How Does a Smoke Detector Work?

Very often confused with smoke detectors that simply sense smoke and then trigger a separate alarm system, a self-contained smoke alarm senses smoke and triggers its own alarm contained inside the unit. It is a smoke alarm, not a smoke detector, that is found in most homes, and an alarm that's properly functioning gives early warning that makes escape more likely. Having smoke alarms properly installed and regularly checked so they can do their job when needed is a critical part of home safety.

Here's everything you need to know about how smoke alarms work.

Smoke alarms have two main components: the sensor and the alarm. Both components play an important role in the overall purpose of the unit, which is to sense fires as soon as possible and alert anyone in the home so they can get out. These components are equally important. Sensing the fire does no good if the alarm isn't loud and functioning. A poor sensor fails to send out the alert soon enough to get people out of the home.

A smoke alarm's initial function is detecting that smoke or fire is present. The way it senses the fire depends on the type of unit it is. A photoelectric model has a light source that shines into a sensing chamber containing a sensor. The light sits at an angle to the sensor. When smoke enters the chamber, it causes the light to reflect onto the sensor, triggering the alarm. These units are best at detecting smoldering fires that start with smoke before eventually breaking into flames.

An ionization model uses radioactive material positioned between two electrically charged plates to ionize the air inside it. This creates a current flow between the two plates. Smoke sets off the alarm when it disrupts the ion flow. These alarms are most responsive to fires with blazing flames.

Photoelectric and ionization smoke alarms both have certain advantages that make them more effective in specific areas. For example, a photoelectric model quickly detects smoke from smoldering bedding or furniture in a bedroom, living room or family room, while ionization units give fast warning of a blaze triggered by flammables in a utility room or a stove fire in a kitchen. Because each type is ideal for a certain type of fire, it's a good idea to have at least one of each in your home. You can also get a dual-sensing model that uses both photoelectric and ionization methods for detection.

Once a smoke alarm senses smoke, its next job is to let people know about it. That warning can make a critical difference in how much time occupants have to escape. Effective units should have an alarm decibel level of at least 85 at 10 feet so they can be heard throughout the home. Alarms that are powered by the home's electrical system — called hardwired alarms — are typically interconnected so that if one alarm is activated, all of the other alarms in the house go off simultaneously. Some battery-powered units also have interconnection capability for enhanced safety.

Smoke alarms are often located outside bedroom doors. That means the alert sound needs to be loud enough to go through doors and wake up sleeping people in case of a fire at night. Alarms should also have a low-battery notification function that makes a repetitive beep when the battery power is nearly drained.

Smoke alarms do not last forever. It's a good idea to replace them every 10 years (or when they reach their expiration date) to make sure you have reliable detection of fires. Most models have a test function that lets you periodically check them to catch potential problems early. Test your smoke alarms monthly and immediately replace any units that do not respond as they should.

Get new alarms after a decade even if the old models still seem to be functioning. Older units may sound when you press the test button, but that doesn't tell you how well the sensor works. The test response simply tells you that the electronic components and sound parts of the alarm still work. You can test the sensor by using smoke alarm test spray, but it's still a good idea to change out any units older than 10 years. The sensor could stop working at any time.

Not sure how old your unit is? Look on the back side for a date, where you should find the manufacture date and/or expiration date. If the former, simply add 10 years to determine the expiration of the unit. Be sure to properly dispose of old smoke alarms.