Smoke Alarms FAQ

First Alert Smoke Alarms, Smoke Detectors & Fire Alarm

What should I know about Smoke Alarms?

The primary thing you should know about smoke alarms is that there are two basic types: Ionization and photoelectric smoke alarms.

Ionization Smoke Alarms are able to quickly detect fast moving fires. Think of a candle catching a curtain on fire- it happens quickly. A Photoelectric Smoke Alarm is quicker in sensing a slower, smoky fire- such as an electrical fire that starts within your walls.

A Dual Sensor Smoke Alarm combines the two types: photoelectric and ionization.

Because there is no way of telling what type of fire may occur in your home, it is strongly recommended by the USFA to have both an ionization smoke alarm and photoelectric alarm- or a dual sensor smoke alarm to detect both types of fires.

For those with hearing disabilities, First Alert also has smoke alarms that utilize flashing strobe lights and vibrations to alert them to danger.

Where should I place my Smoke Alarms?

You should have a smoke alarm in every bedroom, and at least one on each level of your home.

How Do Smoke Alarms Work?

All smoke alarms consist of two basic parts: a sensor to detect the smoke and a very loud electronic horn to alert the surrounding area. Smoke detectors can run off of a 9-volt battery or 120-volt house current. Let's examine the two most common types of smoke alarms used today: photoelectric detectors and ionization detectors. And, we'll also take a look inside an ionization detector. We'll start with photoelectric detectors.

Photoelectric Smoke Detectors

Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photodetector that can "see" the light. When you cross the beam of light, you block it. The photo detector senses the lack of light and triggers a bell. You can imagine how this same type of sensor could act as a smoke detector. If it ever got smoky enough in the store to block the light beam sufficiently, the bell would go off. But there are two problems here:

  • It's a pretty big smoke detector.
  • It is not very sensitive.

There would have to be a LOT of smoke before the alarm would go off -- the smoke would have to be thick enough to completely block out the light. It takes quite a bit of smoke to do that.

Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are positioned at 90-degree angles to one another, like this:

Smoke Alarm - Photoelectric

smoke alarm - bullets

In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor:

photoelectric smoke alarm


The sensor then sets off the horn in the smoke detector.
Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.

Ionization Smoke Alarms: Ionizing Radiation

Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.

Inside an ionization detector is a small amount (perhaps 1/5000th of a gram) of americium-241. The radioactive element americium has a half-life of 432 years, and is a good source of alpha particles.

Another way to talk about the amount of americium in the detector is to say that a typical detector contains 0.9 microcurie of americium-241. A curie is a unit of measure for nuclear material. If you are holding a curie of something in your hand, you are holding an amount of material that undergoes 37,000,000,000 nuclear transformations per second. Generally, that means that 37 billion atoms in the sample are decaying and emitting a particle of nuclear radiation (such as an alpha particle) per second. One gram of the element radium generates approximately 1 curie of activity (Marie Curie, the woman after whom the curie is named, did much of her research using radium).
Let's take a look now at the ionization chamber.

Ionization Detectors: Ionization Chamber

An ionization chamber is very simple. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation, like this:

ionization smoke alarm

The alpha particles generated by the americium have the following property: They ionize the oxygen and nitrogen atoms of the air in the chamber. To "ionize" means to "knock an electron off of." When you knock an electron off of an atom, you end up with a free electron (with a negative charge) and an atom missing one electron (with a positive charge). The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent.

When smoke enters the ionization chamber, it disrupts this current -- the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn.

Speaking of alarms, whenever the words "nuclear radiation" are used an alarm goes off in many people's minds. The amount of radiation in a smoke detector is extremely small. It is also predominantly alpha radiation. Alpha radiation cannot penetrate a sheet of paper, and it is blocked by several centimeters of air. The americium in the smoke detector could only pose a danger if you were to inhale it. Therefore, you do not want to be playing with the americium in a smoke detector, poking at it, or disturbing it in any way, because you don't want it to become airborne.


I'm getting a false alarm from my smoke alarm. Why does my smoke alarm sound when I can't see smoke?

