Smoke detectors are a crucial component of fire alarm systems, designed to detect the presence of smoke and trigger an alarm to alert occupants and emergency services. There are two main types of smoke detectors: ionization and photoelectric. Ionization smoke detectors are particularly effective at detecting fast-flaming fires, which produce small combustion particles. These detectors use a small amount of radioactive material to ionize the air within a sensing chamber. When smoke enters the chamber, it disrupts the ionization process, causing the detector to trigger an alarm.

Photoelectric smoke detectors, on the other hand, are more effective at detecting slow-smoldering fires, which produce larger smoke particles. These detectors use a light source and a light sensor. When smoke enters the chamber, it scatters the light beam, causing the sensor to detect the presence of smoke and trigger an alarm.

In larger buildings, it may be necessary to use a combination of ionization and photoelectric detectors to ensure comprehensive coverage. Regular testing and maintenance are also critical to ensure that smoke detectors remain functional and reliable.

Proper placement of smoke detectors is essential to ensure they function correctly and do not cause false alarms. Detectors should be installed on ceilings or high on walls, as smoke rises and accumulates in these areas first. They should be placed at least three feet away from air vents, windows, and doors to avoid interference from air currents. Additionally, smoke detectors should not be installed near kitchens or bathrooms where cooking smoke or steam might cause false alarms.

NFPA 72 Section 29.11.3 contains the following specific location requirements for smoke alarms and smoke detectors:

29.11.3.1 Peaked Ceilings. Smoke alarms or smoke detectors mounted on a peaked ceiling shall be located within 36 in. (910 mm) horizontally of the peak, but not closer than 4 in. (100 mm) vertically to the peak.

29.11.3.2 Sloped Ceilings. Smoke alarms or smoke detectors mounted on a sloped ceiling having a rise greater than 1 ft in 8 ft (1 m in 8 m) horizontally shall be located within 36 in. (910 mm) of the high side of the ceiling, but not closer than 4 in. (100 mm) from the adjoining wall surface.

29.11.3.3 Wall Mounting. Smoke alarms or smoke detectors mounted on walls shall be located not farther than 12 in. (300 mm) from the adjoining ceiling surface.

29.11.3.4 Specific Location Requirements. The installation of smoke alarms and smoke detectors shall comply with the following requirements:

(1) Smoke alarms and smoke detectors shall not be located where ambient conditions, including humidity and temperature, are outside the limits specified by the manufacturer's published instructions.

(2) Smoke alarms and smoke detectors shall not be located within unfinished attics or garages or in other spaces where temperatures can fall below 40ºF (4.4ºC) or exceed 100ºF (38ºC).

(3) Where the mounting surface could become considerably warmer or cooler than the room, such as a poorly insulated ceiling below an unfinished attic or an exterior wall, smoke alarms and smoke detectors shall be mounted on an inside wall.

(4) Smoke alarms and smoke detectors shall not be installed between 10 ft (3.0 m) and 20 ft (6.1 m) along a horizontal flow path from a stationary or fixed cooking appliance unless the devices comply with the following:

(a) Prior to January 1, 2025, smoke alarms and smoke detectors shall be equipped with an alarm silencing means, use photoelectric detection, or be listed for resistance to common nuisance sources from cooking in accordance with the 8th edition of UL 217, Smoke Alarms, the 7th edition of UL 268, Smoke Detectors for Fire Alarm Systems, or subsequent editions.

(b) Effective January 1, 2025, smoke alarms and smoke detectors shall be listed for resistance to common nuisance sources from cooking in accordance with the 8th edition of UL 217, the 7th edition of UL 268, or subsequent editions.

(5) Smoke alarms and smoke detectors shall not be installed within an area of exclusion determined by a 10 ft (3.0 m) radial distance along a horizontal flow path from a stationary or fixed cooking appliance. When the 10 ft (3.0 m) area of exclusion would prohibit the placement of a smoke alarm or smoke detector required by other sections of this Code, and when the kitchen or cooking area and adjacent spaces have no clear interior partitions or headers, smoke alarms or smoke detectors shall be permitted for installation at a radial distance between 6 ft (1.8 m) and 10 ft (3.0 m) from any stationary or fixed cooking appliance unless the devices comply with the following:

(a) Prior to January 1, 2025, the devices shall use photoelectric detection or be listed for resistance to common nuisance sources from cooking in accordance with the 8th edition of UL 217, the 7th edition of UL 268, or subsequent editions.

(b) Effective January 1, 2025, the devices shall be listed for resistance to common nuisance sources from cooking nuisance alarms in accordance with the 8th edition of UL 217, the 7th edition of UL 268, or subsequent editions.

(6) Smoke alarms and smoke detectors shall not be installed within a 36 in. (910 mm) horizontal path from a door to a bathroom containing a shower or tub unless listed for installation in close proximity to such locations.

(7) Smoke alarms and smoke detectors shall not be installed within a 36 in. (910 mm) horizontal path from the supply registers of a forced air heating or cooling system and shall be installed outside of the direct airflow from those registers.

(8) Smoke alarms and smoke detectors shall not be installed within a 36 in. (910 mm) horizontal path from the tip of the blade of a ceiling-suspended (paddle) fan unless the room configuration restricts meeting this requirement.

(9) Where stairs lead to other occupiable levels, a smoke alarm or smoke detector shall be located so that smoke rising in the stairway cannot be prevented from reaching the smoke alarm or smoke detector by an intervening door or obstruction.

(10) For stairways leading up from a basement, smoke alarms or smoke detectors shall be located on the basement ceiling near the entry to the stairs.

(11) For tray-shaped ceilings (coffered ceilings), smoke alarms and smoke detectors shall be installed on the highest portion of the ceiling or on the sloped portion of the ceiling within 12 in. (300 mm) vertically down from the highest point.

(12) Smoke alarms and detectors installed in rooms with joists or beams shall comply with the requirements of 17.7.4.2.4.

(13) Heat alarms and detectors installed in rooms with joists or beams shall comply with the requirements of 17.6.3.

By understanding the different types of smoke detectors and their proper placement, installers can significantly reduce the risk of false alarms and ensure the safety of building occupants.