The essentials in 30 seconds
Carbon monoxide (CO) is a poisonous gas released when fuel burns: furnaces, water heaters, ranges, room heaters, fireplaces, generators and cars. Fit battery-operated or battery backup CO alarms on every level of the home and outside each separate sleeping area, choose models listed to UL 2034 (look for a UL or CSA listing on the box), test them once a month and replace them on the manufacturer's schedule or when the end-of-life signal sounds. Four beeps and a pause means CO: get everyone outside and call 911 from the fresh air. One chirp every 30 to 60 seconds usually means the battery is low.
- A CO alarm is a backup; yearly servicing of fuel-burning appliances is what prevents the gas in the first place.
- A smoke alarm and a CO alarm detect different things; a combination unit does both.
- Write the installation date on each alarm so you know when to replace it.
What a carbon monoxide detector does and does not do
A CO alarm contains a sensor that reacts to carbon monoxide building up in the air and sounds before levels become life-threatening. It is a different device from a smoke alarm, which reacts to smoke from a fire, and from a natural gas or propane detector, which reacts to a fuel leak. Combination smoke and CO alarms put both sensors in one case and use different sound patterns for each.
The gas comes from anything that burns fuel: a furnace or boiler with a cracked heat exchanger or blocked flue, a water heater, a gas range or oven, a room heater, a fireplace or wood stove, charcoal burned indoors, a portable generator or a car left running in an attached garage. CPSC and NFPA stress that proper installation, operation and maintenance of fuel-burning appliances is the most important factor in reducing CO poisoning, so the alarm is your safety net, not the fix.
Only about two out of five US households reported having a working CO alarm in the American Housing Survey quoted by CPSC and NFPA, compared with almost every home having a smoke alarm.
Where to put a carbon monoxide detector
| Location | Why | Notes |
|---|---|---|
| Outside each separate sleeping area | Wakes people who would otherwise sleep through rising CO | If bedrooms are in different parts of the home, each area needs its own alarm |
| Every level of the home | CO from a basement furnace or an attached garage spreads upward and through the house | Include a finished basement where people spend time |
| Rooms with a fuel-burning appliance (UK guidance) | Gives early warning close to the source | UK guidance: about 1 to 3 meters from the appliance, at head height on a wall or shelf |
Read the manual before you drill: manufacturers give minimum distances from cooking appliances, bathrooms, heating and cooling vents, fans and windows, because steam, drafts and cooking fumes can cause nuisance alarms or dilute the air reaching the sensor. Keep alarms out from behind curtains and furniture.
How many CO detectors you need
Count one per level plus one outside each separate sleeping area. For example, a two-story house with a furnace in the basement and all bedrooms along one upstairs hallway needs at least three: basement, main floor and the upstairs hallway. If the main bedroom is downstairs and the children's rooms are upstairs, add one outside the downstairs bedroom. An apartment on one level with one bedroom corridor needs at least one, and more if it has a fireplace or gas appliances in rooms far from the bedrooms.
Which type of CO detector to buy
| Type | Good for | Watch out for |
|---|---|---|
| Battery-operated | Any room, quick to fit, works in a power outage | Replace batteries on the manufacturer's schedule; some have sealed long-life batteries |
| Plug-in with battery backup | Rooms with a free outlet at a sensible height | Choose one with battery backup so it keeps working when the power goes out |
| Hardwired with battery backup | New builds and remodels, often interconnected | Installed by an electrician; the battery covers outages |
| Combination smoke and CO | Hallways outside bedrooms, saving a second device | Learn both sound patterns; replace the whole unit at end of life |
| With digital readout | Seeing the peak CO level since the last reset | The display is extra information; the alarm still decides when to warn you |
| Interconnected (wired or wireless) | Large or multi-story homes | When one alarm sounds, they all sound |
Whatever type you choose, CPSC recommends alarms that meet the UL 2034 safety standard, with a UL or CSA listing on the packaging. In the UK the matching standard is BS EN 50291. Battery backup matters most during storms, when people run generators and the risk of CO poisoning rises; our power outage guide covers generator safety.