If you feel like you are getting a false alarm from your smoke alarm (not a low battery chirp) often, please review the following known reasons:

  • The cover or sensor chamber is covered by dust or dirt. Alarms may look clean, but dust can accumulate inside the cover, especially in newly built homes. Gently vacuum smoke alarms regularly using the soft brush attachment. Be sure electricians install the provided dust cover to keep alarm clean during construction.
  • Power interruptions or loose connections to AC/DC hardwired smoke alarms. Smoke alarms may sound off briefly when power is interrupted, then restored. Power interruptions are common in areas where utility companies switch grids in the early hours of the morning.
  • When the furnace is turned on for first use or the alarm is too close to the furnace.
  • Humidity - Ionization smoke alarms are more susceptible to nuisance alarms when placed near a bathroom or other potentially high humidity areas.
  • Cooking - According to a survey performed by First Alert, nearly 7 out of 10 Americans have had a smoke detector alarm because of smoke from cooking.
  • Smoke Alarm May Need to be Relocated - If possible, install smoke alarms at least 20 feet from appliances like furnaces and ovens, which produce combustion particles. Alarms should be at least 10 feet from high humidity areas like showers and laundry rooms, and at least 3 feet from heat/AC vents and fluorescent lights whenever possible. In areas where a 20-foot (6 meter) distance is not possible - in modular, mobile, or smaller homes, for example; it is recommended the Smoke Alarm be placed as far from these fuel-burning sources as possible. The placement recommendations are intended to keep these Alarms at a reasonable distance from a fuel-burning source, and thus reduce false alarms.

My smoke alarm keeps chirping and beeping. Why does my smoke alarm chirp intermittently?

It is likely that the reason your smoke alarm keeps chirping and beeping is that the battery is low. Whenever your smoke alarm keeps chirping, replace the battery immediately.

  • A different device or appliance such as a security system, monitor, carbon monoxide alarm, or other device which has a similar low battery or alert signal.
  • Some of the same factors that cause unwanted alarms can cause intermittent alarms: dust and insects in the alarm or power interruptions in hardwired alarms.
  • Improper wiring on AC or AC/DC smoke alarms. AC alarms will chirp every 5 seconds if the interconnect wire is grounded. The orange interconnect wire should NEVER be grounded; it should only be used to interconnect other smoke alarms or compatible devices.

Why doesn't my smoke alarm sound when I push the test button?

It is important that you frequently test your smoke alarms. When you are testing your smoke alarm, there are a number of reasons why the alarm might not sound:

  • You may not be holding the test button down long enough. Try holding it down for up to 10 seconds (20 seconds on photoelectric models).
  • Your battery may not be installed properly or snapped all the way in place. Even if the alarm sounded briefly when the battery touched the terminals, you still need to make sure it is snapped securely in place. If the battery is loose, in cannot power the smoke alarm properly. After installing new batteries, be sure to test your smoke detector.
  • Your AC power may not be on. AC and AC/DC units will have a power indicator light (red or green) that shines continuously when they are receiving electrical power.
  • If you have a 10-Year model, the smoke alarm may not have been properly activated. If the tab broke away before the alarm was activated, you can use a toothpick to move the switch over to test the alarm.

Why does my smoke alarm go off when I install a battery or turn on the AC power?

It is normal for smoke alarms to go off and sound briefly (up to 5-10 seconds) when you install a new battery or when they are powered up. If the alarm continues to go off and no smoke is present, the cause may be one of the following:

  • There may be insufficient battery power, try new batteries.
  • Problems with voltage or insufficient electrical power (brown out) may cause a continuous weak sounding alarm. For AC or AC/DC models, temporarily disconnect power at the service panel until the brown out is over. If you do not restore the AC power, your smoke alarms cannot warn you of a fire.
  • Incompatible warning device. If an incompatible alarm or auxiliary device is linked into a series of AC or AC/DC smoke alarms it may cause the system inadvertently go off.

My smoke alarm keeps chirping, even with a new battery. What is causing this?

There are a number of possible causes for your smoke alarm to keep chirping even with a new battery.

  • It is possible that your smoke alarm "silence" button was pushed by mistake. The alarm will now "chirp" once a minute for up to 15 minutes before resetting.
  • Are you sure it's the smoke alarm? Funny to ask, but other devices have similar low battery chirps or warning tones.
  • Your "new" batteries may not be fresh. If batteries are stored, especially in cold areas like refrigerators, they quickly lose their charge. Always check the freshness date on the package when buying new batteries. Keep plenty of replacement batteries on hand so that you are sure to always be protected by your smoke alarms.

I'm ready to change my smoke alarm battery - what replacement batteries can I use?

Check your User's Manual or the nameplate on the back of the alarm. Different smoke detectors use different kinds of batteries - 9V, AA, AAA - it all depends on the particular model you have. Use quality batteries like lithium smoke detector batteries - having plenty of power is worth any extra cost. Never use rechargeable batteries because they may not always provide a consistent charge.

I lost my First Alert Smoke Alarm Owner's Manual. How can I get a new one?

First Alert smoke alarm owner's manuals are available online for download at no cost. Find your alarm in our Smoke Alarms section.

