How often should I replace smoke detectors?

This is the one people get wrong in a specific and dangerous way: they assume a detector that beeps when tested is a detector that works. The test button confirms the battery, horn, and circuit. It does not confirm the sensor.

Why ten years

The sensing element degrades over time — dust, humidity, and in ionization detectors the slow decay of the sensing source itself. Manufacturers set a 10-year service life because past that point sensitivity has drifted enough that detection is no longer reliable. Every unit has a manufacture date printed on the back or side. That is the clock, not your install date; a detector can sit in a warehouse for a year before you buy it.

The schedule

• Test every detector monthly with the test button.
• Replace batteries annually in battery units — a familiar date like the start or end of daylight saving makes it stick. Sealed 10-year-battery units skip this and get replaced whole.
• Vacuum the exterior vents a couple of times a year; dust is a leading cause of both missed detections and false alarms.
• Replace the entire unit at 10 years from the manufacture date.

Where they go

One inside every bedroom, one outside each sleeping area, and at least one on every level including the basement. Mount them on the ceiling or high on a wall, since smoke rises. Keep them a few feet clear of kitchens and bathrooms — steam and cooking smoke cause the nuisance alarms that lead people to pull the battery out, which is how detectors end up disabled.

A worthwhile upgrade

If you are replacing a whole set anyway, interconnected detectors — where one triggering sounds all of them — are a genuine safety improvement in a multi-storey house. A basement fire detected by a basement detector alone may not wake anyone upstairs.

When you install a new set, write the install year on the unit in marker. Ten years is long enough that you will not otherwise remember.