Photoelectric vs. Ionization Smoke Alarms
Two sensors, two kinds of fire. Here is what each one is good at.
Walk down the smoke alarm aisle and you will see two kinds of sensors, plus alarms that combine them. They detect smoke in different ways and each one is faster at a different type of fire.
Photoelectric alarms
A photoelectric alarm shines a small light inside a chamber. When smoke drifts in, it scatters the light onto a sensor and the alarm sounds. These alarms are generally quicker to react to slow, smoldering fires, the kind that start in a couch, a mattress or a pile of clothes and fill a room with smoke before there are big flames.
Photoelectric alarms are also less likely to go off from normal cooking. That makes them the better choice for spots near the kitchen.
Ionization alarms
An ionization alarm uses a tiny amount of radioactive material to create a small electrical current between two plates. Smoke particles disrupt that current and set off the alarm. These are generally quicker to react to fast, flaming fires, like burning paper or a grease fire that flares up.
The tradeoff is that ionization alarms react more easily to cooking smoke and steam. That is a big reason people pull batteries out of them, which leaves the home unprotected. Keep them well away from the kitchen and bathrooms.
About the kitchen. No smoke alarm should be right next to the stove. Keep every alarm at least 10 feet from a cooking appliance. In the zone near the kitchen, choose a photoelectric alarm or one listed as resistant to cooking nuisance alarms. Newer alarms built to the latest UL 217 standard are designed to tell cooking smoke from a real fire.
Which one should you buy?
Since nobody knows what kind of fire they will have, fire safety groups including the NFPA and the U.S. Fire Administration recommend protection from both technologies. You can do that with dual-sensor alarms, which have both sensors in one unit, or by mixing photoelectric and ionization alarms around the house.
| Type | Fastest at | Cooking false alarms | Good places |
|---|---|---|---|
| Photoelectric | Smoldering fires | Less likely | Bedrooms, living areas, hallways near the kitchen |
| Ionization | Fast flaming fires | More likely | Hallways and living areas away from kitchens and bathrooms |
| Dual-sensor | Both | Depends on model | Anywhere you want the widest coverage |
Other features worth knowing
- 10-year sealed battery. The battery lasts the life of the alarm. No battery swaps, and no temptation to borrow it for the TV remote. When it reaches end of life, you replace the whole unit.
- Hardwired with battery backup. Powered by your home's wiring, with a battery that keeps it running in a power outage. Installing or replacing these is electrical work. If you are not comfortable with it, hire a licensed electrician.
- Interconnected. When one alarm senses smoke, every alarm in the house sounds. Hardwired systems do this through the wiring, and many newer battery alarms do it wirelessly. A fire in the basement wakes up the bedrooms.
- Smoke and carbon monoxide combination. One unit that watches for both. Carbon monoxide is a separate danger and needs its own detection, so a combo unit can save ceiling space.
- Alarms for people who are deaf or hard of hearing. Options include strobe lights, bed shakers and low-frequency sound alarms that are easier to hear while asleep.
Whatever you buy, look for the mark of a recognized testing lab such as UL on the package. Then put it in the right spot.