Where to Place Smoke Detectors?

Where to place smoke detectors? *

Detector placement is critical to early warning functions. To provide effective early warning of a developing fire situation, smoke detectors should be installed in all areas of the protected premises. Total coverage as defined by NFPA 72 should include all rooms, halls, storage areas, basements, attics, lofts, and spaces above suspended ceilings including plenum areas utilized as part of the HVAC (Heating, Ventilating and Air Conditioning) system. In addition, this should include all closets, elevator shafts, enclosed stairways, dumbwaiter shafts, chutes and other subdivisions and accessible spaces.

Fire detection systems installed to meet local codes or ordinances may not be adequate for early warning of fire. Some codes or ordinances have minimum objectives such as capturing elevators or preventing circulation of smoke through the HVAC systems instead of early detection of fire.

A user should weigh the costs against the benefits of installing a complete fire detection system when any detection system is being installed. The location, quantity and zoning of detectors should be determined by what objectives are desired rather than the minimum requirements of any local codes or ordinances.

“Total coverage”, as defined in NFPA 72, is the definition of a complete fire detection system. In some of the specified areas of coverage, such as attics, closets, under open loading docks or platforms, a heat detector may be more appropriate than a smoke detector. Careful consideration should be given to the detector manufacturer’s instructions and the following recommendations in this guide.

In general, when only one detector is required in a room or space, the detector should be placed as close to the center of the ceiling as possible. Central location of the detector is best for sensing fires in any part of the room. If a center location is not possible, it may be placed no closer than 4 inches from the wall, or if listed for wall mounting, it may be mounted on the wall. Wall-mounted detectors should be located not less than 4 inches nor more than 12 inches from the ceiling to the top of the detector, and at least 4 inches (10 cm) from any corner wall junction.

When air supply and/or air return ducts are present in a room or space, the detector(s) should not be placed in the path of the air flow supply or return duct (NFPA 72, 1999 Edition).

Smoke tests are helpful in determining proper placement. Special attention should be given to smoke travel directions and velocity, since either can affect detector performance.

Placement of detectors near air conditioning or incoming air vents can also cause excessive accumulation of dust and dirt on the detectors. This dirt can cause detectors to malfunction and cause unwanted alarms. Detectors should not be located closer than 3 feet from an air supply diffuser or an air return vent.

Spot type detectors, in properly engineered systems, may also be placed in return air ducts, or in approved duct detector housings designed for this application. Although duct detectors are not a substitute for open area detectors, they can provide an effective method of initiating building control functions to prevent smoke from being transported from the fire area to other parts of a building.

Where not to place smoke detectors?

One of the major causes of unwanted alarms is improper placement of detectors. The best way to avoid unwanted alarms is not to install detectors in environments that can cause them to malfunction, or to install detectors specially designed for those environments. Examples follow:

• Excessively Dusty or Dirty Areas: In excessively dusty or dirty areas consider using the Filtrex™ smoke detector. This detector incorporates a microprocessor-controlled air intake fan and filter that allows the unit to be installed in areas where ordinary detectors cannot be used. Filtrex is an intelligent smoke detector that removes airborne particles before they reach the sensing chamber. It is ideal for textile mills, dusty manufacturing facilities, paper mills, and recycling centers.

• Outdoors: Avoid using detectors outdoors, in open storage sheds, or other open structures affected by dust, air currents, or excessive humidity and temperature extremes.

• Wet or Excessively Humid Areas: Avoid damp, wet or excessively humid areas, or next to bathrooms with showers.

• Elevator Lobbies: Do not place over ashtrays or where people will smoke while waiting for the elevator.

• Extreme Cold or Hot Environments: Avoid very cold or very hot environments, or unheated buildings or rooms where the temperature can fall below or exceed the operating temperature range of the detector. At temperatures above or below the operating range of the detector*, its internal components may not function properly. *Manufacturers’ specifications should list acceptable temperatures in these ranges.

• Areas with Combustion Particles: Avoid areas where particles of combustion are normally present, such as in kitchens or other areas with ovens and burners; in garages, where particles of combustion are present in vehicle exhausts. When a detector must be located in or adjacent to such an area, a heat detector may be appropriate.

• Manufacturing Areas: Avoid manufacturing areas, battery rooms, or other areas where substantial quantities of vapors, gases, or fumes may be present. Strong vapors can make detectors overly sensitive or less sensitive than normal. In very large concentrations, gases heavier than air, such as carbon dioxide, may make detectors more sensitive, while gases lighter than air, such as helium, may make them less sensitive. Aerosol particles may collect on detector chamber surfaces and cause nuisance alarms.

• Fluorescent Light Fixtures: Avoid placement near fluorescent light fixtures. Electrical noise generated by fluorescent light fixtures may cause unwanted alarms. Install detectors at least 1 foot (0.3 m) away from such light fixtures.

* The guidelines on this page are adapted from Standards published by the National Fire Protection Association, Quincy, Massachusetts, USA. These standards include NFPA 72, National Fire Alarm Code; NFPA 70, “National Electrical Code”, Article 760; and NFPA 90A, “Installation of Air Conditioning and Ventilating Systems”.