The expiry date nobody reads: your alarms have a shelf life

There is a date printed on the back of every smoke and CO alarm in your home, and almost nobody has ever read it. Not an installation date — a retire-by date. Alarms are sensors, sensors are consumables, and an alarm past its life can chirp cheerfully on test while detecting progressively less every year.

Smoke alarms: ten years, from manufacture

Fire-safety guidance across the US and UK converges on the same number: replace smoke alarms after ten years from the date of manufacture — which is printed or stamped on the base, and can be years before you bought it. Warehouse stock sits on shelves; a "new" alarm can already be two or three years into its life. Inside the chamber, dust and contaminants degrade the sensor's sensitivity even in an alarm that has never once gone off. The failure mode is the cruel one: the alarm still beeps on test, so it feels reliable, while the margin it would need to wake you at 3am has quietly eroded.

CO sensors: the shorter clock

Carbon monoxide alarms use electrochemical cells that deplete by design — the sensor is a chemical reaction running down, and its working life is typically in the five-to-ten-year band depending on the maker. The replace-by or replace-on date is printed on the body; when it passes, the whole unit is replaced, because on domestic alarms the sensor isn't a serviceable part. Combination smoke-plus-CO units follow the shorter of the two lives — which means a "10-year" combination alarm is usually governed by its CO cell, not its smoke side. If yours has never been opened from its box... it still expires: the clock runs from manufacture.

The signs you've already left it late

An end-of-life chirp is different from a low-battery chirp — three beeps repeating means smoke or CO and everyone gets out; a single periodic chirp usually means battery; but a chirp that persists after a fresh battery on an older unit is the end-of-life signal on many models. Yellowing plastic, a manufacture date you have to squint at, or any alarm that survived an actual fire event should simply be retired. Sealed long-life battery alarms chirp end-of-life too — the difference is the answer is a new alarm, not a new battery.

The five-minute audit

Tonight, twist each alarm off its bracket and read the date. Manufactured more than ten years ago (smoke) or past its printed replace-by (CO): replace it — and count how many you actually need while you're up there (the counting guide). When you buy, put the new retire-by date in your phone with a reminder a month ahead. And put the test in your calendar too, because expiry is only half the story — the monthly habit is the other half, placement is the rest, and for UK landlords the legal minimums are in the law guide. Ten minutes, once, and every date in the house is on your side.

Sources: NFPA and US state fire-marshal replacement guidance (10-year smoke alarm rule); manufacturer replace-by labelling conventions for electrochemical CO sensors. Last reviewed September 2026 · general information, not professional advice.