Air supply for combustion appliances

1.2 Combustion appliances require ventilation to supply them with air for combustion. Ventilation is also required to ensure the proper operation of flues or, in the case of flueless appliances, to ensure that the products of combustion are safely dispersed to the outside air. Installation of room-sealed appliances or those with a directly connected ducted external air supply will minimise ventilation energy losses from the room and the risk of cold draughts. In some cases, combustion appliances may also require air for cooling control systems and/or to ensure that casings remain safe to touch (see Diagram 8). General guidance on where it may be necessary to install air vents for these purposes is given below.

1.3 Air vent sizes, which are dependent upon the type of fuel burned, are given in Sections 2, 3 and 4 and are for one combustion appliance only.

The air supply provisions will usually need to be increased where a room contains more than one appliance (such as a kitchen containing an open-flued boiler and an open-flued cooker).

Permanently open ventilation of rooms

1.4 A room containing an open-flued appliance may need permanently open air vents. An openflued appliance must receive a certain amount of air from outside (‘combustion air’ in Diagram 8) dependent upon its type and rating. Infiltration through the building fabric may be sufficient but for certain appliance ratings and forms of construction, permanent openings are necessary (see Diagram 8).

Permanent ventilation of appliance compartments

1.5 Appliance compartments that enclose open-flued combustion appliances should be provided with vents large enough to admit all of the air required by the appliance for combustion and proper flue operation, whether the compartment draws its air from a room or directly from outside (see Diagram 8 (b) and (c)).

1.6 Where appliances require cooling air, appliance compartments should be large enough to enable air to circulate and high- and low-level vents should be provided (see Diagram 8 (d), (e), (f) and (g)).

1.7 Where appliances are to be installed within balanced compartments (see paragraph 0.4(2)), special provisions will be necessary and the appliance and ventilation system manufacturer’s instructions should be followed.

Ventilation of other rooms or spaces

1.8 If an appliance is room-sealed but takes its combustion air from another space in the building (such as the roof void) or if a flue has a permanent opening to another space in the building (such as where it feeds a secondary flue in the roof void), that space should have ventilation openings directly to outside. Where the roof-space is to be used as a source of air for a combustion installation serving a dwelling, the dwelling roof ventilation provisions suggested in Approved Document C would normally be satisfactory.

1.9 Where flued appliances are supplied with combustion air through air vents which open into adjoining rooms or spaces, the adjoining rooms or spaces should have air vent openings of at least the same size direct to the outside. Air vents for flueless appliances, however, should open directly to the outside air.

Diagram 8 General air supply to a combustion appliance (for sizes see Sections 2, 3 and 4)

Diagram 9 Ventilator free areas

Permanently open air vents

1.10 Permanently open air vents should be non-adjustable, sized to admit sufficient air for the purpose intended and positioned where they are unlikely to become blocked. Ventilators should be installed so that building occupants are not provoked into sealing them against draughts or noise. Ventilation openings should not be made in fire-resisting walls other than external walls (although they should not penetrate those parts of external walls shielding LPG tanks). Air vents should not be located within a fireplace recess except on the basis of specialist advice.

1.11 A way of meeting the requirement would be to size permanently open air vents so that their equivalent area is sufficient for the appliance(s) to be installed (taking account where necessary of obstructions such as grilles and anti-vermin mesh), and to site them:

a. outside fireplace recesses and beyond the hearths of open fires so that dust or ash will not be disturbed by draughts; and

b. in a location unlikely to cause discomfort from cold draughts.

1.12 Where ventilation is to be provided via a single proprietary assembly, for example when it is proposed to use a proprietary ventilator with integral grilles to bridge a cavity wall, the equivalent area of the ventilator should be taken as that declared by the manufacturer having been measured by the method in BS EN 13141-1:2004.

1.13 Where two or more components are to be used to provide a non-proprietary assembly, the assembly should be kept as simple and smooth as possible. The assembly should be taken to have an equivalent area equal to that of the component with the smallest equivalent area in the assembly.

1.14 The equivalent area stated in the ventilator manufacturer’s literature or marked on the air vent should be used whenever it is available, as this can differ considerably from the free area measured at one end of the air vent. When this is not available the equivalent area of a simple ventilator with no internal baffles can be taken as the total unobstructed cross-sectional area, measured in the plane where this area is at a minimum and at right angles to the direction of air flow. For an airbrick, grille or louvre with apertures no smaller than 5mm, it will be the aggregate free area of the individual apertures as shown Diagram 9.

