What is a smoke detector?
Updated August 2026
A smoke detector is a ceiling- or wall-mounted alarm that watches a small sensing chamber for smoke particles and sounds an 85 decibel horn when it finds them. Two technologies split the work. Ionization units pass a faint current across the chamber and notice when smoke disrupts it, which favors fast flaming fires. Photoelectric units shine a light beam past a sensor and notice when smoke scatters that beam into view, which favors slow smoldering fires such as a cigarette in upholstery. Dual-sensor models carry both.
Where it is
Coverage rules are specific: one inside every bedroom, one immediately outside each sleeping area, and at least one on every level including the basement. A three bedroom two story house therefore needs six alarms, not two. Mount on the ceiling at least 4 inches from the nearest wall, or high on a wall 4 to 12 inches below the ceiling, and keep 10 feet of distance from the stove and the bathroom door to cut nuisance alarms. Interconnect them where the wiring allows, so a basement fire wakes the upstairs bedrooms.
What it does
An alarm buys minutes, and modern furnishings burn fast enough that those minutes are the entire margin. Two things erode it: dust and age. The sensing chamber collects lint, cobwebs, and cooking grease, which both mutes real smoke and provokes false alarms, while the sensor drifts across a decade whether or not it ever meets a fire. Every listed unit carries a manufacture date on the back, and 10 years past that date the device is expired no matter how clean it looks or how brightly its green light glows. A dead alarm and a working one are visually identical, which is the whole argument for trusting the calendar over the eyeball.
Maintenance
Three intervals, no exceptions. Press the test button monthly and vacuum the vents with a brush attachment while you are already on the ladder, per the monthly test routine. Swap 9 volt batteries once a year, or buy sealed 10 year lithium units and skip the ladder entirely. Replace the alarm itself at 10 years from the printed manufacture date, following the replacement schedule. Running all three in one pass alongside the CO alarms is exactly what the combined detector checklist is for.
What it costs to replace
A replacement smoke detector runs $15–50 per alarm, $25–60 for a sealed 10 year model, or $70–120 for a hardwired smoke and CO combination.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Ionization or photoelectric: which should I buy?
Buy dual-sensor and stop thinking about it; the price gap is roughly $10 an alarm. Forced to choose one, photoelectric is the safer default for bedrooms and hallways, because most fatal house fires smolder for a long stretch before they flame, and photoelectric units also false-alarm far less on shower steam and burnt toast. Ionization keeps a small edge on fast flaming fires, which is why many fire departments still want both types present.
How do I find out how old my smoke alarm is?
Twist it off the ceiling mount and read the back. Every listed alarm has a manufacture date stamped or printed there, sometimes as a plain date, sometimes as a four digit year and week code. If there is no date at all, the unit predates the labeling requirement and is well past due. Write the replacement year on the face in marker when you install the new one, so the next check takes five seconds.
Why does my smoke alarm chirp at 3 in the morning?
Temperature. A house hits its coldest point in the small hours, battery voltage sags with the drop, and the alarm crosses its low-battery threshold right then. It is the most reliable trick in home ownership. Fit a fresh battery rather than pulling the old one out, and if a new battery does not stop the chirp, check the date on the back: an expired unit chirps on its own schedule and only a replacement silences it.
Do hardwired smoke alarms still need batteries?
Yes. The battery backs up exactly the scenario you care about: a fire that takes out the circuit, or a power cut during a storm. Hardwired alarms with replaceable batteries need a fresh one every year, while newer models ship with a sealed 10 year cell that retires with the unit. Hardwired alarms are also interconnected, so replace them as a set, and never mix brands on the same interconnect wire.