Where CO alarms go: every level, outside sleeping areas.
Carbon monoxide is colorless and odorless. The placement rule from federal safety agencies is simple enough to remember: put a working CO alarm on every level of the home and outside sleeping areas. Then keep smoke alarms where smoke guidance says they belong.
Our take: install battery-operated CO alarms or CO alarms with battery backup on every level and outside sleeping areas, and interconnect them if you can so when one sounds, they all sound. Follow the manufacturer for mounting height. Skip kitchens and the wall above fuel-burning appliances, which CPSC does not recommend. Keep separate smoke alarms inside bedrooms; CO guidance centers on outside sleeping areas, and combination units exist if you want both sensors in one device. Test at least monthly (EPA’s battery-unit page also says weekly), replace batteries on schedule, and replace the whole alarm per the maker or, per CDC, every 5 years if the maker does not say otherwise. If it sounds, get to fresh air outside and call 911 before you hunt for a source. [S1] [S2] [S3] [S4] [S5]
Where the CO pin goes
Where they go
CPSC says to install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas, and that interconnected alarms are best because when one sounds, they all sound. NFPA uses the same core map: a central location outside each sleeping area and on every level, with interconnection for the best protection. CDC says to install battery-operated or battery back-up detectors near every sleeping area. EPA adds that each floor needs a separate detector, and that if you only have one, place it near sleeping areas and make sure it is loud enough to wake you. [S1] [S5] [S3] [S4]
CPSC’s Invisible Killer guide repeats every level and outside sleeping areas, and says the alarms should be certified to the most recent UL, IAS or CSA standard for CO alarms. CPSC’s Q&A points to UL 2034 for consumer CO alarms. NFPA says to choose an alarm with the label of a recognized testing laboratory, and to follow the manufacturer for placement and mounting height. Hard-wired or plug-in units should have battery backup. [S7] [S1] [S5]
Choose your layout to highlight matching guidance.
One level still needs outside-bedroom coverage
Put a CO alarm outside the sleeping area on that level. If the home is only one living level, that single level is your “every level” requirement. Still place it where it can wake sleepers, not buried in a closet. EPA says if you only buy one detector, put it near sleeping areas and confirm it is loud enough to wake you. Interconnect if you add more later. [S2] [S4]
Alarm on each level, plus hallway outside bedrooms
CPSC and NFPA both call for every level and outside sleeping areas. That usually means at least one on the bedroom floor in a central hall, plus another on the main living level, even if nobody sleeps downstairs. Interconnected units are best so a downstairs reading still wakes upstairs sleepers. [S1] [S5]
Basements and attics only if someone sleeps there
CPSC says CO alarms should not be installed in attics or basements unless they include a sleeping area. If a basement bedroom or attic loft is used for sleep, treat that space like any other sleeping level: CO outside that sleeping area, and keep smoke coverage inside bedrooms per smoke guidance. Unused storage attics or unfinished basements without sleeping are not the placement target in CPSC’s CO Alarms page. Local code or the manufacturer can still add requirements, so follow those too. [S2]
Put the first one near sleepers, then fill levels
EPA says each floor needs a separate detector, and that if you are getting a single carbon monoxide detector, place it near the sleeping areas and make certain the alarm is loud enough to wake you up. Plan the next purchase for any other living level. A CO alarm is not a substitute for proper appliance use and upkeep. [S4] [S1]
Start with sleepers, then walk every level
Walk the house once: mark every place people sleep, then every living level. Put CO coverage outside each sleeping area and on each living level. Skip unused attic/basement space unless someone sleeps there. Check local building-code rules (many jurisdictions require CO alarms) and follow the booklet that came with the alarm for height and wiring. [S1] [S2] [S5]
These branches summarize agency guidance. They are not a home inspection of your floor plan.
Avoid these spots
CPSC says to install CO alarms according to the manufacturer’s instructions, into a plug-in receptacle or high on the wall, and to avoid locations near heating vents or that can be covered by furniture or draperies. CPSC does not recommend installing CO alarms in kitchens or above fuel-burning appliances. Keep the path clear so the alarm can sample room air and so you can hear it. [S1] [S6]
Smoke vs CO: both, not either/or
EPA says CO alarms and smoke alarms detect different hazards, so install both UL Listed CO alarms and smoke detectors. CPSC’s CO Alarms page draws the placement line clearly: smoke alarms go on every level, outside sleeping areas and inside each bedroom; CO alarms go on each level and outside sleeping areas, and should not go in attics or basements unless those spaces include a sleeping area. Combination smoke and CO alarms are available. Do not drop bedroom smoke coverage because you added a hallway CO alarm. [S4] [S2]
Test & replace
CPSC recommends testing smoke and CO alarms monthly and replacing batteries annually. NFPA says to test CO alarms at least once a month and replace them according to the manufacturer’s instructions. EPA’s placement FAQ says that if the unit operates off a battery, test the detector weekly and replace the battery at least once a year, and to follow the manufacturer’s use and care booklet exactly. CDC suggests checking CO detector batteries when you change clocks in spring and fall, considering a digital readout that shows the highest level, and replacing the detector following the manufacturer’s instructions or every 5 years, with a calendar reminder. CPSC notes that the test button checks circuitry, not sensor accuracy, and that replacement age is in the product literature; newer alarms may have an end-of-life signal. [S2] [S5] [S4] [S3] [S1] [S6]
Enter the number of months to compare it with monthly test guidance.