How long will the battery last in my smoke alarm?

Actual battery service life depends on the particular design of your smoke or carbon monoxide alarm and the environment in which it is installed. All kinds of alarm batteries specified in the user's manual are acceptable replacement batteries. Regardless of the manufacturer's suggested battery life, you MUST replace the batteries immediately once the unit starts "chirping" (the "low battery warning"). It is recommended that you replace the batteries in your alarms when you change your clocks for daylight saving time. Also consider replacing your current alarms with 10-Year Life alarms that never require a costly battery replacement for the ten year life of the alarm. This 10-Year series is available in smoke, carbon monoxide and combination alarms.

What is the proper placement of smoke alarms?

It is important that you have the proper placement for your smoke alarms. Install your alarms at least 20 feet from appliances like furnaces and ovens, which produce combustion particles. Alarms should be at least 10 feet from high humidity areas like showers and laundry rooms, and at least 3 feet from heat/AC vents. Be sure to install a smoke alarm in each bedroom, one at the top of each stairwell, and one on every level.

Why does the National Fire Protection Association (NFPA) recommend that home smoke alarms be replaced after 10 years?

Smoke alarms have a limited life. Although each smoke alarm and all of its parts have passed many stringent tests and are designed to be as reliable as possible, any of these parts could fail over time. Therefore, you must test the devices weekly. The unit should be replaced immediately if it is not operating properly. The performance of smoke alarms older than 10 years is simply not reliable. To ensure your family's safety, all carbon monoxide and smoke/CO combination alarms need to be replaced every 5-7 years. All smoke alarms need to be replaced every 10 years.

If it's time to replace your alarms, consider the 10-Year Life series and never have to worry about a battery replacement for the life of the alarm. 10-Year alarms are available in smoke, carbon monoxide and combination alarms.

Ionization smoke alarms vs. Photoelectric smoke alarms - what is the difference?

There are generally two types of smoke alarms - ionization smoke alarms and photoelectric smoke alarms. Smoke particles of a varying number and size are produced in all fires. Ionization smoke alarms are generally more sensitive than photoelectric smoke alarms at sensing small particles, which tend to be produced in greater amounts by hot, flaming fires that are consuming combustible materials rapidly and may spread quickly. Sources of these fires may include paper burning in a wastebasket, or a grease fire in the kitchen. Photoelectric smoke alarms are generally more sensitive than ionization smoke detectors at sensing large smoke particles, which tend to be produced in greater amounts by smoldering fires, which may smolder for hours before bursting into flame. Sources of these fires may include cigarettes burning in couches or bedding. For maximum protection, use both types of technology, such as in the Dual Sensor Smoke Alarm, on each level and in every bedroom of your home.

The following is a collection of First Alert Smoke Alarms and Detectors that are readily available for purchase here at the First Alert Store:

First Alert Store Dual Ionization & Photoelectric Smoke Alarm

Battery operated Dual Ionization and Photoelectric Sensor Smoke Alarms offer maximum protection against both primary types of fires. This alarm actively seeks out flaming and smoldering fires with tremendous precision. A Remote Controlled Mute button silences false and nuisance alarms such as cooking smoke and shower steam. This button can be accessed from a standard household remote control (remote not included). This can also be used to test alarm functions. This alarm features an easily accessible battery drawer. You'll never have any need to remove alarms from the ceiling to change its battery. This alarm is accompanied by a 10-year limited warranty. Smoke Alarm meets UL Standards.

This smoke & fire alarm is for you if:

  • You want the convenience of a "point & click" feature that turns off nuisance alarms
  • You want optimal protection for your family from both types of primary fire - flaming and smoldering

What are the primary differences between First Alert Smoke Alarms?

First Alert has created an elaborate series of Smoke Detectors and Alarms. While they all feature the essential function of alerting you of a fire, there are a few other notable differences that separate them from one another. Features such as a muting option, testing the unit and easy access battery doors are included in all models. Other features such as strobe lights for the hearing impaired and remote control access are model specific.

10-Year Battery Smoke Alarm

This battery operated Ionization Smoke Alarm comes equipped with an extended lithium battery. This long-life battery is warranted for 10 years! You'll never have reason to change the battery for the duration of the alarm's lifespan. This is especially convenient for property owners who don't necessarily trust their tenants to replace batteries annually. This is also the ultimate in peace of mind when it comes to reassuring a decade's worth of protection for those who may be forgetful! This alarm comes equipped with a mute feature that will silence your alarm in the instance of cooking or shower steam.

This is the right smoke alarm for you if:

  • You want the convenience of never changing the battery on your alarm.
  • You want to mute unwanted alarms with the press of a button.

