Where CO alarms go
NFPA's 2026 educational messages say to install and maintain carbon monoxide alarms in three kinds of places:
- Every room or area where a person sleeps.
- Outside each separate sleeping area, such as the hallway that serves the bedrooms.
- Every level of the home, including the basement.
For the exact spot and mounting height, follow the manufacturer's instructions; that is where NFPA sends you too. Local codes can add their own wording. A New Jersey township, for example, requires alarms in the immediate vicinity of the sleeping area and lets them go on the wall, the ceiling or another location the manufacturer specifies. Pick an alarm listed by a nationally recognized testing laboratory, which means it was tested to safety standards.
How many alarms a home needs
Count one per sleeping room, one outside each separate sleeping area, and one on each level that has neither. The examples assume the hallway alarm also covers its own level.
| Home | Rooms | Outside | Other levels | Total |
|---|---|---|---|---|
| Apartment, 2 bedrooms off one hall | 2 | 1 | 0 | 3 |
| Ranch, 3 bedrooms in one wing, no basement | 3 | 1 | 0 | 4 |
| Ranch, split bedrooms, basement | 3 | 2 | 1 | 6 |
| Two stories plus basement, 4 bedrooms upstairs | 4 | 1 | 2 | 7 |
A split-bedroom ranch needs two hallway alarms because its bedrooms form two separate sleeping areas. If you are drawing a floor plan to mark the spots, the square footage calculator helps with the room sizes.
Interconnected and combination alarms
NFPA says the best protection comes from CO alarms that are interconnected throughout the home, so when one sounds, they all sound. A combination smoke and CO alarm is one device that does both jobs, and NFPA says to install it following the directions for smoke alarm installation. That means the distances from kitchens, bathrooms and vents in the smoke alarm placement guide apply to a combination unit as well.
A plain CO alarm does not detect smoke, and a plain smoke alarm does not detect CO. NFPA is explicit that neither is a substitute for the other.
Battery, plug-in or hardwired
The New Jersey township rule lists three types: battery operated, hard-wired and plug-in. Each has a trade-off.
| Type | Good for | Watch for |
|---|---|---|
| Battery | Any spot, no outlet needed | Battery changes and the low-battery chirp |
| Plug-in | Rooms with a free outlet at the right spot | The outlet location sets the alarm location |
| Hardwired | Permanent power from a house circuit | Rules on which circuit may feed it |
For hardwired alarms the township rule says the power comes from a dedicated branch circuit or the unswitched part of a lighting or power circuit, and it must not be possible to cut it with a switch other than a circuit breaker, or with a ground-fault circuit interrupter (see GFCI vs AFCI for what those devices do). Where a deaf or hard-of-hearing person lives, it requires a visual alarm.
Testing and replacement
NFPA says to test CO alarms at least once a month and replace any that fail to respond. The sensors have a limited life, so replace the alarm according to the manufacturer's instructions or when the end-of-life signal sounds. Clean it the way the manufacturer's instructions say. The City of DuPont fire department in Washington gives a typical window of every 5 to 7 years depending on the manufacturer, and batteries at least once a year.
| Installed | Replace between | In 2026 |
|---|---|---|
| 2018 | 2023 and 2025 | Past the window |
| 2019 | 2024 and 2026 | In the window now |
| 2020 | 2025 and 2027 | In the window now |
| 2021 | 2026 and 2028 | In the window now |
| 2022 | 2027 and 2029 | Not before 2027 |
| 2024 | 2029 and 2031 | Not before 2029 |
| 2026 | 2031 and 2033 | Not before 2031 |
The manual for your model wins over this table. Write the install date on the alarm when you mount it.
What the sounds mean
NFPA asks everyone in the home to know the difference between a CO alarm, a smoke alarm and a low battery. Its Fire Prevention Week sheet describes the patterns:
| Sound | Meaning |
|---|---|
| Four loud beeps, repeating | CO is present |
| Three loud beeps, repeating | Smoke or fire |
| One chirp every 30 or 60 seconds | Low battery |
| Chirping after a new battery | End of life, replace the alarm |
A low-battery chirp at those intervals comes 120 or 60 times an hour, far slower than an alarm pattern. If you are not sure which you hear, time the gap with a countdown timer. End-of-life sounds vary by manufacturer, so look up your model's user guide.
