackets, and terminals are available.

Some metal chimneys are specifically designed for use with gas-fired appliances and should not be used for solid fuel appliances because of the higher temperatures and greater corrosion risk.

Metal system chimneys, with the following designations, should be constructed in accordance with the recommendations in BS EN 1856-1:

  1. T400 N1 D V3 (or Vm - Lxxxxx) Gxx, for solid fuel appliances

  2. T400 P2 D V3 (or Vm - Lxxxxx) Gxx where it serves an oil-firing appliance producing a flue gas temperature of not more than 400ºC, e.g. burning Class D oil (gas oil)

  3. T250 N2 D V2 (or Vm - Lxxxxx) Oxx where it serves an oil-firing appliance producing a flue gas temperature of not more than 250ºC, e.g. burning Class C2 oil (kerosene)

  4. T250 N2 D V1 (or Vm - Lxxxxx) Oxx where it serves a gas appliance.

The corrosion resistance may be specified, according to BS EN 1856-1, by:

  1. a corrosion test method, which leads to a value of either V1, V2 or V3, or

  2. by a material specification code Vm - Lxxxxx where the first 2 digits represent a material type as quoted in BS EN 1856-1, Table 4 and the last 3 digits represent the material thickness.

Acceptable material specifications may be taken from the national Annex to BS EN 1856-1. For example, an acceptable material code for solid fuel, oil or gas, would be Vm - L50040 representing a material type 50 with a thickness of 0.40mm.

A metal chimney should not pass through a compartment wall, compartment floor, separating wall or separating floor. However they may if the chimney, or a non-combustible casing totally enclosing the chimney, is constructed in such a way that, in the event of a fire, the fire resistance of the compartment wall, compartment floor, separating wall or separating floor is maintained (see Section 2, Fire).

A metal chimney should only pass through a storage space, cupboard or roof space provided any flammable material is shielded from the chimney by a removable, imperforate casing. Also where the chimney passes through the roof space, such as an attic, it should be surrounded be a rigid mesh that will prevent vermin from building a nest beside the warm chimney. Mesh should prevent an 8mm diameter sphere from passing.

There should be no joints within any wall, floor or ceiling that make accessing the chimney for maintenance purposes difficult.

3.18.5 Flue-pipes

A flue-pipe should be of a material that will safely discharge the products of combustion into the flue under all conditions that will be encountered. A flue-pipe serving a solid fuel appliance should be non-combustible and of a material and construction capable of withstanding the effects of a chimney fire without any structural change that would impair the stability and performance of the flue-pipe.

Flue-pipes should be manufactured from the materials noted below:

  1. cast iron pipe to BS 41: 1973 (1988)

  2. mild steel at least 3mm thick, to Section 1.1 of BS 1449: Part 1: 1991

  3. vitreous enamelled steel to BS 6999: 1989

  4. stainless steel designated Vm - L50100, in accordance with BS EN 1856-2: 2005 or Vm - Lxxxxx for oil or gas applications

  5. any other material approved and tested under the relevant conditions of a notified body.

Flue-pipes should have the same diameter or equivalent cross sectional area as that of the appliance flue outlet and should be to the size recommended by the appliance manufacturer. It should be noted that oversized flue-pipes can cause condensation problems in modern, highly efficient oil and gas fired boilers.

A flue-pipe connecting a solid fuel appliance to a chimney should not pass through:

  1. a roof space

  2. an internal wall, although it is acceptable to discharge a flue-pipe into a flue in a chimney formed wholly or partly by a non-combustible wall

  3. a ceiling or floor. However it is acceptable for a flue-pipe to pass through a ceiling or floor where they are non-combustible and the flue-pipe discharges into a chimney immediately above.

3.18.6 Flue liners

A flue liner is the wall of the chimney that is in contact with the products of combustion. It can generally be of concrete, clay, metal or plastic depending on the designation of the application.

All new chimneys will have flue liners installed and there are several types, as follows:

  • rigid sections of clay or refractory liner

  • rigid sections of concrete liner

  • rigid metal pipes.

Flue liners suitable for solid fuel appliances, and therefore generally suitable for other fuels, should have a performance at least equal to that corresponding to the designation T400 N2 D 3 G as described in BS EN 1443: 2003 and manufactured from the following materials:

  1. clay flue liners with rebates or sockets for jointing and meeting the requirements for Class A1 N2 or Class A1 N1 as described in BS EN 1457: 1999, or

  2. concrete flue liners meeting the requirements for the classification Type A1, Type A2, Type B1 or Type B2 as described in BS EN 1857: 2003, or

  3. any other material approved and tested under the relevant conditions of a notified body.

Stainless steel flexible flue liners meeting BS EN 1856-2: 2005 may be used for lining or relining flues for oil and gas appliances, and for lining flues for solid fuel applications provided that the designation is in accordance with the intended application. These should be installed in accordance with their manufacturer’s instructions.

Single skin, stainless steel flexible flue liners may be used for lining flues for gas and oil appliances. These should be installed in accordance with their manufacturer’s instructions.

Double skin, stainless steel flexible flue liners for multi-fuel use should be installed in accordance with their manufacturer’s instructions.

Existing custom-built masonry chimneys may be lined or re-lined by one of the following flue liners:

  • flexible, continuous length, single-skin stainless steel for lining or re-lining chimney flues for C2 oil and gas installations designated T250

  • flexible, continuous length, double-skin stainless steel for lining or re-lining systems designated T400 for multi-fuel installations

  • insulating concrete pumped in around an inflatable former

  • spray-on or brush-on coating by specialist.

Existing chimneys for solid fuel applications may also be relined using approved rigid metal liners or single-walled chimney products, an approved cast-insitu technique or an approved spray-on or brush-on coating. Approved products are listed in the HETAS Guide.

Masonry liners for use in existing chimneys should be installed in accordance with their manufacturer’s instructions. Appropriate components should be selected to form the flue without cutting and to keep joints to a minimum. Bends and offsets should only be formed with factory-made components. Liners should be placed with the sockets or rebate ends uppermost to contain moisture and other condensates in the flue. In the absence of specific liner manufacturer’s instructions to the contrary, the space between the lining and the surrounding masonry could be filled with a weak insulating concrete.

The corrosion resistance of a metal liner may be specified, according to BS EN 1856-1, by either:

  1. a corrosion test method, which leads to a value of either V1, V2 or V3, or

  2. by a material specification code Vm - Lxxxxx where the first 2 digits represent a material type as quoted in BS EN 1856-1, Table 4 and the last 3 digits represent the material thickness.

Acceptable material specifications may be taken from the national Annex to BS EN 1856-1. For example, an acceptable material code for solid fuel, oil or gas, would be Vm - L50040 representing a material type 50 with a thickness of 0.40mm.