120VAC Hardwired Smoke Alarm with Battery Backup

The 120VAC Hardwired Smoke Alarm wires directly into your home's electrical system and protects you from ionization smoke. This alarm comes equipped with a battery backup and a mute button. You won't ever have to worry about alarm failure due to a power outage. The built-in battery provides assurance of maximum coverage. This unit also features the easily accessible battery door and convenient mute feature.

This is the right smoke alarm for you if:

  • You require alarms wired directly into your home's electrical system. This allows for additional safety in case of a power outage.
  • You want to easily mute unwanted alarms with the press of a button.
  • 120vac hardwired, easily replace existing or install new hardwired alarms into your home's electrical.

Basic Smoke Alarm

These are the standard Smoke Alarms found all over the country and feature an Ionization Smoke Sensor. Additions include: Battery Operation, Mute Button and Test Function. These are often utilized for their reliability.

This is the right alarm for you if:

  • You require a basic and standard means of fire protection.
  • You need access to a Mute Button that quickly dismisses the sound produced by unwanted alarms.

Dual Ionization & Photoelectric Smoke Alarm

Dual Ionization & Smoke Alarms are the only smoke detectors to feature both an Ionization and Photoelectric Sensor! Dual Alarms provide excellent protection from both slow and rapid fires alike. These units also come equipped with a remote control feature (not included) to silence nuisance alarms brought on by cooking smoke and shower steam. Simply pressing any button on a standard household remote control will give you access to muting your alarm. These models also incorporate Smart Technology that effectively deciphers between cooking smoke/shower steam and true fires.

This is the right alarm for you if:

  • You require the convenience of a remote control to instantly silence nuisance alarms.
  • You're looking for protection from both types of fire-flaming and smoldering.

Escape Light Smoke Alarm

This Smoke Alarm comes equipped with a light to assist you in escaping a fire. The presence of Heavy Smoke often carries the threat of reducing your visibility. The addition of a light is perfect for revealing your path of escape.

This is the right smoke detector for you if:

  • You want the added guidance and protection of a light source to assist your family in escape during fire.
  • You want to easily mute unwanted alarms with the press of a button.

Hearing Impaired Strobe Light LED Smoke Alarm

These Smoke Alarms were specifically designed for those suffering from a hearing impairment. Featuring a powerful 177 candela strobe light, these smoke alarms meet all of the requirements penned in the Americans with Disabilities Act. The 1120VAC Hardwired Alarms wire directly into your home's electrical system.

This smoke & fire alarm is for you if:

  • You need to install smoke or CO notification for the hearing impaired that meet ADA requirements.
  • You already have or are purchasing First Alert compatible hard wired smoke or carbon monoxide alarms.

Photoelectric Sensor Smoke Alarm

The Photoelectric Sensor Smoke Alarm is an eco friendly option that features a completely recyclable battery. The utilization of Photoelectric Smoke Sensing Technology has been realized and successfully distinguishes between cooking smoke, shower steam and legitimate threats. This unit also features the ever popular means of accessing a mute feature to instantly silence false alarms.

This is the right smoke detector for you if:

  • You want to reduce nuisance alarms like those caused by cooking fires and shower steam.
  • You want to protect your family and the environment.

Photoelectric Sensor Smoke Alarm with Escape Light

This Photoelectric Sensor Smoke Alarm comes equipped with a fully functional escape light to greatly assist you in securing a path away from danger. This unit is also eco friendly and can be completely recycled.

This is the right smoke detector for you if:

  • You require the added guidance and peace of mind that an emergency light has to offer.
  • You want to protect your family and the environment.

Tamperproof 10-Year Smoke Alarm

The First Alert Tamperproof Lithium Powered Detector was the first available smoke alarm proven to last ten years without battery replacement. This feature makes it a favorite amongst landlord/tenant users who may forget to replace their regular powered alarms annually. This Smoke Alarm also includes an Ionization Smoke Sensor.

This is the right smoke & fire alarm for you if:

  • You need to install tamper-resistant alarms that meet local codes for rental properties.
  • You want the convenience of never changing batteries.

First Alert 10 Year Photoelectric Combo Smoke & CO Alarm, Designer Series

The First Alert 10 Year Photoelectric Combo Smoke & CO Alarm features an unprecedented ten year life span that complies with the Current 10-Year Legislation. A leap in technology has eliminated the need to ever power or charge this unit with batteries. An end of life signal will readily alert owners once the unit has reached its ten year capacity.

This smoke and CO detector is for you if:

  • You're searching for the ultimate in peace of mind and ten years of protection.
  • You require an alarm that senses both Smoke and CO.