Short answer: when to replace smoke detectors comes down to one date, the date of manufacture printed on the back of each unit. The National Fire Protection Association (NFPA) says to replace all smoke alarms when they are 10 years old, and some models are rated for less. Carbon monoxide (CO) alarms usually wear out sooner, often somewhere between 7 and 10 years depending on the model. Replace any alarm early if it fails a test, gives an end-of-life signal, or has been through a fire.
This guide is independent and brand-neutral. It explains why alarms expire, how to find the real date on each one, and how to run a quick check of every alarm in your home in one afternoon.
Why smoke and CO alarms expire at all
An alarm that still beeps when you press the button can look perfectly healthy. The problem is inside, where you cannot see it.
Kidde, a smoke and CO alarm manufacturer, puts it plainly on its alarm replacement page (read 2026-09-25): smoke sensors lose sensitivity over time, and replacing alarms on schedule matters even if the test button still works. A slow sensor is the worst kind of fault because nothing about the alarm tells you it is happening.
That is the reason the expiry date exists. It is not about the battery. It is the point at which the maker no longer stands behind the sensor.
The stakes are high. The NFPA's Smoke Alarms at Home tip sheet (read 2026-09-25) states that roughly 3 out of 5 fire deaths happen in homes with no smoke alarms or no working smoke alarms. An alarm that is past its life is the easiest way for a home that looks protected to end up in the "no working smoke alarms" group without anyone noticing.
When to replace smoke detectors and CO alarms: the dates
| Alarm type | Replace by | Source |
|---|---|---|
| Smoke alarm (any power type) | 10 years old, or sooner if the model says so | NFPA tip sheet; Kidde says 5 to 10 years depending on model |
| Combination smoke and CO alarm | 5 to 10 years from manufacture, depending on model | Kidde |
| Standalone CO alarm | 7 to 10 years, based on the model | Kidde |
| Any alarm that fails a test, gives an end-of-life signal, or was exposed to heavy smoke or fire | Now | Kidde |
Two details in that table catch people out.
First, the clock starts on the date of manufacture, not the day you installed it. Kidde says smoke and combination alarms are good for 5 to 10 years "from the date it was manufactured." An alarm that sat in a drawer or on a store shelf has already used some of its life.
Second, a combination alarm runs on its own clock, which can be shorter than the 10-year smoke alarm rule. If your home relies on combination smoke and CO units, go by the model's rated life in the manual.
How to find the date on your alarm
You do not need tools for this on a battery alarm. Work through each unit:
- Take the alarm off its mounting plate. Most battery alarms twist off the ceiling bracket. Your manual shows how.
- Read the label on the back. Kidde says you can find the date in the user guide or product manual, or on the back of the alarm. Look for a manufacture date or a "replace by" date.
- Write it down, room by room. A simple list on your phone, such as "hallway, made 2019," is enough.
- Put the alarm back and press the test button. The U.S. Fire Administration (USFA) says to test alarms monthly, and this is a good moment to do it.
If you cannot find a date anywhere on the alarm or in its manual, you have no way to confirm its age. Replacing it is the safe call.
Hardwired alarms are different. If the alarm has wires running into it from the ceiling, do not unplug it or take it apart to find the date. The USFA states that only qualified electricians should install hardwired smoke alarms. Ask an electrician to check the dates and replace any expired units. That same line between homeowner work and trade work is covered in our guide to where DIY security installation should stop.
Signs an alarm needs replacing before its date
The date is the outer limit. Kidde lists three reasons to replace an alarm early:
- It fails a test. Pressing the test button does not trigger a loud alarm. On an alarm with a replaceable battery, try a fresh battery of the type the manual lists first. If it still fails, replace the alarm.
- It gives end-of-life chirps. Kidde says a single chirp every 30 to 60 seconds often signals that the alarm has reached the end of its service life.
- It has been through heavy smoke or fire. Kidde says alarms can be damaged during a fire and should be replaced immediately.
That chirp needs a word of caution, because a low battery can also cause chirping. The back of the alarm or the manual explains which sound means what for that model. If you have just fitted a new battery and the chirping continues, check the manual to see whether the alarm is telling you it has reached the end of its life.
