Smoke Detector & Heat Detector Mounting Heights

Smoke Detector & Heat Detector Mounting Heights BS5839

There are essentially two types of initiating devices used in a typical fire alarm system: automatic and manual.

Typical automatic devices include spot-type smoke detectors, rate-of-rise heat sensors, fixed-temperature sensors, photoelectric smoke detectors, sprinkler heads and duct-type smoke detectors.

Manual activation is commonly accomplished using a manual fire pull, and automatic detection happens without human intervention. NFPA 72, also called the National Fire Alarm Code (NFAC), is the primary document used when an engineer designs, and an EC or fire alarm technician installs, a fire alarm system.

BS5839 makes reference to recommendations of maximum mounting heights for different types of detectors.  This is to ensure that the detector is still effective when mounted at high levels.  The recommendations of BS5839 split the mounting heights up into two categories.  These categories are standard BS5839 systems and systems with a 5 minute fire service response time.

Below is a series of recommended mounting heights for fire alarm system equipment including different types of smoke detector (including Optical Beam Detectors) and Heat Detectors.

The table below also shows heights which detectors are permitted to be mounted at if 10% or less of the ceiling is the height shown.

Mounting Heights 

Detector Type	Maximum Ceiling Height (m) Cat L/P	5 Minute Response (m) Cat P
Rate of Rise Heat Detector	9 Meters (10.5 meters for 10% of Ceiling)	13.5 Meters (15 meters for 10% of Ceiling)
Fixed Temperature Heat Detectors	7.5 Meters (10.5  meters  for 10% of Ceiling)	12 Meters (15 meters for 10% of Ceiling)
Smoke Detectors	10.5 Meters (12.5  meters  for 10% of Ceiling)	15 Meters (18 meters for 10% of Ceiling)
CO Point Detectors	10.5 Meters (12.5  meters  for 10% of Ceiling)	15 Meters (18 meters for 10% of Ceiling)
Optical Beam Detectors	25 Meters (25  meters  for 10% of Ceiling)	40 Meters (40 meters for 10% of Ceiling)

Automatic smoke detector placement is another important aspect of a fire alarm installation. Unless every smoke detector is placed properly, false alarm, or even no alarms, can occur. Either way, the client suffers, and the low-voltage contractor looks bad in the process.

The most common type of smoke detector used in residential and commercial settings is the spot-type detector. According to Section 5.7.3.2, NFPA 72, 2007 Edition, spot-type smoke detectors can be mounted either on the side wall or ceiling. When using the side wall, mount the device no closer than 4 inches from the ceiling.

The maximum distance is 12 inches. The distance from detector to side wall when mounting the same smoke detector on a ceiling is the same 4 inches. Ceiling-mounted units also must be placed no closer than 4 inches from a side wall.

Spacing between smoke detectors is likewise important. On a smooth ceiling, the common distance between each smoke detector is 30 feet. According to Section 5.7.3.2.3.5, all points on a smooth ceiling should have a smoke detector within 0.7 times the selected spacing or 21 feet.

Not all ceilings are smooth; thus, the spacing between smoke detectors will vary. To ensure optimum performance and compliance, refer to Chapter 5 of NFPA 72, 2007 Edition, for a specific situation. Read also Codes & Standards, page 140.

Space and placement both matter when planning your fire alarm package. Thorough preparation often leads to a successful installation.

As per IS: 2189:2008 under clause 6.3.2 Siting and spacing of detectors (common to all types of smoke and heat detectors):