1.15 Grilles or meshes protecting air vents from the entry of animals or birds should have aperture dimensions no smaller than 5mm.

1.16 Discomfort from cold draughts can be avoided by supplying air directly to appliances, locating vents close to appliances (for example by using floor vents), by drawing air from intermediate spaces such as hallways or by ensuring good mixing of incoming cold air by placing external air vents close to ceilings (see Diagrams 10 and 11). In noisy areas it may be necessary to install noise-attenuated ventilators to limit the entry of noise into the building. Transfer or connecting ventilation should be at low level to reduce the transfer of smoke in the event of a fire and otherwise meet the guidance given in Approved Document B.

1.17 Buildings may have air-tight membranes in their floors to isolate them from the ground below. Ventilation ducts or vents installed to supply air to combustion appliances should not penetrate these membranes in a way that will render them ineffective. Such membranes (including radonproof membranes) are described in BRE Report BR 414 (2001) and BRE Report BR 211 (2007), which give guidance when service penetrations are necessary.

Provisions complying with both Part F and Part J

1.18 Rooms or spaces intended to contain open-flued combustion appliances may need permanent ventilation to comply with Part J and adjustable ventilation to comply with Part F. Permanently open air vents for combustion appliances can be accepted in place of some or all of the adjustable background ventilation for health, dependent upon opening area and location. However adjustable vents installed to meet the requirements of Part F cannot be used as substitutes for the ventilation openings needed to comply with Part J unless they are fixed permanently open.

1.19 Rooms or spaces intended to contain flueless appliances may need: permanent ventilation and purge ventilation (such as openable windows) to comply with Part J; and adjustable ventilation and rapid ventilation to comply with Part F. Permanent ventilation provisions to comply with Part J may be acceptable in place of adjustable ventilation provisions for Part F subject to the limitations described in Paragraph 1.18. Openable elements installed for the rapid ventilation of rooms and other provisions made for the rapid ventilation of kitchens, in order to comply with Part F, may be acceptable in place of openable elements for the rapid ventilation of rooms or spaces containing flueless appliances.

Diagram 10 Location of permanent air ventilation

1.19 Rooms or spaces intended to contain flueless appliances may need: permanent ventilation and purge ventilation (such as openable windows) to comply with Part J; and adjustable ventilation and rapid ventilation to comply with Part F. Permanent ventilation provisions to comply with Part J may be acceptable in place of adjustable ventilation provisions for Part F subject to the limitations described in Paragraph 1.18. Openable elements installed for the rapid ventilation of rooms and other provisions made for the rapid ventilation of kitchens, in order to comply with Part F, may be acceptable in place of openable elements for the rapid ventilation of rooms or spaces containing flueless appliances.

Diagram 11 Provision of permanent air ventilation

Interaction of mechanical extract ventilation and open-flued combustion appliances

1.20 Extract fans lower the pressure in a building, which can cause the spillage of combustion products from open-flued appliances. This can occur even if the appliance and the fan are in different rooms. Ceiling fans produce air currents and hence local depressurisation, which can also cause the spillage of flue gases from open-flued appliances or from solid fuel open fires. In buildings where it is intended to install open-flued combustion appliances and extract fans, the combustion appliances should be able to operate safely whether or not the fans are running. A way of showing compliance in these circumstances would be to follow the installation guidance below, and to show by tests that combustion appliances operate safely whether or not fans are running.

a. For gas appliances: where a kitchen contains an open-flued appliance, the extract rate of the kitchen extract fan should not exceed 20 litres/second (72m³/hour).

b. For oil appliances: where a room contains an open-flued appliance the extract rate should be limited to 40 litres/second for an appliance with a pressure jet burner and 20 litres/second for an appliance with a vaporising burner.

c. For solid fuel appliances: avoid installing extract ventilation in the same room. An open-flued appliance in a kitchen may satisfy the requirements of Part F through passive stack ventilation. Refer to Approved Document F. If mechanical extraction is unavoidable then seek specialist advice to ensure safe operation of the appliance.

d. For commercial and industrial installations, specialist advice may be necessary regarding the possible need for the interlocking of gas heaters and any mechanical ventilation systems.

e. When fans are used to extract radon from below a building follow the guidance in BRE Good Building Guide GBG 25.