Battery changes are not alarm changes
A lot of confusion comes from mixing up two schedules.
- Replaceable-battery alarms need a new battery regularly. The USFA says to replace the batteries at least once every year.
- Sealed 10-year battery alarms never get a new battery. The USFA says to replace the entire smoke alarm according to the manufacturer's instructions.
- Every alarm, whatever powers it, still has its own replacement date.
A fresh battery in a 12-year-old alarm gives you a 12-year-old sensor with a fresh battery. It does not reset the clock.
On sealed alarms, the battery was built to last the life of the unit, so a low-battery warning usually arrives near the end of that life anyway. When a sealed alarm starts warning, follow its manual, which will normally mean replacing the whole unit.
A one-afternoon check for the whole house
Most homes have alarms installed at different times, which is why dates drift out of step. A single walk-through fixes that.
Step 1: Count your alarms. The NFPA tip sheet says to install smoke alarms in every bedroom, outside each sleeping area and on every level of the home, including the basement. The Consumer Product Safety Commission (CPSC) advises CO alarms on every level of the home and outside sleeping areas. If your count comes up short, note where the gaps are.
Step 2: Record each date. Use the steps above for battery alarms. Leave hardwired units for an electrician.
Step 3: Sort into three groups. Past date, due within a year, and fine. Anything past date moves to the top of the list.
Step 4: Test everything that is staying. Press and hold the test button on each alarm. On an interconnected set, listen for the others too.
Step 5: Label the new ones. Kidde suggests marking the installation date on your alarms so you can track when replacement is needed. A small note on the back or inside the battery door works well, alongside the manufacture date that is already printed there.
If an older relative lives alone, this check is worth doing for them. Our guide to smart home safety for elderly parents covers other ways to keep an eye on things from a distance. It also fits naturally into a pre-trip routine like our home security checklist before a vacation.
Replacing a linked or smart set
If your alarms are interconnected, so that one sounding sets off the others, a replacement has to be compatible with the rest of the set. Brands, and even models within one brand, do not always link to each other, so check the manufacturer's compatibility guide before you buy. When several units in a linked set are close to their date, replacing them together keeps every alarm on the same schedule.
Smart or connected alarms follow the same rules as any other alarm. An app that sends phone alerts does not extend the life of the sensor, so a connected alarm still needs to be checked against its manufacture date.
On cost, think in tiers rather than price tags. Basic battery alarms cover the core job. Mid-tier models add sealed long-life batteries, combined smoke and CO sensing, or voice alerts. Connected models add phone notifications. Every tier has an expiry date, and none of them is exempt from the monthly test.
What to do with an expired alarm
Take the old alarm down and replace it on the same day. A gap in coverage, even for a week, is the outcome the whole schedule is trying to avoid.
For disposal, follow the instructions in the manual and your local waste rules. Your local fire department is the right contact for questions about local alarm requirements, which vary by location, and a replacement is a good time to ask whether your home's current setup meets them.
Frequently Asked Questions
How do I know when to replace smoke detectors?
Check the date of manufacture on the back of each alarm. The NFPA says to replace all smoke alarms when they are 10 years old. Replace sooner if the model's manual gives a shorter life, if the alarm fails a test, or if it gives an end-of-life signal.
Do CO alarms last as long as smoke alarms?
Often not. Kidde says standalone CO alarms should be replaced every 7 to 10 years, based on the model. Combination smoke and CO alarms follow the model's rated life, which Kidde puts at 5 to 10 years from manufacture.
Does a 10-year battery mean I never need to replace the alarm?
No. The battery is built to last the life of the alarm, and then the whole alarm is replaced. The USFA says to replace sealed-battery alarms according to the manufacturer's instructions.
My alarm still works when I press the test button. Do I still need to replace it?
Yes, if it is past its date. Kidde says smoke sensors lose sensitivity over time and that replacement matters even if the test button still works.
Can I replace a hardwired smoke detector myself?
The USFA states that only qualified electricians should install hardwired smoke alarms. Battery alarms are a homeowner job; hardwired ones are work for an electrician.
I’m John Smith, the founder of Homegadgetguru.com. I use my IT background to extensively test home automation setups in my own house, giving you the hands-on, practical advice you need to build a smarter, more connected home.
