A smoke alarm, also known as a smoke detector, is a device that detects smoke and issues an audible sound and/or a visual signal to alert residents to a potential fire.
Facts and Figures
According to the Consumer Product Safety Commission:
- Almost two-thirds of reported deaths caused by home fires from 2003 to 2006 resulted from fires in homes that lacked working smoke alarms.
- Older homes are more likely to lack an adequate number of smoke alarms because they were built before requirements increased.
- In 23% of home fire deaths, smoke alarms were present but did not sound. Sixty percent of these failures were caused by the power supplies having been deliberately removed due to false alarms.
- Every year people lose their lives in residential fires. Most of these deaths are caused by smoke inhalation, rather than as a result of burns.
Smoke Alarm Types
Ionization and photoelectric are the two main designs of smoke detectors. Both types must pass the same tests to be certified to the voluntary standard for smoke alarms, but they perform differently in different types of fires. Detectors may be equipped with one or both types of sensors — known as dual-sensor smoke alarms — and possibly a heat detector, as well. These sensors are described as follows:
- Ionization smoke sensors are the most common and economical design, and are available at most hardware stores. They house a chamber sided by small metal plates that irradiate the air so that it conducts electricity. When smoke enters the chamber, the current flow becomes interrupted, which triggers an alarm to sound. These sensors will quickly detect flaming-type fires but may be slower to react to smoldering fires.
- Photoelectric smoke sensors use a light-sensitive photocell to detect smoke inside the detector. They shine a beam of light that will be reflected by smoke toward the photocell, triggering the alarm. These sensor types work best on smoldering fires but react more slowly to flaming fires. They often must be hard-wired into the house’s electrical system, so some models can be installed only in particular locations.
While heat detectors are not technically classified as smoke detectors, they are useful in certain situations where smoke alarms are likely to sound false alarms. Dirty, dusty industrial environments, as well as the area surrounding cooking appliances, are a few places where false alarms are more likely and where heat detectors may be more useful.
Individual authorities having jurisdiction (AHJs) may have their own requirements for smoke-alarm placement, so inspectors and homeowners can check with their local building codes if they need specific instructions. The following guidelines, however, can be helpful.
Smoke alarms should be installed in the following locations:
- on the ceiling or wall outside of each separate sleeping area in the vicinity of bedrooms;
- in each bedroom, as most fires occur during sleeping hours;
- in the basement, preferably on the ceiling near the basement stairs;
- in the garage, due to all the combustible materials commonly stored there;
- on the ceiling or on the wall with the top of the detector between 6 to 12 inches from the ceiling; and/or
- in each story within a building, including basements and cellars, but not crawlspaces or uninhabited attics.
Smoke alarms should not be installed in the following locations:
- near heating or air-conditioning supply and return vents;
- near a kitchen appliance;
- near windows, ceiling fans or bathrooms equipped with a shower or tub;
- where ambient conditions, including humidity and temperature, are outside the limits specified by the manufacturer’s instructions;
- within unfinished attics or garages, or in other spaces where temperatures can rise or fall beyond the limits set by the manufacturer;
- where the mounting surface could become considerably warmer or cooler than the rest of the room, such as an inadequately insulated ceiling below an unfinished attic; or
- in dead-air spots, such as the top of a peaked roof or a ceiling-to-wall corner.
Power and Interconnection
Power for the smoke alarms may be hard-wired directly into the building’s electrical system, or it may come from just a battery. Hard-wired smoke detectors are more reliable because the power source cannot be removed or drained, although they will not function in a power outage. Battery-operated units often fail because the battery can be easily removed, dislodged or drained, although these units can be installed almost anywhere. Older buildings might be restricted to battery-powered designs, while newer homes generally offer more options for power sources. If possible, homeowners should install smoke alarms that are hard-wired with a battery backup, especially during a renovation or remodeling project.
Smoke alarms may also be interconnected so that if one becomes triggered, they all sound in unison. Interconnected smoke alarms are typically connected with a wire, but new technology allows them to be interconnected wirelessly. The National Fire Protection Agency requires that smoke alarms be AFCI-protected.
In summary, smoke alarms are invaluable, life-saving appliances when they are installed properly and adequately maintained.