1.21 A suitable test would be to check for spillage when appliances are subjected to the greatest possible depressurisation. A prerequisite for this condition is that all external doors, windows and other adjustable ventilators to outside are closed. The depressurisation at the appliance will depend on the particular combination of fans in operation (fans in the room containing the appliance and fans elsewhere in the building) and the pattern of open internal doors, hatches etc. which is established at the time of the test (when fans should be on their maximum useable setting), and the specific combination causing the greatest depressurisation at the appliance depends upon the circumstances in each case. Several tests (which should include a test with the door leading into the room of installation closed and all fans in that room switched on) may therefore be necessary to demonstrate the safe operation of the appliance with reasonable certainty. The effect of ceiling fans should be checked during the tests.

1.22 The presence of some fans may be obvious, such as those on view in kitchens, but others may be less obvious: fans installed in domestic appliances such as tumble dryers and fans fitted to other open-flued combustion appliances can also contribute to depressurisation. In addition, fans may also be provided to draw radon gas from the ground below a building (see Paragraph 1.17).

1.23 The appliance manufacturer’s installation instructions may describe a suitable spillage test for gas appliances but the procedure in BS 5440-1:2008 can be used. For oil-fired appliances the effects of fans can be checked and, where spillage or flue draught interference is identified, it may be necessary to add additional ventilation to the room or space. A flue draught interference test for oil-fired appliances is described in OFTEC Technical Books 2, 4 and 5.

Provision of flues

1.24 Appliances other than flueless appliances should incorporate or be connected to suitable flues which discharge to the outside air.

1.25 This Approved Document provides guidance on how to meet the requirements in terms of constructing a flue or chimney, where each flue serves one appliance only. Flues designed to serve more than one appliance can meet the requirements by following the guidance in BS 5410-1:1997 for oil- and BS 5440-1:2008 for gas-fired systems. However, each solid fuel appliance should have its own flue.

Condensates in flues

1.26 Chimneys and flues should provide satisfactory control of water condensation. Ways of providing satisfactory control include:

a. for chimneys that do not serve condensing appliances, by insulating flues so that flue gases do not condense in normal operation

b. for chimneys that do serve condensing appliances:

by using lining components that are impervious to condensates and suitably resistant to corrosion (BS EN 1443:2003 ‘W’ designation) and by making appropriate provisions for draining, avoiding ledges, crevices, etc
making provisions for the disposal of condensate from condensing appliances.

Construction of masonry chimneys

1.27 New masonry chimneys should be constructed with flue liners and masonry suitable for the intended application. Ways of meeting the requirement would be to use bricks, medium weight concrete blocks or stone (with wall thicknesses as given in Section 2, 3 or 4 according to the intended fuel) with suitable mortar joints for the masonry and suitably supported and caulked liners. Liners suitable for solid fuel appliances (and generally suitable for other fuels) could be:

a. liners whose performance is at least equal to that corresponding to the designation T400 N2 D 3 G, as described in BS EN 1443:2003, such as:

clay flue liners with rebates or sockets for jointing meeting the requirements for Class A1 N2 or Class A1 N1 as described in BS EN 1457:2009; or
concrete flue liners meeting the requirements for the classification Type A1, Type A2, Type B1 or Type B2 as described in BS EN 857:2003; or
other products that meet the criteria in a).

1.28 Liners 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 be formed only with matching factory-made components. Liners need to be placed with the sockets or rebate ends uppermost to contain moisture and other condensates in the flue. Joints should be sealed with fire cement, refractory mortar or installed in accordance with their manufacturer’s instructions. Spaces between the lining and the surrounding masonry should not be filled with ordinary mortar. In the absence of liner manufacturer’s instructions, the space could be filled with a weak insulating concrete such as mixtures of:

a. one part ordinary Portland cement to 20 parts suitable lightweight expanded clay aggregate, minimally wetted; or

b. one part ordinary Portland cement to 6 parts Vermiculite; or

c. one part ordinary Portland cement to 10 parts Perlite.

