22.1 Commentary

Heat and Smoke Detectors:

Depend on convection to transport hot gases and smoke.
Should be sited at the highest parts of enclosed areas where hot gases and smoke accumulate.
Ceiling height affects detection; higher ceilings dilute smoke, needing more sensitive detectors.
Optical beams and aspirating smoke detectors are less affected by ceiling height.

Stratification:

Occurs when the plume of smoke and hot gases reaches ambient temperature before the ceiling.
Difficult to predict, depending on fire's heat output and temperature profile.
Multi-level detection can mitigate stratification effects.

Flame Detectors:

Detects radiation, does not rely on convection, unaffected by stratification.
Sensitivity decreases with distance; require clear line-of-sight.
Can be mounted at various heights for optimal sensitivity and coverage.

Localized Protection:

Additional detectors can provide "spot" protection for specific items (e.g., plant or cabling).
Detectors should be close to the potential fire source.

Obstructions:

Detectors should not be too close to obstructions that block hot gases or flame radiation.
Avoid "dead space" at wall-ceiling junctions.
Aspirating systems can have flush-mounted sampling points; point detectors should not be flush-mounted.

Air Movement:

Air conditioning and ventilation systems can affect detector response.
Specialist advice might be needed for high air change rates.

Ceiling Voids and Ducts:

Still air assumed in ceiling voids; airflow affects detector spacing and siting.
Smoke detectors in ventilation ducts prevent smoke spread; not reliable for primary fire detection.
High-sensitivity smoke detection systems can detect diluted smoke in return air.

Testing and Maintenance:

Consider feasibility of testing and maintenance for remotely situated or hard-to-access equipment during the design stage.

22.2 Recommendations for provision of automatic fire detectors

The following recommendations are applicable.

General Placement:

Fire detectors should be installed as per section 8.2.

Stairways:

Detectors at the top and on each main landing.

Flue-like Structures and Shafts:

Detectors at the top and within 1.5 m of ceiling penetrations on each level (except L4, L5, and P2 systems).

NOTE 1 For any of these flue-like structures a fire detector within 1.5 m is needed only in the areas protected by the category of system in question.

Horizontal Voids (800 mm or more):

Protect voids unless:
Fire or smoke can spread extensively before detection.
Fire risk assessment warrants protection.
Voids less than 800 mm protected only if:
Extensive spread of fire or smoke possible.
Fire risk assessment justifies it.

NOTE 2 In Category P systems, it is common practice to protect all floor voids in electronic data processing installations, regardless of their depth, if the voids contain cables. Usually, this is on the basis of the probability of ignition and the amount of combustible material within the void, in conjunction with the consequences of a fire within the void in respect of business continuity. Guidance on protection of electronic data processing installations is given in BS 6266.

NOTE 3 In a Category L2 or L3 system, detectors in rooms opening onto escape routes are provided only to give a warning of fire before it affects the escape routes. Detection in voids above or below the rooms might not be necessary for this purpose (e.g. if fire-resisting construction separates a void from the adjacent escape route).

NOTE 4 If the fire risk within a void of 800 mm or more is considered to be low, consideration might be given to omission of fire detection from the void, subject to the agreement of the interested parties (see Clause 6), but this ought to be recorded as a variation on the relevant system certificate. This might arise, for example, if the probability of ignition and development of fire in the void were very low, or if the void were limited in extent so that spread of fire beyond the room of origin, via the void, were unlikely.

Electronic Data Processing Installations:

Protect all floor voids containing cables, regardless of depth.

Category L2 and L3 Systems:

Detects rooms opening onto escape routes; void detection may not be necessary if fire-resisting construction is present.

Low Fire Risk Voids:

Detection can be omitted with agreement and documented as a variation.

Lantern-lights (800 mm or more or used for ventilation): (see 3.31)

Install a detector in the lantern-light.

22.3 Recommendations for provision of automatic fire detectors

The following recommendations are applicable.

Flat Ceilings:

a) Max horizontal distance from any point to nearest detector:

7.5 m for smoke detectors.
5.3 m for heat detectors.

b) Corridors ≤ 2 m wide:

Smoke detectors at 15 m intervals.
Heat detectors at 10.6 m intervals.

c) Follow BS 7273-4 if conflicts arise (e.g., L4 and L5 systems).

NOTE 1 In corridors of width not exceeding 2 m, only points close to the centre line of the corridor need be considered, and accordingly the above recommendations will be satisfied if smoke or heat detectors are sited at intervals of 15 m and 10.6 m respectively, with the maximum dimension from the end wall being 7.5 m and 5.3 m respectively.

NOTE 2 Where it is the design intention regularly to disable sensors in a multi-sensor fire detection system, such that the sensors rely solely or substantially on a heat detector, the location of sensors is governed by 22.3a)2).

Pitched Ceilings:

a) Increase max distance by 1% per degree of slope, up to 25%, for detectors at/near apex.

b) Treat as flat if apex height difference is:

< 600 mm for smoke detectors.
< 150 mm for heat detectors.

c) Site detectors at/near each apex.

