Just like any electrical appliance, the working components of smoke alarms wear out over time. When the smoke alarm reaches 10 years of use, the potential of failing to detect a fire increases. Replacing them after 10 years reduces the likelihood of failure.
My smoke alarms are wired into my electrical system.
If the home is large, use more than one on each level. The closer the alarm is to the fire source, the faster it will work, so extra units give you more safety. Some homes may have more than one bedroom area, so they should have extra smoke alarms.
For existing homes, the NFPA only requires a smoke alarm outside the bedroom area and one on each level of the home. However, it also recommends that homeowners install additional smoke alarms like you would find in new homes. It makes sense to install a smoke alarm in each bedroom. A good number of fires start in bedrooms, and the closer the smoke alarm is to the fire, the faster it will alert you. The recommendations are located in Appendix A of the document.
Are there places where I should not put a smoke alarm? Smoke alarms are not designed to work in extreme heat or cold, or in areas where smoke and dust are common. They are not recommended for unheated attics or similar spaces. They should not be used in garages because they are usually not heated or cooled. Also, the smoke from engine exhaust fumes will cause nuisance alarms and clog the smoke alarm.
Another area of concern is the kitchen. If a smoke alarm is installed too close to cooking and baking, this may result in nuisance alarms. The NFPA is addressing this problem and will soon require that smoke alarms installed within 20 feet of cooking be photoelectric or have a silencing button. We have a section below on the two types of smoke alarms for residences.
The reference in the appendix talks about heat alarms for areas in the home where smoke alarms are not recommended. The examples include garages, attics, unheated crawl spaces and kitchens. The air in the spaces like garages can become too hot or cold for smoke alarms to operate properly. Smoke alarms are not recommended for kitchens if the cooking causes nuisance alarms. On the other hand, if the smoke alarm does not cause nuisance alarms, then its location is fine.
For all other spaces like living rooms, dens, dining rooms and bedrooms, the standard recommends smoke alarms. It cautions you against depending primarily on heat alarms. The Consumer Product Safety Commission takes a similar position.
Is there more than one type of smoke alarm, and how do they differ? There are two types of smoke alarms currently available for homes. One type is called an ionization alarm because it monitors "ions," or electrically charged particles in the air. Smoke particles entering the sensing chamber change the electrical balance of the air. The alarm's horn will sound when the change in electrical balance reaches a preset level.
The other type of alarm is called photoelectric because its sensing chamber uses a beam of light and a light sensor. Smoke particles entering the chamber change the amount of light that reaches the light sensor. The alarm sounds when the smoke density reaches a preset level.
Fire research shows that a fire with a lot of small smoke particles will cause an ionization smoke alarm to sound sooner than a photoelectric. The time delay between the two is relatively small. On the other hand, a fire with a lot of large smoke particles will cause a photoelectric smoke alarm to sound sooner than an ionization. In this case the time delay between the two can be relatively long.
Because the ionization type alarm is sensitive to small smoke particles, it will respond more quickly to cooking. If you experience this problem, you have several options:
1) Move the unit farther away from the cooking area. The cooking gases will be more diluted in the air when they reach the smoke alarm.
2) Install a photoelectric smoke alarm.
3) Install a combination ionization/photoelectric alarm. When the two sensors are combined in one unit, the ionization sensor is set at a slightly less sensitive level. Thus the unit is less prone to nuisance alarms and you have a unit with greater sensitivity to both fast-flaming and slow smoldering fires.
4) Install an ionization smoke alarm that has a hush button. This allows you to silence the unit for 15 minutes while you remove the source of the nuisance alarm (e.g., burnt toast) and clear the air. This is not as convenient as options 2 or 3, but it will work as long as you can reach the button and don't get aggravated enough to remove the battery.
The distance from a fire can also change the smoke, and this is where the difference between the ionization and photoelectric smoke alarms becomes important. What can take place is called "smoke aging." As smoke particles move away from the fire, the heat dissipates and the smoke cools. As smoke gets cooler, the smoke particles and coagulate into larger particles. The farther the distance from the fire to the smoke alarm, the cooler the smoke and the larger the smoke particles. Ionization smoke alarms are better at detecting smaller particles, while the photoelectric respond better to larger particles. In these situations, photoelectric smoke alarms have an advantage.
Happily, you no longer have to choose between an ionization or photoelectric smoke alarm. Units with both types of sensors are also available. Because the unit has both types of sensors, the manufacturers do not have to make the ionization sensor as sensitive, thus making it less prone to nuisance alarms from cooking.
Another option is to replace the ionization-type alarm with a photoelectric alarm. This alarm is less sensitive to smaller smoke particles and thus is less affected by cooking smoke.
You can also select a combination ionization/photoelectric alarm (both sensors in one unit). It will be less sensitive to cooking and responds faster to smoldering fires than ionization units. The reason that it is less sensitive to cooking is because the manufacturer can make the ionization sensor less sensitive when a photoelectric sensor is also present.
NOTE: Ten-year batteries will not last for their stated service life in photoelectric smoke alarms because this type of smoke alarm uses more power than an ionization type. However, there is nothing wrong with installing a long-life battery in them as long as you remember to replace the smoke alarm itself when it is ten years old.
When you read claims that ionization smoke alarms are hazardous, be aware that competitors or experts working for competitors have been known to make assertions about the so-called radiation hazard. We have found that they have no basis in fact. When ionization smoke alarms were first manufactured, the federal government monitored their use to assure that they would not pose a risk to consumers. After several years of monitoring, the government was confident that they were safe and halted its monitoring program.
SMOKE ALARMS, SMOKE DETECTORS
MERRIMAC FIRE DEPARTMENT