Construction of flueblock chimneys

1.29 Flueblock chimneys should be constructed of factory-made components suitable for the intended application installed in accordance with the manufacturer’s instructions. Ways of meeting the requirement for solid fuel appliances (and generally suitable for other fuels) include using:

a. flueblocks whose performance is at least equal to that corresponding to the designation T400 N2 D 3 G, as described in BS EN 1443:2003, such as:

clay flueblocks at least meeting the requirements for Class FB1 N2 as described in BS EN 1806:2006
other products that meet the criteria in a).

b. blocks suitable for the purpose lined in accordance with Paragraph 1.27.

1.30 Joints should be sealed in accordance with the flueblock manufacturer’s instructions. Bends and offsets should be formed only with matching factory-made components.

Material change of use

1.31 Where a building is to be altered for different use (e.g. it is being converted into flats) the fire resistance of walls of existing masonry chimneys may need to be improved as shown in Diagram 12.

Connecting fluepipes

1.32 Satisfactory components for constructing connecting fluepipes include:

a. cast iron fluepipes complying with BS 41:1973 (1998)

b. metal flue pipes appropriately designated in accordance with BS EN1856-2:2004 to suit the appliance and types of fuels to be burnt – refer to detailed guidance in Sections 2, 3 and 4.

c. vitreous enamelled steel pipe complying with BS 6999:1989 (1996)

d. other fluepipes having the necessary performance designation for use with the intended appliance.

Diagram 12 Material change of use: fire protection of chimneys passing through other dwellings

1.33 Fluepipes with spigot and socket joints should be fitted with the socket facing upwards to contain moisture and other condensates in the flue. Joints should be made gas-tight. A satisfactory way of achieving this would be to use proprietary jointing accessories or, where appropriate, by packing joints with noncombustible rope and fire cement.

Repair of flues

1.34 It is important to the health and safety of building occupants that renovations, refurbishments or repairs to flue liners should result in flues that comply with the requirements of J2 to J5. The test procedures referred to in paragraph 1.55 and in Appendix E can be used to check this.

1.35 Flues are controlled services as defined in Regulation 2 of the Building Regulations, that is to say they are services in relation to which Part J of Schedule 1 imposes requirements. If renovation, refurbishment or repair amounts to or involves the provision of a new or replacement flue liner, it is ‘building work’ within the meaning of Regulation 3 of the Building Regulations. ‘Building work’ and must not be undertaken without prior notification to the local authority. Examples of work that would need to be notified include:

a. relining work comprising the creation of new flue walls by the insertion of new linings such as rigid or flexible prefabricated components

b. a cast in situ liner that significantly alters the flue’s internal dimensions.

Anyone in doubt about whether or not any renovation, refurbishment or repair work involving a flue is notifiable ‘building work’, could consult the building control department of their local authority, or an approved inspector.

Re-use of existing flues

1.36 Where it is proposed to bring a flue in an existing chimney back into use or to re-use a flue with a different type or rating of appliance, the flue and the chimney should be checked and, if necessary, altered to ensure that they satisfy the requirements for the proposed use. A way of checking before and/or after remedial work would be to test the flue using the procedures in Appendix E.

1.37 A way of refurbishing defective flues would be to line them using the materials and components described in Sections 2, 3, and 4 dependent upon the type of combustion appliance proposed. Before relining flues, they should be swept to remove deposits.

1.38 A flue may also need to be lined to reduce the flue area to suit the intended appliance. Oversize flues can be unsafe.

1.39 If a chimney has been relined in the past using a metal lining system and the appliance is being replaced, the metal liner should also be replaced unless the metal liner can be proven to be recently installed and can be seen to be in good condition.

Use of flexible metal flue liners for the relining of chimneys

1.40 A way of relining a chimney would be to use a flexible metal flue liner, appropriately designated in accordance with BS EN1856-2:2004 to suit the appliance, fuel and flue gas characteristics. Flexible flue liners should be used only to reline a chimney and should not be used as the primary liner of a new chimney. They can be used to connect gas back boilers to chimneys where the appliance is located in a fireplace recess.

Use of plastic fluepipe systems

1.41 A way of using plastic flue systems and liners would be to use a plastic flue, appropriately designated in accordance with BS EN 14471:2005 to suite the appliance, fuel and flue characteristics. Plastic fluepipe systems can be acceptable in some cases, for example with condensing boiler installations, where the fluepipes are supplied by or specified by the appliance manufacturer as being suitable for purpose.