If the alarm sounds
NFPA's instruction: go outside immediately, call 9-1-1 once you are out, and stay outside until emergency personnel say it is safe to go back in. If the low-battery signal sounds, replace the batteries or the device; if the CO alarm itself still sounds, get to fresh air and call 9-1-1.
Apartments, boats and travel
- Apartments. A home without its own fuel-burning appliance can still need an alarm. The New Jersey township rule requires one near the sleeping areas of a unit that has no fuel-burning appliance or attached garage of its own when a unit above or below it does.
- Boats. NFPA says to install CO alarms inside the boat and test them before each trip, again choosing a listed alarm.
- Hotels and trips. NFPA suggests choosing a hotel or motel that has smoke alarms, CO alarms and fire sprinklers, and packing a travel CO alarm.
One placement rule outside the house
Alarms are only part of the setup. NFPA says portable generators belong outdoors in well-ventilated areas at least 20 feet from all doors, windows and vent openings, never in an attached garage even with the door open. NFPA writes it as 20 feet (7 meters); the exact conversion is 6.1 m, which the length converter shows, so the metric figure rounds up.
Checklist
- An alarm in every sleeping room, outside each sleeping area and on every level.
- Listed by a nationally recognized testing laboratory.
- Interconnected if you can.
- Tested monthly, batteries changed on schedule, install date written on it.
- Everyone knows four beeps from three, and to go outside and call 9-1-1.
Frequently asked questions
Where should carbon monoxide alarms be installed?
NFPA says in every room or area where a person sleeps, outside each separate sleeping area, and on every level of the home, including the basement. Follow the manufacturer's instructions for the exact placement and mounting, and choose an alarm listed by a nationally recognized testing laboratory.
How often should CO alarms be replaced?
NFPA says CO alarm sensors have a limited life and the alarm should be replaced according to the manufacturer's instructions or when the end-of-life signal sounds. One city fire department puts the typical range at every 5 to 7 years, depending on the manufacturer. Write the install date on the alarm so you know when it is due.
What does a carbon monoxide alarm sound like?
NFPA describes a continuous set of four loud beeps for a CO alarm, compared with three loud beeps for a smoke alarm. A single chirp every 30 or 60 seconds means the battery is low. End-of-life sounds vary by manufacturer, so check the user guide for your model.
What should I do if the CO alarm goes off?
NFPA says to go outside immediately, call 9-1-1 once you are outside, and stay out until emergency personnel say it is safe to go back in. If only the low-battery chirp sounds, replace the batteries or the alarm.
Can a combination smoke and CO alarm replace both?
Yes, one device can do both jobs, but NFPA says to install combination alarms following the directions for smoke alarm installation, and to replace them on the manufacturer's schedule. A plain CO alarm is not a substitute for a smoke alarm, and a smoke alarm is not a substitute for a CO alarm.
Sources
- NFPA Educational Messages Desk Reference, 2026 edition, chapter 3 Carbon Monoxide (3.4 installation: listed alarms, every sleeping room, outside each sleeping area, every level including the basement, interconnection, combination alarms follow smoke alarm directions, not substitutes, travel alarm; 3.5 monthly test, limited sensor life, end-of-life signal, low-battery signal, cleaning; 3.9 generators 20 feet away; 3.10 alarms inside boats, tested before each trip; 5.1.1 hotels with CO alarms; 3.11 go outside and call 9-1-1)
- NFPA Fire Prevention Week sheet on alarm sounds, 2021, hosted by the Village of Penn Yan (four loud beeps for CO, three for smoke; chirp every 30 or 60 seconds for a low battery; end-of-life sounds vary by manufacturer)
- Plainsboro Township, New Jersey: Carbon monoxide alarms guidelines (battery, hard-wired or plug-in; hard-wired power not interruptible by a switch or GFCI; immediate vicinity of sleeping areas; units next to a unit with a fuel-burning appliance or attached garage; visual alarm for deaf or hard-of-hearing residents)
- City of DuPont, Washington: Smoke and carbon monoxide alarms (replace CO alarms every 5 to 7 years depending on the manufacturer; batteries at least once a year; four loud beeps with a pause)