NOTE 3 This might arise only in the case of Category L4 and L5 systems in which there are electronically held-open fire doors; in certain circumstances, the maximum spacing between smoke detectors in corridors recommended in BS 7273-4 is less than the maximum specified in this part of BS 5839.

Ceiling Mounting:

a) Sensitive elements:

25-600 mm below ceilings for smoke detectors.
25-150 mm for heat detectors.

b) Category L3 rooms:

Wall-mounted detectors 150-300 mm below ceiling, above door opening level.

Voids:

a) Unventilated voids ≤ 1.5 m deep:

● Detectors within top 10% or top 125 mm.

b) Voids > 1.5 m deep are treated as rooms.

NOTE 4 Detectors may be regarded as “near the apex” referred to in 22.3c) and 22.3d) if the vertical distance from the apex to the detector is within the figures given in 22.3d)1) and 22.3d)2) for the appropriate type of detector.

Proximity and Obstructions:

Detectors ≥ 500 mm from walls/partitions/obstructions > 250 mm deep.
Avoid mounting detectors closer to obstructions than twice their depth.

Partitions and Racks:

Partitions/racks within 300 mm of ceiling treated as walls.

NOTE 5 Particular care is necessary in rooms with a high ceiling height (for example height exceeding 4 m).

NOTE 6 Where an automatic sprinkler installation initiates a fire alarm signal, via the fire detection and fire alarm system, on flow of water from a single sprinkler head, a sprinkler head within a room may be regarded as an automatic fire detector for the purpose of this recommendation.

NOTE 7 Wall mounting may also be adopted in those rooms or areas in which the protection afforded by a Category L2 system reflects only the recommendations for a Category L3 system, in that the purpose is protection of the adjacent escape route (i.e. other than in areas in which detector siting needs to reflect the objective of protecting the occupant of the room of fire origin, as is the case in bedrooms intended for disabled people, or giving warning in areas in which the likelihood of fire is high).

Ceiling Obstructions:

Obstructions deeper than 10% of ceiling height are treated as walls.

Honeycomb Ceilings:

Detectors spaced per Table 1 and Figure 14 for small cells.
For elongated cells, spacing (M) as per Table 2 and Figure 15.

NOTE 8 If an enclosed area has no horizontal dimension greater than 1 m, it is impossible to comply with this recommendation; this need not be regarded as a variation if the detector is sited as close as possible to the centre of the space.

Perforated False Ceilings:

Detectors above perforated ceiling if:
Perforations > 40% of surface.
Each perforation ≥ 10 mm.
Ceiling thickness ≤ 3 times perforation size.
Otherwise, detectors below the false ceiling.

NOTE 9 In some circumstances, compliance with this recommendation might result in non-compliance with 22.3 h). In such circumstances, non-compliance with 22.3h) [or 22.3i)] need not be regarded as a variation.

Ventilation Systems:

Detectors ≥ 1 m from air supply points.
Imperforate radius of 600 mm around each detector for forced air systems.

NOTE 10 Where a rack contains high-value or high-risk materials or where the height of the rack exceeds 8 m, the use of in-rack detection might be considered. For more information, see the FIA Code of Practice for Design, Installation, Commissioning and Maintenance of Aspirating Smoke Detector (ASD) Systems [7].

Clear Space:

Maintain 500 mm clear space below each detector.

NOTE 11 Within horizontal voids, beams or obstructions that are deeper than 10% of the overall depth of the void, regardless of whether the void is above the ceiling or below the floor, ought to be treated as walls that subdivide the void.

NOTE 12 For a cellular structure or closely spaced beams in a void, see 22.3l).

Figure 7 — Smoke detectors in pitched roofs [see 22.3d)]

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Figure 8 — Detectors in top 10% of a void [see 22.3g)]

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Figure 9 — Proximity of detectors to ceiling fittings [see 22.3i)]

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Figure 10 — Partitions or storage on racks

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Figure 11 — Ceilings

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Figure 11 — Ceilings (Continued)

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Figure 12 — Detector on perforated ceiling [see 22.3o)]

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Figure 13 — Clear space around a detector[see 22.3p)

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Figure 14 — Spacing and siting of detectors on honeycomb and similar ceilings

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Table 1 — Spacing and siting of detectors on honeycomb and similar ceilings

Figure 15 — Spacing and siting of detectors on ceilings with closely spaced structural beams or joists

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Table 2 — Spacing and siting of detectors on ceilings with closely spaced structural beams or joist

22.4 Recommendations for siting of carbon monoxide fire detectors

Carbon monoxide fire detectors should be sited in accordance with all recommendations of 22.3 for smoke detectors.

22.5 Recommendations for siting of optical beam smoke detectors

The following recommendations are applicable.

1) Spacing and Distance:

Horizontal distance from any point to the beam: max 7.5 m.
For pitched ceilings, increase distance by 1% per degree of slope, up to 25%.

NOTE 1 Where there is no discernible fire load above such a layer of detectors it may be justifiable to forego the provision of detection at or near the ceiling.

2) Installation Exceptions:

More than 600 mm below ceiling in high spaces or atriums: width of protected area is 12.5% of beam height above the fire.
At angles from the ceiling for stratified smoke: multiple beams improve detection.
Near the apex of a pitched roof: vertical distance from apex ≤ H/10 or 2.5 m.
When physical features prevent standard installation: minimize distance from ceiling and apply compensatory measures.