Factory-made metal chimneys

1.42 Ways of meeting the requirements when proposing factory-made metal chimneys include:

a. using component systems appropriately designated in accordance with BS EN18561:2003 to suit the appliance and types of fuels to be burnt and installing them in accordance with the relevant recommendations of BS EN 15287-1:2007;

b. for gas and for oil appliances where flue temperatures will not normally exceed 250°C, using twin-walled component systems (and, for gas, single-walled component systems) appropriately designated in accordance with BS EN1856-1:2003 to suit the appliance and types of fuels to be burnt and installing gas appliances in accordance with BS 54401:2008;

c. using any other chimney system that is suitable for the intended purpose and installed in accordance with the relevant recommendations in BS EN 15287-1:2007 or BS 5440-1:2008, as appropriate to the type of appliance being installed.

1.43 Where a factory-made metal chimney passes through a wall, sleeves should be provided to prevent damage to the flue or building through thermal expansion. To facilitate the checking of gas-tightness, joints between chimney sections should not be concealed within ceiling joist spaces or within the thicknesses of walls without proper access being provided (see paragraph 1.47).

1.44 When providing a factory-made metal chimney, provision should be made to withdraw the appliance without the need to dismantle the chimney.

1.45 Factory-made metal chimneys should be kept a suitable distance away from combustible materials. Ways of meeting the requirement for chimneys designated to BS EN 1856-1:2003 comprise:

a. locating the chimney not less than distance ‘xx’ from combustible material, where ‘xx’ is defined in BS EN 1856-1:2003 as shown in Diagram 13;

b. where a chimney passes through a cupboard, storage space or roof space, providing a guard placed no closer to the outer wall of the chimney than the distance in a) above.

1.46 Where a factory-made metal chimney penetrates a fire compartment wall or floor, it must not breach the fire separation requirements of Part B. See Approved Document B for more guidance but the requirements may be met by:

a. using a factory-made metal chimney of the appropriate level of fire resistance installed in accordance with BS EN 1856-1:2003 Annex NA; or

b. casing the chimney in non-combustible
material giving at least half the fire resistance recommended for the fire compartment wall or floor.

Diagram 13 The separation of combustible material from a factory-made metal chimney designated to BS EN

Concealed flues

1.47 Where a flue is routed within a void, appropriate means of access at strategic locations should be provided to allow the following aspects to be visually checked and confirmed. This is necessary both when an appliance is first installed and subsequently when the appliance is serviced:

the flue is continuous throughout its length
all joints appear correctly assembled and are appropriately sealed
the flue is adequately supported throughout its length
any required gradient of fall back to the boiler (required to recover the condensate produced as part of the combustion process) and any other required drain points have been provided.

Means of access for flues needs to be sufficiently sized and positioned to allow a visual inspection to be undertaken of the flue, particularly at any joints in the flue. It is not intended that the means of access should be sized to allow full physical access to the flue system. Diagram 14 shows an acceptable approach for a flue in a ceiling void. Flues should not pass through another dwelling since access for inspection may not always be available to that dwelling and chimney system running through it. Flues may pass through communal areas including purpose-designed ducts where inspection access is provided.

Any ‘means of access’ should not impair any fire, thermal or acoustic requirements of the Building Regulations. Refer to the relevant guidance in Approved Documents B, L and E. Where necessary, inspection panels or hatches should be fitted with resilient seals and provide the similar standards of fire, thermal and acoustic isolation to the surrounding structure.

Access hatches should be at least 300mm x 300mm or larger where necessary to allow sufficient access to the void to look along the length of the flue. Digram 14 shows an acceptable approach to providing access to a horizontal flue located within a ceiling void.

Diagram 14 Example locations of access panels for concealed horizontal flues

Configuration of natural draught flues serving open-flued appliances

1.48 Flue systems should offer least resistance to the passage of flue gases by minimising changes in direction or horizontal length. A way of meeting the requirement would be to build flues so that they are straight and vertical except for the connections to combustion appliances with rear outlets where the horizontal section should not exceed 150mm. Where bends are essential, they should be angled at no more than 45° to the vertical.