1. Under flat ceilings, the horizontal distance between any point in a protected area and the detector nearest to that point shall not exceed 7.5 m in case of smoke detector, and 5.3 m in case of heat detector.
2. In case of a sloping roof or pitched ceiling (where the distance between the top of apex and bottom of the roof exceeds 600 mm), spacing of detectors at or in the vicinity of apex may be spaced between 7.5 m and 8.5 m for smoke detectors.
3. Detectors shall not be mounted within 500 mm of any walls, partitions or obstructions to flow of smoke or hot gases, such as structural beams and duct work, where the obstructions are greater than 250 mm in depth.
4. Where structural beams or duct work for light fittings or any other ceiling attachments, not greater than 250 mm depth, create obstacles to the flow of smoke, detectors shall not be mounted closer to the obstruction than twice the depth of the obstruction.
5. Where partitions or storage racks that reach within 300 mm of the ceiling, they shall be construed as walls that extend to the ceiling for the purpose of siting the detectors.
6. Similarly, ceiling obstructions, such as structural beams, deeper than 10 percent of the overall ceiling height shall be construed as walls for the purpose of siting the detectors, that is, each bay formed by such beams shall be treated as separate enclosure for provision/spacing of detectors.
7. Detectors shall not be mounted within 1m of any air inlet (supply air inlets of H VAC system) or a forced ventilation system.
8. Detector siting shall be such that a clear space of 500 mm is maintained below each detector.
9. Where detectors are constrained to be fixed to the wall, they shall be sited in such a way that the top of the detection element is between 150 mm and 300 mm below the ceiling and the bottom of the detection element is above the level of door opening. Additional detector shall be placed on the ceiling at a position 1.5 m from any opening which might act like a flue.
10. A detector shall be placed on the protected side of the premises on the ceiling 1.5 m from any door, window or any opening in the wall partitions separating the protected premises from the other premises.
11. All stairwells, lift shafts, other utility shafts, etc, shall have a detector at the top. Lift machine rooms shall be provided with a detector.
12. All unenclosed staircase shall have one detector at each main landing within the staircase.
13. The detector shall also be provided in cable tunnels, ducts, false floors, AC and AHU room, long AC return ducts and distribution boards.
14. No detector shall be subjected to any interior decoration treatment, that is, painting, alteration of exterior cover, etc, to conform with the environment.
15. Every enclosure (that is, room or cabin) shall have a detector at ceiling level and also under false ceiling, if provided.
16. Where there is more than one such enclosure per floor, a response indicator shall be installed at the entrance to such enclosures to indicate where the detector has actuated. This arrangement shall also be followed in case of all concealed detectors in false floors, plenums, shafts, tunnels, etc.
17. Voids as in false ceiling/flooring more than 800 mm shall be protected with detectors with spacing like in normal installation. However, voids as in false ceiling/flooring less than 800 mm height need not necessarily have independent coverage unless the void is such that the spread of fire products between the rooms or compartments take place through it. Bathroom, lavatories, WC, etc, however, need not be protected.
18. For irregular shaped areas, the spacing between the detectors may be greater than the determined spacing provided the maximum spacing from the detector to the farthest point of a side wall or comer within its zone of protection is not greater than 0.7 times the determined spacing.

As per IS: 2189:2008 under clause 6.3.3 Compensation to the Spacing of Detectors

1. Height consideration

Spacing of 7.5 m for smoke detectors is valid up to a height of 7 m only and that of 5.3 m for heat detectors is valid only up to a height of 5 m. beyond these heights, spacing between the detectors shall be adjusted as follows:

1. Smoke detectors for heights between 7 m and 10 m - 5 m spacing

Beyond 10m height - Only beam detectors or aspirating type detection systems

2. Heat detectors for heights between 5 m and 7 m - 3.5 m spacing

Beyond7m height- Not allowed to install heat detectors

2. High air movement consideration
1. Spacing between detectors shall be suitably reduced in areas where high air movement or where high air changes prevail.
2. Detectors shall not be located in the vicinity of supply air diffusers. Minimum distance between the detector and the air inlets/diffusers shall be at least 1.5 m.
3. Detectors shall be so mounted as to favour the air flow towards return air openings.
4. The above provisions shall not disturb the normal population (count) of detectors, which is provided assuming that air-handling systems are off.
5. After designing the detector spacing, it shall be cross-checked to ensure that there is at least one smoke detector for every 100 m² or one heat detector for every 50 m’ of the compartment area.