NOTE 2 The spacing recommendations for angled optical beam detectors within the three-dimensional space below a ceiling is beyond the scope of this code of practice. However, while it is possible that a single angled optical beam detector will eventually detect a spreading layer of stratified smoke, the installation of more beams, strategically positioned through the three-dimensional space, will provide a substantially better chance of detecting stratifying smoke and reduce detection times. In general terms, for every one optical beam detector at the ceiling a further two or three detectors angled through the space might be appropriate.

3) Obstructions and Clearance:

Avoid positioning beams within 500 mm of walls, partitions, or obstructions.
Height above floor: ≥ 2.7 m to avoid obstruction by people or equipment.
Mount on solid construction to prevent alignment issues.

4) Path Length and Coverage:

Follow manufacturer’s recommended path length.
Coverage area should not exceed the limits of a single detection zone.

NOTE 3 Those parts of the optical beam within 500 mm of the beam’s transmitter and receiver or reflector(s) need not be considered for the purpose of this recommendation.

NOTE 4 It is also necessary to comply with 22.5d).

NOTE 5 Where the optical beam detector uses a reflector, the path length is the distance from the transmitter to the reflector and back to the receiver.

Figure 16 — Siting optical beam detectors

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22.6 Recommendations for siting of line heat detectors

The following recommendations are applicable.

Spacing and Distance:

No point in the protected space should be more than 5.3 m horizontally from the nearest detector.
For pitched ceilings, increase distance by 1% per degree of slope, up to 25%.

Obstructions and Clearance:

Avoid positioning detectors within 500 mm of walls, partitions, or obstructions.
Sections of detectors within 500 mm of control equipment are not considered for detection.

Targeted Protection:

For protecting specific items (e.g., plant or cabling), mount detectors as close as possible to the risk area, either above the risk or in thermal contact with it.

Apply General Recommendations:

Follow specific recommendations (22.3d, 22.3e, 22.3g, 22.3j, 22.3k, 22.3m).

NOTE Those parts of a line heat detector within 500 mm of the control equipment need not be considered for the purpose of this recommendation.

22.7 Recommendations for siting of aspirating smoke detection systems

General Area Protection:

Each sampling point is treated as a point smoke detector if it has equivalent sensitivity.
Follow standard siting guidelines (22.3), except for 22.3e); sampling points can be flush with the ceiling.

Very Early Warning:

For supplementary detection (e.g., monitoring air conditioning return air or specific equipment), follow the manufacturer's guidance.

High Ceilings:

For spaces with ceilings over 25 m where smoke may stratify, use multiple levels of sampling with drop-pipe arrangements at the walls and within the main space where possible.

NOTE For more information see the FIA Code of Practice for Design, Installation, Commissioning and Maintenance of Aspirating Smoke Detector (ASD) Systems [7].

22.8 Recommendations for siting of flame detectors.

The following recommendations are applicable.

Spacing:

Follow manufacturer’s maximum spacing limits.

General Area Protection:

Ensure a clear line of sight between all points in the protected area and one or more detectors.

Specific Hazard Protection:

Ensure a clear line of sight between the specific hazard(s) and one or more detectors.

22.9 Recommendations for limits of ceiling height

The following recommendations are applicable.

Ceiling Height Limits:

Detectors should not be mounted on ceilings higher than limits in column 1 of Table 3.
Small sections (≤ 10% of ceiling area) exceeding these limits are acceptable if within column 2 limits.

Special Structures:

For large, open areas, use Table 3 limits.
For vertical or flue-like structures (e.g., lift shafts, stairwells), use the limits as guidance.
Very high shafts (> 25 m) are better protected by multiple levels of detection and/or higher sensitivity at the ceiling.

Adapting Recommendations:

For vertical structures, the Class and number of ASD sampling holes can be adapted.
Example: 15 Class C sample points on a 200 m² ceiling can be replaced by 3 points with 5 times higher sensitivity.

NOTE 1 A very high shaft (e.g. exceeding 25 m) would be better protected by detection at several levels and/or greater sensitivity at the ceiling.

NOTE 2 The recommendations in Table 3 for the Class and minimum number of ASD sampling holes may be adapted for vertical or flue-like structures. For example, the detection provided by 15 Class C sample points on a ceiling covering just 200 m² can be achieved using fewer sample points at a higher sensitivity, e.g. just 3 sample points which are 5 times more sensitive than the Class C.

Table 3 — Limits of ceiling height

22.10 Smoke detectors in ventilation duct

Where the specification requires smoke detectors within air extraction ducts, the following recommendations are applicable.

Installation Location:

Install in straight stretches of ductwork, at least three times the duct width downstream from bends, corners, or junctions.

Suitability:

Ensure the smoke detector is suitable for ductwork applications based on manufacturer’s guidance.

Probe Arrangement:

Arrange the sampling inlet probe and its holes to cover as much of the duct as possible, following manufacturer’s instructions.

22 Spacing and siting of automatic fire detectors