1.49 Provisions should be made to enable flues to be swept and inspected. A way of making reasonable provision would be to limit the number of changes of direction between the combustion appliance outlet and the flue outlet to not more than four (each up to 45°), with not more than two of these being between an intended point of access for sweeping and either another point of access for sweeping or the flue outlet. (90° factory-made bends, elbows or Tee pieces in fluepipes may be treated as being equal to two 45° bends (see Diagram 15)).

Diagram 15 Bends in flues

Inspection and cleaning openings in flues

1.50 A flue should not have openings into more than one room or space except for the purposes of: a. inspection or cleaning; or b. fitting an explosion door, draught break, draught stabiliser or draught diverter.

1.51 Openings for inspection and cleaning should be formed using purpose factory-made components compatible with the flue system, having an access cover that has the same level of gas-tightness as the flue system and an equal level of thermal insulation. Openings for cleaning the flue should allow easy passage of the sweeping brush. Covers should also be noncombustible except where fitted to a combustible fluepipe (such as a plastic fluepipe). After the appliance has been installed, it should be possible to sweep the whole flue.

Flues discharging at low level near boundaries

1.52 Flues discharging at low level near boundaries should do so at positions where the building owner will always be able to ensure safe flue gas dispersal. A way of achieving this where owners of adjacent land could build up to the boundary would be to adopt the suggestions in Diagram 34 or 41, as relevant.

Dry lining around fireplace openings

1.53 Where a decorative treatment, such as a fireplace surround, masonry cladding or dry lining, is provided around a fireplace opening, any gaps that could allow flue gases to escape from the fireplace opening into the void behind the decorative treatment should be sealed to prevent such leakage. The sealing material should be capable of remaining in place despite any relative movement between the decorative treatment and the fireplace recess.

Condition of combustion installations at completion

1.54 Responsibility for achieving compliance with the requirements of Part J rests with the person carrying out the work. That ‘person’ may be, e.g., a specialist firm directly engaged by a private client or it may be a developer or main contractor who has carried out work subject to Part J or engaged a sub-contractor to carry it out. In order to document the steps taken to achieve compliance with the requirements, a report should be drawn up showing that materials and components appropriate to the intended application have been used and that flues have passed appropriate tests. A suggested checklist for such a report is given at Appendix A and guidance on testing is given at Appendix E. Other forms of report may be acceptable. Specialist firms should provide the report to the client, developer or main contractor, who may be asked for documentation by the Building Control Body.

1.55 Flues should be checked at completion to show that they are free from obstructions, satisfactorily gas-tight and constructed with materials and components of sizes which suit the intended application. Where the building work includes the installation of a combustion appliance, tests should cover fluepipes and [the gas-tightness of] joints between fluepipes and combustion appliance outlets. A spillage test to check for compliance with J2 should be carried out with the appliance under fire, as part of the process of commissioning to check for compliance with Part L, and (in relevant cases) as required by the Gas Safety (Installation and Use) Regulations.

1.56 Hearths should be constructed with materials and components of sizes to suit the intended application and should show the area where combustible materials should not intrude.

Notice plates for hearths and flues (Requirement J5)

1.57 Where a hearth, fireplace (including a flue box), flue or chimney is provided or extended (including cases where a flue is provided as part of the refurbishment work), information essential to the correct application and use of these facilities should be permanently posted in the building. A way of meeting this requirement would be to provide a notice plate as shown in Diagram 16 conveying the following information:

a. the location of the hearth, fireplace (or flue box) or the location of the beginning of the flue;

b. the category of the flue and generic types of appliances that can be safely accommodated;

c. the type and size of the flue (or its liner if it
has been relined) and the manufacturer’s name;

d. the installation date.

1.58 Notice plates should be robust, indelibly marked and securely fixed in an unobtrusive but obvious position within the building such as:

a. next to the electricity consumer unit; or

b. next to the chimney or hearth described; or

c. next to the water supply stop-cock.

1.59 For chimney products whose performance characteristics have been assessed in accordance with a European Standard (EN) and which are supplied or marked with a designation, the installer may optionally include this designation on the label as shown in Diagram 16.

Access to combustion appliances for maintenance

1.60 There should be a permanent means of safe access to appliances for maintenance. Roof space installations of gas-fired appliances should comply with the requirements of BS 6798:2009.

Diagram 16 Example notice plate for hearths and flues

Section 1: Provisions which apply generally to combustion installations