Smoke and Carbon Monoxide Detectors:
where they go, how long they last, and how to test them
They are the cheapest safety system in your house and the one almost nobody tracks. Heating season is the right time to check them, and the clocks changing on November 1 is the easiest reminder you will get all year.
The short version
- Smoke alarms belong inside every bedroom, outside each sleeping area, and on every level of the home, including the basement.
- Carbon monoxide alarms belong outside each sleeping area and on every level, especially if you have gas appliances, a fireplace, or an attached garage.
- Replace smoke alarms 10 years from the manufacture date on the back, even if they still pass the test button. CO alarms typically last 5 to 10 years.
- Press the test button monthly. Change batteries in replaceable-battery units at least once a year; the fall clock change is a good anchor.
- A steady chirp means a low battery or an alarm at end of life. Neither should be ignored or silenced by pulling the battery.
Ask most homeowners how old their smoke detectors are and you will get a shrug. That is understandable. Detectors come with the house, they sit quietly on the ceiling, and the only time anyone thinks about them is when one starts chirping at 3 a.m.
But they are also the one home system where the stakes are measured in minutes, not dollars. The good news is that getting them right takes about an hour and very little money. Here is what to check.
What actually matters
If a carbon monoxide alarm sounds, do not try to find the source. Get everyone, including pets, outside into fresh air, then call 911 or your fire department from outside. Headache, dizziness, or nausea that improves when you leave the house is a sign to take it seriously.
Where Smoke Alarms Should Go
Older homes were often built with one or two detectors in a hallway. Current guidance from the National Fire Protection Association calls for more coverage than that, and many local codes now require it when a home is sold or renovated.
Inside every bedroom. Bedroom doors are usually closed at night, and a closed door can keep a hallway alarm from waking someone in time.
Outside each sleeping area. One in the hallway that connects the bedrooms, so a fire elsewhere in the house is caught before it reaches them.
On every level, including the basement. Basements are where furnaces, water heaters, dryers, and storage live. Put the basement alarm on the ceiling near the bottom of the stairs.
High, and away from the kitchen and bathrooms. Mount on the ceiling or high on a wall, since smoke rises. Keep alarms at least 10 feet from the stove to cut down on nuisance alarms, and away from bathroom doors where shower steam can set them off.
Where Carbon Monoxide Alarms Should Go
Carbon monoxide is odorless and colorless, and it comes from anything that burns fuel: a gas furnace, a gas water heater, a fireplace, a gas range, or a car idling in an attached garage. Heating season is when CO exposure peaks, because those appliances are running and the windows are closed.
Put a CO alarm outside each sleeping area and on every level of the home. If you have no fuel-burning appliances and no attached garage, your risk is lower, but CO alarms are inexpensive and many states require them in any home that is sold. One more thing worth knowing: a smoke alarm does not detect carbon monoxide unless the label specifically says it is a combination smoke and CO unit.
How Long They Actually Last
This is the part almost everyone misses. Detectors wear out even when they seem fine.
Smoke alarms: 10 years. Replace the whole unit 10 years after the manufacture date printed on the back. The test button checks the horn and the battery, not how sensitive the sensor still is, so an old alarm can pass the test and still be slow to respond to real smoke.
Carbon monoxide alarms: 5 to 10 years. It depends on the model. Most list a replace-by date on the back, and many newer units chirp or show an end-of-life message when the sensor expires.
Hardwired alarms wear out too. Alarms wired into the house have a backup battery and the same sensor lifespan. Replacing one is usually a simple swap of the same brand, since the wiring connector stays in place.
If you just bought a home, assume the detectors are as old as the last renovation until you check. Twist one off its base and look at the date. If there is no date at all, it is old enough to replace.
The Twice-a-Year Routine
Daylight saving time ends on Sunday, November 1 this year, and it starts again in March. Those two weekends make an easy schedule for the bigger checks. Here is the whole routine:
Monthly: press the test button. Every alarm, every month. It takes two minutes. If the sound is weak or nothing happens, change the battery and test again.
Twice a year: change batteries and vacuum. For alarms with replaceable batteries, put in fresh ones at least once a year, and use the clock change as your reminder. While you are up there, vacuum the vents gently; dust and cobwebs cause false alarms.
Once a year: check the dates. Read the manufacture date on every unit. Write down which ones are due in the next year so you can replace them on your schedule instead of after a chirp in the middle of the night.
Consider interconnected alarms. Interconnected alarms all sound when any one of them detects smoke, so a fire in the basement wakes someone upstairs. Newer homes are often hardwired this way, and wireless interconnected sets are available for older homes.
What a Chirp Is Telling You
A single short chirp every 30 to 60 seconds almost always means a low battery. Swap it and the chirp should stop. If the alarm has a sealed 10-year battery, or a fresh battery does not fix it, the unit is telling you it has reached end of life. Replace it rather than taking it down and leaving a gap on the ceiling.
Detectors chirp most often at night because the house cools down, and a weak battery loses voltage as it gets colder. That is also why it pays to replace batteries on a schedule instead of waiting for the chirp.
Part of a Bigger Heating-Season Checklist
Detectors are the last line of defense, not the first. The other half of heating-season safety is making sure the equipment that produces heat and carbon monoxide is working the way it should: an annual furnace service, a chimney inspection if you burn wood, and clear vents outside. If your heat is giving you trouble this month, our guide to what to check when your furnace won't turn on walks through it in order, and the fall home maintenance checklist covers the rest of the season.
The hard part with detectors is never the work itself. It is remembering how old each one is and when it is due. BTLR keeps that kind of detail for you. Upload your inspection report and your home's systems, safety items included, get tracked in one place with reminders that arrive before something is due, rolled into a single BTLR Score that shows what needs attention next.
Frequently Asked Questions
How often should smoke detectors be replaced?
Every 10 years from the date of manufacture, which is printed on the back of the unit. That applies even if the alarm still beeps when you press the test button. The sensors inside lose sensitivity with age, so a 12-year-old alarm can pass a button test and still respond slowly to real smoke.
How long do carbon monoxide detectors last?
Most carbon monoxide alarms last 5 to 10 years depending on the model, and many newer units chirp or display an end-of-life warning when the sensor expires. Check the manufacture date and the replacement date printed on the back, and replace the whole unit when it reaches that date.
Where should carbon monoxide detectors be placed?
Outside each sleeping area and on every level of the home, including the basement. If you have an attached garage or fuel-burning appliances like a gas furnace, water heater, or fireplace, those detectors matter most. Follow the manufacturer's height guidance printed on the unit.
Why is my smoke detector chirping?
A single chirp every 30 to 60 seconds almost always means a low battery. If a fresh battery does not stop it, or the unit is sealed with a 10-year battery, the alarm has reached end of life and needs to be replaced. Check the manufacture date on the back to confirm.
Does a smoke detector also detect carbon monoxide?
Not unless it says so. Smoke alarms and carbon monoxide alarms use different sensors. Combination units exist and are labeled as smoke and CO alarms. If the label only mentions smoke, you need a separate carbon monoxide alarm.
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