We have all had experience with smoke detectors. Specifying the right smoke detector for the application will improve the reliability of fire alarm systems tremendously. Of course, the detector must be installed correctly to prevent problems.
Basically, there are two types of smoke detectors: ionization and photoelectric. Knowing how they operate and what outside influences affect their operation will greatly improve reliability. Ionization smoke detectors, which have been around the longest, respond best to small, hot smoke particles, typically less than 1 micron in size. If detecting flaming fires is the goal, ionization smoke detectors are probably the best choice.
However, a number of environmental factors affect the operation of ionization smoke detectors that both designers and installers need to consider. A designer must be aware of the protected area’s environment and the type of materials used there to be able to specify the correct type of smoke detector. For example, ionization smoke detectors are affected by gusts of wind or excessive air velocity. So that would not be a good choice for a computer room. These detectors also are highly susceptible to airborne fumes, such as paint spray, car exhaust, cooking fumes, hair sprays, etc. You would never want to use an ionization smoke detector in an elevator lobby in a parking garage. That is asking for false alarms.
Other misapplications include installing ionization smoke detectors in corridors near electric doors to the outside, near stairwell doors or in elevator hoistways due to wind gusts. Ionization smoke detectors also should not be installed in office breakrooms where there is a microwave oven. The fumes from food cooking would likely cause a nuisance alarm.
Photoelectric smoke detectors respond best to large, cool smoke particles, such as those produced by smoldering fires. Since these operate on a light-reflection or light-refraction principle, these detectors are best used when light-colored smoke will be present initially. Remember, smoke detectors are primarily used in occupied buildings to allow people more time to escape from a fire. Photoelectric smoke detectors do not respond well to dark smoke (since there is no light reflection) or where there is an excess of airborne dust or lint. Also, remember that if bugs can get into the detection chamber, it is possible the light beam could reflect off the bugs onto the receiving sensor, causing nuisance alarms.
Another common misapplication is installing smoke detectors (either type) too close to air vents. First, the movement of air could prevent smoke from reaching the detector, slowing response. If the detector is installed directly in the airflow, airborne dust and dirt from the air diffuser could cause nuisance alarms.
NFPA 72 recommends installing smoke detectors a minimum of 3 feet from any air supply or return air grill. They may need to be installed farther away when air velocity is higher than normal. Near the return grills, smoke detectors may not respond as quickly as they should because of air turbulence near the grill, which is caused by air from the room being forced to enter a relatively small opening.
In general, the same applies to duct smoke detectors. NFPA 72 has an
annex note that states that photoelectric smoke detectors are probably the best choice. This is due not only to the moving air in the ductwork, but also to the fact that as the air moves away from the fire, the particles will cool and collide, growing larger, making it more likely to be sensed by the photoelectric detector. Return-duct smoke detectors are not usually installed close to where the fire starts, so smoke must travel farther to reach the detector. On the supply side of the air handler, where the primary purpose is to detect a fire in the fan or filter, either type would be effective since the detector is located closer to the source of the fire. Duct detectors should not be installed on the outside of air ducts located on a roof unless installed in a manufacturer’s approved enclosure that can keep the detector within its listed temperature and humidity range when the air handler unit is running.
There is much attention paid in the codes recently to the type of smoke alarms to be installed in homes. From a fire detection point of view, it is best to have combination ionization and photoelectric alarms because they can detect any type of fire. Most smoke alarms installed in homes today are the ionization type because they are the least expensive. Unfortunately, many are installed too close to kitchens and will cause nuisance alarms due to the cooking fumes. The recommendation is to either install the detector farther from the kitchen, or if not possible, install photoelectric smoke detectors, since they do not respond to most cooking fumes.
Remember, smoke detectors serve a very important purpose as part of the fire protection plan. So be sure to use the right type, and install it correctly for a more reliable system.
HAMMERBERG is currently the president/executive director of the Automatic Fire Alarm Association Inc. headquartered in Jasper, Ga. He serves on a number of NFPA committees, including the NFPA 72 Technical Correlating Committee and the Protected Premises Technical Committee. He can be reached at TomHammerberg@afaa.org.