Where to put smoke and carbon monoxide detectors

Smoke alarms and carbon monoxide (CO) alarms only help if they're in the right places and still working. This guide covers where each one goes, where not to put them, the kinds you can buy and how long they last. It follows guidance from the National Fire Protection Association (NFPA), the U.S. Fire Administration (USFA) and the Consumer Product Safety Commission (CPSC).

The words "detector" and "alarm" get used for the same thing. We use "alarm" here.

Where smoke alarms go

The NFPA says to install smoke alarms [1]:

Inside every bedroom
Outside each sleeping area, such as the hallway outside the bedrooms
On every level of the home, including the basement

Large homes may need extra alarms [1]. In a basement, an alarm should be close to the stairway to the floor above [4].

The NFPA and USFA both recommend interconnected alarms, so when one sounds, they all sound [1, 3]. That way an alarm in the basement can wake someone sleeping upstairs.

Why so many? The NFPA says roughly 3 out of 5 home fire deaths happen in homes with no smoke alarms or no working smoke alarms [1].

Mounting height

Smoke rises, so put smoke alarms on the ceiling or high on a wall [1]. The City of Lafayette, Indiana, gives these common measurements in its fire safety guidance [4]:

On a ceiling: at least 4 inches from the wall
On a wall: the top of the alarm 4 to 12 inches below the ceiling
On a sloped ceiling: on the high side

Always check the manufacturer's instructions too, since they can differ by model [3].

Spots to avoid

Some spots cause false alarms or slow the alarm down. Avoid these [1, 4]:

Within 10 feet of the stove. Cooking smoke sets off alarms that are too close. The NFPA says alarms installed closer than that must be photoelectric or have a hush button [2].
Within 3 feet of a bathroom door with a tub or shower, because steam can trigger them.
Near windows, and within 3 feet of heating or cooling vents. Moving air can blow smoke away from the alarm.
Between an air return and a bedroom door.
Garages, furnace rooms, workshops and other spaces that get colder than 32°F or hotter than 100°F.
Places you can't reach to test, like the top of a tall foyer or high over a stairway.

If a home report says an alarm is in the wrong location, it's usually one of these. See moving smoke detectors.

Where CO alarms go

Carbon monoxide is a gas you can't see or smell. It can come from equipment that burns fuel, such as gas furnaces, water heaters, stoves and fireplaces, and from cars or generators running in or near the home. The CPSC says more than 200 people in the U.S. die every year from accidental CO poisoning not related to fires [5].

The CPSC says to install CO alarms that run on batteries or have a battery backup [5]:

On every level of the home
Outside sleeping areas

Like smoke alarms, interconnected CO alarms are best [5]. Guidance from the North Carolina Office of State Fire Marshal says CO alarms belong in any home with heating or cooking equipment that burns fuel, in a central spot outside each sleeping area on every level. It also says to follow the manufacturer's instructions for placement and mounting height [6]. CO alarm instructions vary more than smoke alarm instructions, so read them before you mount one.

On a report, a home with fuel-burning equipment and no CO alarms shows up as CO detectors needed.

If a CO alarm sounds and anyone feels sick, with a headache, dizziness, nausea, sleepiness or confusion, the CPSC says to get outside to fresh air right away and call 911 [5]. Have a licensed HVAC or gas professional check fuel-burning equipment before you go back to using it.

Battery, hardwired or combination?

Power source. The USFA describes three choices [3]:

Type	How it's powered	Upkeep
Sealed 10-year battery	A battery you can't replace	Replace the whole alarm at the end of its life
Replaceable battery	Usually a 9-volt battery	Replace the battery at least once a year
Hardwired	The home's electrical system, usually with a backup battery	Replace the backup battery about once a year

The USFA says only qualified electricians should install hardwired smoke alarms [3]. If your home already has hardwired alarms, have the electrician replace them with compatible models so they stay connected to each other.

Smoke sensor type. Smoke alarms use two main types of sensor: ionization and photoelectric. The NFPA says photoelectric alarms are generally more responsive to smoldering fires [2]. Both the NFPA and USFA recommend having both kinds, or dual-sensor alarms that have both [2, 3].

Combination alarms. Some alarms detect both smoke and CO in one unit [1]. They can cut down on the number of devices on the ceiling. Make sure each one is in a spot that works for both: a combination alarm still needs to meet the smoke alarm placement rules above.

Testing and replacing

Test every alarm once a month with the test button [1, 3, 6].
Replace smoke alarms when they are 10 years old [1, 3]. The manufacture date is printed on the back [7].
Replace CO alarms on the manufacturer's schedule. Kidde, a major alarm maker, says its CO alarms should be replaced every 7 to 10 years depending on the model [7].
Know the end-of-life signal. Many alarms chirp or show an error when they reach the end of their life. Kidde says changing the battery won't stop an end-of-life warning: the alarm has to be replaced [7].

Never take an alarm down or cover it to stop false alarms. On a report, that shows up as a taped smoke detector. Move the alarm or replace it with a better-suited type instead.

Rules differ by place

Building codes for new homes and remodels, and laws for rental homes, set their own alarm requirements. These vary by state and city, including across North Carolina, Georgia and Texas. For what's required in your home or project, check with your local building department or fire marshal.

Next steps

Walk through your home with this list: every bedroom, outside each sleeping area, every level, and a CO alarm on every level if anything in the home burns fuel. Check each alarm's date on the back.

For what HomeCloud reports price this work at, see smoke detectors and carbon monoxide detectors. If you're also looking at air quality, read radon test results explained. HomeCloud is a licensed general contractor that evaluates homes and prices and does the repairs, and you can request an estimate.