How often should you replace smoke and CO alarms?

Short answer

Replace the alarms every 10 years for smoke alarms, every 5 to 10 years for CO alarms. The date that matters is the manufacture date printed on the back of the alarm, not the day it was installed, and an alarm with no date on it is old enough to replace now. A new alarm costs $15 to $50, and swapping one takes 15 to 30 minutes.

Why it matters

The sensor inside a smoke alarm ages whether the alarm has ever gone off or not. Dust settles in the chamber, the electronics drift, and the sensing element itself degrades, until the alarm sounds slower in smoke or not at all. The test button doesn't catch this, because it checks the horn and the circuit, not the sensor's sensitivity. That's why alarm makers put a date on the back and tell you to replace the unit at 10 years regardless of how it tests.

A carbon monoxide sensor wears faster. The chemical cell that reacts to the gas is used up over time, which is why CO alarms carry a shorter life, 5 to 10 years depending on the model, and why many of them start chirping an end-of-life warning on their own. An alarm that's past its date is a device that looks like protection and may not be.

When to do it more often

The schedule is fixed by the date on the alarm, and there's one case where it moves to now: an alarm with no date on the back. Alarms have carried a manufacture date for a long time, so one without is older than any of the intervals above.

The other reasons to replace early are the ones the monthly test turns up: an alarm that fails the test with a fresh battery, one that's yellowed from age, or one that sounds false alarms often enough that the household has started ignoring it. A smoke alarm that cries wolf is as dangerous as one that's silent.

What you need

A screwdriver, for the mounting plate and for the wiring connector on a hardwired alarm
A step ladder
New alarms, matched to the old ones: battery or hardwired, and for hardwired ones, a model that fits the existing connector or comes with an adapter

Steps at a glance

For a hardwired alarm, switch off the breaker that feeds it and confirm with the test button that it's dead.
Twist the old alarm off its mounting plate and unplug its wiring connector, or take out its battery.
Unscrew the old plate and fit the new one, using the same holes where they line up.
Plug the new alarm's connector into the house wiring, or fit its battery.
Twist the alarm onto the plate, restore the breaker, and press the test button.
For interconnected alarms, test one and listen for the rest to sound.
Write the install date on the new alarm's edge, and set a reminder for its replacement date.

You're done when every new alarm sounds on its test button and, if interconnected, sets off the others.

Signs it's overdue

A manufacture date on the back more than 10 years ago.
A housing that's yellowed, which is the plastic aging.
An alarm that fails its test with a fresh battery.
Frequent false alarms with no smoke, as the sensor becomes unstable.

DIY or pro

Replacing a battery-powered alarm is an easy job: a new plate, a couple of screws, and a battery. A hardwired alarm is a step up, but still within reach for anyone comfortable turning off a breaker and plugging in a connector. Most replacements use the same plate and connector as the old alarm, so the house wiring isn't touched.

An electrician charges $50 to $150 per alarm to replace hardwired units, which is worth it if the connector doesn't match and the wiring has to be redone, if the alarms are on a high ceiling, or if you're replacing a whole house of interconnected alarms at once and want them tested as a system. Replace all the alarms in a house together, since they were probably installed together and are aging together.

Safety

Electricity: a hardwired alarm is connected to house wiring. Switch off its breaker before you unplug the old alarm, and check that the alarm is dead before you touch the connector. If the wiring looks damaged or the connector doesn't match, stop and have an electrician finish the job.

FAQ

How do I know how old my smoke alarm is?

Twist it off its plate and read the manufacture date printed on the back. If there's no date, replace it.

Do smoke alarms really expire?

Yes. The sensor degrades over about 10 years whether the alarm has ever gone off or not, and the test button doesn't measure the sensor.

Can I replace a hardwired smoke alarm myself?

Yes, if the new alarm fits the existing connector. Turn off the breaker first. If the connector or the wiring needs changing, that's an electrician's job.

Should smoke and CO alarms be replaced at the same time?

Replace each by its own date. Smoke alarms go 10 years and CO alarms 5 to 10, so a combined unit follows the shorter life.