This tool only compares your number with published test intervals. It can’t hear the alarm, and nothing you type is saved or sent.
Where the sources pull in different directions
How often to test. CPSC and NFPA say at least monthly. EPA’s battery-unit FAQ says test weekly and replace the battery at least once a year. We present both: follow the manufacturer’s booklet, treat monthly testing as the common agency floor, and note EPA’s weekly line for battery units if you want a tighter habit. [S2] [S5] [S4]
Smoke inside bedrooms vs CO outside them. CPSC puts smoke alarms inside each bedroom and CO alarms outside sleeping areas (plus every level). Combination units exist. Keep both hazard protections; do not skip bedroom smoke alarms. [S2] [S4]
Basements and attics. “Every level” means living and sleeping levels. CPSC says do not install CO alarms in attics or basements unless they include a sleeping area. If someone sleeps there, cover that sleeping area; if it is unused storage space, CPSC’s CO Alarms page does not put a CO alarm there by default. Still follow local code and the manufacturer. [S2]
When to replace the whole alarm. Most agency pages say follow the manufacturer. CDC also offers “or every 5 years” and a calendar reminder. Use the maker’s life span when it is printed; otherwise CDC’s five-year outer reminder is a useful backup. Newer units may chirp an end-of-life signal. [S3] [S1] [S6]
What to do when it sounds. CPSC’s primary path is: immediately move outside to fresh air, call 911 or the fire department, do a head count, and do not reenter until responders say it is OK. Do not try to find the source first. NFPA says immediately move to a fresh air location outdoors or by an open window or door, account for everyone, call for help from fresh air, and stay there until emergency personnel. We lead with CPSC’s get-outside-and-call path; NFPA’s open-window/door wording is another way to reach fresh air, not a reason to delay calling for help. [S1] [S5]
If it sounds
Never ignore a CO alarm. CPSC says: do not try to find the source of the CO; immediately move outside to fresh air; call emergency services, the fire department or 911; after calling, do a head count; do not reenter until responders give permission. If a malfunctioning appliance is the source, do not operate it again until trained personnel service it. CPSC’s Information Center and CDC both say that if you suspect CO poisoning, get outside to fresh air immediately, then call 911. Symptoms can look flu-like (headache, dizziness, weakness, nausea, vomiting, chest pain, confusion), and people who are sleeping can die before they notice symptoms. [S1] [S8] [S3]
Placement and upkeep checklist
Before you move an alarm
Can I put a CO alarm only in the furnace room?
CPSC does not recommend installing CO alarms in kitchens or above fuel-burning appliances. Put coverage on every level and outside sleeping areas instead, and keep up with appliance maintenance. [S1]
Do I still need smoke alarms if I buy a combination unit?
Combination smoke and CO alarms are available, per CPSC. EPA still says the two sensors address different hazards. Whatever hardware you choose, keep smoke protection inside bedrooms and CO protection outside sleeping areas and on each level. [S2] [S4]
How far should a generator be from the house?
CDC says operate generators outdoors more than 20 feet from windows, doors and vents, never inside a home or garage even with doors and windows open, and to use a battery-powered or battery-backup CO detector when a generator is in use. CPSC’s Information Center and Invisible Killer guide match the outdoors-and-away message (at least 20 feet, exhaust facing away). [S3] [S8] [S7]
Does the test button prove the sensor works?
No. CPSC says the test button tests whether the circuitry is operating correctly, not the accuracy of the sensor. Follow the manufacturer for replacement age. [S1]
What if I feel flu-like symptoms at home but feel better away?
That pattern is one of the Invisible Killer clues CPSC lists. Get to fresh air, seek medical attention, and tell clinicians you suspect CO. Do not wait for an alarm if you already feel sick and suspect CO. [S7] [S3]
Are brands ranked here?
No. This guide cites agency placement and response rules only. CPSC says a CO alarm is no substitute for proper use and upkeep of fuel-burning appliances, and we do not rank or endorse models. [S1]
Sources
CPSC, CDC, EPA and NFPA guidance, checked October 16, 2026, and rechecked October 21, 2026. Recommendations here are editorial interpretation of that guidance; no hands-on results are claimed.
- S1 · CPSC: Carbon Monoxide Questions and Answers
- S2 · CPSC: CO Alarms
- S3 · CDC: Carbon Monoxide Poisoning Basics
- S4 · EPA: Where should I place a carbon monoxide detector?
- S5 · NFPA: Carbon Monoxide Safety
- S6 · CPSC: Carbon Monoxide Fact Sheet
- S7 · CPSC: The Invisible Killer (PDF, May 2024)
- S8 · CPSC: Carbon Monoxide Information Center