On average, there are 2,620 deaths from residential home fires each year, and about one in every 320 households will report a fire within a five year period, according to the National Fire Protection Association. About 60 percent of those deaths are in homes with malfunctioning smoke/fire alarms or no alarms at all, so the lesson is clear: Understanding the mechanics of smoke alarms, and using and maintaining them correctly, can prevent an enormous family tragedy from occurring.
Types of Alarms
Although they are often lumped together as "smoke alarms," there are actually three different types of smoke and fire alarms used in homes. Some actually detect heat and actual fire, some detect smoke, and some do both jobs.
Heat detectors represent fairly old technology, and the first residential fire alarms were of this type. They feature a detecting component inside the unit that activates an alarm when it reaches a predetermined temperature. Heat detectors are sufficient for places where speed of detection isn't critical, or in small, confined spaces.
Heat detectors have a lower false alarm rate, but they are somewhat slower at detecting fires. Studies have shown that in many household fires, smoke kills victims long before the fire temperature is high enough to set off heat alarms.
Smoke alarms will detect most fires more quickly than heat detectors. They use a different technology entirely to sense a fire, and there are three types of smoke alarms currently sold:
- Ionization smoke alarms contain bits of radioactive material that emit ions that create a faint electrical current between electrodes. When smoke of any amount enters the internal chamber, the current flow is interrupted and the alarm sounds. This type of alarm works best for very fast, fierce fires.
- Photoelectric smoke alarms operate using a light source and a photoelectric sensor. When smoke enters the optical chamber and interrupts the path of the light, the light is scattered about by the smoke particles and causes a sensor to activate the alarm. This type of alarm works best for slow, smoldering fires.
- Combination smoke alarms use both ionization and photoelectric technologies. The NFPA recommends using smoke alarms that combine both technologies for the best protection.
Smoke and fire alarms can be powered by household circuit current (hardwired), or they can be powered by batteries. These hardwired smoke alarms usually have an internal battery backup system that allows them to keep operating should the home's electrical service be interrupted.
Although the battery-operated smoke alarms, which usually use 9-volt batteries, are very popular due to their low costs and ease of installation, they only provide good protection if the batteries are checked and replaced on a regular basis. Hundreds of deaths have occurred in homes equipped with smoke detectors that had expired batteries. For this reason, building codes in many areas now require hard-wired smoke alarms that are interconnected, so that if one sensor detects smoke, all the detectors in the chain will sound loud alarms. These alarms generally work on 120-volt household current with a long-life/10-year internal battery for backup.
The proper positioning of smoke alarms may be dictated by your local building code, but there are also standard recommendations from the National Fire Protection Association. These recommendations are the basis for many local code requirements:
- Install smoke alarms inside each bedroom, outside each sleeping area and on every level of the home, including the basement.
- On levels without bedrooms, install alarms in the living room (or den or family room) or near the stairway to the upper level, or in both locations.
- Smoke alarms installed in the basement should be installed on the ceiling at the bottom of the stairs leading to the next level.
- Smoke alarms should be installed at least 10 feet (3 meters) from a cooking appliance to minimize false alarms when cooking.
- Mount smoke alarms high on walls or ceilings (remember, smoke rises). Wall-mounted alarms should be installed not more than 12 inches away from the ceiling (to the top of the alarm).
- If you have ceilings that are pitched, install the alarm within 3 feet of the peak but not within the apex of the peak (four inches down from the peak).
- Don't install smoke alarms near windows, doors, or ducts where drafts might interfere with their operation.
- Never paint smoke alarms. Paint, stickers, or other decorations could keep the alarms from working.
- For the best protection, interconnect all smoke alarms. When one smoke alarm sounds they all sound. Interconnection can be done using hard-wiring or wireless technology.
- When interconnected smoke alarms are installed, it is important that all of the alarms are from the same manufacturer. If the alarms are not compatible, they may not sound.
Many experts point out that smoke alarms should not be placed on the ceiling, but instead high up on the walls. The logic for this is that some types of fire will fill the ceiling areas with chemical gases that push the smoke downward slightly. By positioning the detectors at roughly 12 inches below the ceiling, you can ensure that it will detect smoke at the earliest opportunity.
It is important that every floor of your home have at least one smoke detector. Garages are a very common source of fires, so make sure yours is equipped with a smoke detector. Don't forget basements, sunrooms, and swimming pool or spa areas of the home and be sure to include furnace rooms, laundry rooms, and other utility spaces in your alarm plan.
Hardwired, interconnected smoke detectors are the best option and are now required in new construction. Even where building codes allow older homes to rely on battery-operated detectors, it is a good idea to have a hardwired system installed. If you must rely on battery-operated detectors, make sure to replace the batteries twice a year. The habit of changing the batteries at each change from daylight saving time to standard time is good practice to get into.
There are many combination detectors sold that combine smoke detection with carbon monoxide detection, and these pose some complications for positioning in the home. While smoke from fires tends to rise to near ceiling level, carbon monoxide mixes with air and is often found in greatest concentrations near the floor. Most experts recommend positioning at least one CO detector on each floor of the home, near knee level or at the level of a reclined sleeper.
For these reasons, it is best to keep smoke detection and CO detection systems separate and not rely on combination devices. If you do have combination detectors, mount them at the recommended smoke detection height, then install backup CO-only detectors at knee level on each floor of your home.
While you may prefer a hardwired smoke detector, it's important to remember that a battery-operated smoke detector is safer than no smoke detector at all.
Home Structure Fires report. National Fire Protection Association
Smoke Alarms in U.S. House Fires. National Fire Protection Association