What carbon monoxide detector beeping means
Most alarms follow the same basic code, but fault and end-of-life signals vary between manufacturers. Keep the manual, or note the model number, so you can check the exact pattern.
| What you hear or see | What it usually means | What to do |
|---|---|---|
| Four beeps, a pause, repeating | CO detected | Get everyone outside and call 911 from fresh air |
| Three beeps, a pause, repeating (combination units) | Smoke or fire | Get out, stay out and call 911 |
| One chirp every 30 to 60 seconds | Low battery | Replace the battery, or the unit if the battery is sealed |
| Chirping that continues after a new battery | End of life | Replace the whole alarm |
| Other chirp patterns, flashing or colored lights | Fault, end of life or memory of a past alarm, depending on the model | Check the manual or the label on the back |
UL 2034 requires CO alarms to give an end-of-life warning, so a chirping alarm that is several years old is often telling you it needs replacing. If you are unsure whether a sound is a chirp or an alarm, treat it as an alarm and go outside first.
How to test a CO detector and how long it lasts
-
01
Test monthly
Press and hold the test button until the alarm sounds. CPSC and NFPA advise a test once a month; ask someone to stand in the farthest bedroom to check it can be heard there.
-
02
Check batteries twice a year
CDC suggests checking batteries when you change the clocks in spring and fall. Replace them on the manufacturer's schedule, and straight away if the alarm chirps.
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03
Keep it clean
Dust the vents gently with a soft brush or vacuum attachment, and keep sprays, paint and cleaning products away from the sensor.
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04
Check the date
Look on the back for the manufacture or replace-by date. CDC advises replacing CO detectors according to the manufacturer's instructions or every 5 years, and setting a reminder when you install one.
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05
Replace at end of life
Many newer alarms announce their end of life with a distinct signal. Replace the whole unit then, or sooner if it fails a test or the date label is missing.
What to do when your CO alarm goes off
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01
Get out
Move everyone, including pets, outdoors or to an open doorway away from the source. Leave the door open behind you.
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02
Call for help
Call 911 from outside. Explain that a CO alarm is sounding and whether anyone has symptoms.
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03
Count heads
Make sure everyone is accounted for. Tell responders if anyone is still inside; let them go back in.
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04
Watch for symptoms
Anyone with headache, dizziness, nausea, confusion or chest pain needs medical help. In the UK, call 999 for severe symptoms and NHS 111 for milder ones; the NHS advises against driving yourself to A&E.
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05
Stay out
Wait until the fire department or utility says the home is safe to enter.
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06
Get the source fixed
Have the suspected appliance, flue or chimney checked by a qualified technician before using it again. If the alarm went off with all appliances off, mention it to responders.
If you smell gas rather than hearing a CO alarm, that is a different emergency: leave without touching switches and call the gas utility from outside. In the UK, the national gas emergency number is 0800 111 999.
Carbon monoxide poisoning symptoms
CDC lists the most common symptoms as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, often described as flu-like. High levels can cause fainting and death, and people who are asleep or intoxicated can die before they notice anything. At extremely high levels, CPSC notes that a person can lose consciousness suddenly without milder symptoms first.
One clue the NHS points out: symptoms that ease when you go outside and return when you come back into a particular room.
Everyone is at risk, but CDC says infants, older adults and people with chronic heart disease, anemia or breathing problems are more likely to get sick. Each year in the US, CDC counts more than 400 deaths from unintentional CO poisoning not linked to fires, more than 100,000 emergency department visits and more than 14,000 hospital admissions.
How to prevent carbon monoxide at home
- Service fuel-burning appliances yearly. CPSC and NFPA advise an annual inspection of the heating system by a qualified professional, including the vents, chimneys and flues. Wood-burning fireplaces need their own check; see our chimney cleaning guide.
- Generators outside only. Keep them and any other engine at least 20 feet from the house, never in a garage, basement, crawl space, shed or porch.
- No charcoal or camp stoves indoors, and never heat the home with a gas range or oven.
- No idling cars in an attached garage, even with the door open.
- Keep vents and flues clear and ask the technician to check that exhaust leaves the home properly through vents, chimneys and flues. A leaking or poorly vented water heater is worth a technician's visit.
CO alarms in the UK
In the UK the device is usually called a CO alarm. The NHS advises fitting one in each room with a gas, oil, coal or wood-burning appliance and having boilers, heaters and cookers serviced regularly by a qualified, registered engineer. In England, since 1 October 2022, private and social landlords must fit a CO alarm in any room used as living accommodation that contains a fixed combustion appliance (gas cookers excluded) and must repair or replace alarms once told they are faulty. The government guidance suggests head height on a wall or shelf, about 1 to 3 meters from the appliance, and notes that sealed-for-life batteries are the better option for battery models.
Carbon monoxide detectors: frequently asked questions
Where should a carbon monoxide detector be placed?
On every level of the home and outside each separate sleeping area, following the manual for distances from cooking appliances, bathrooms and vents. UK guidance also puts one in each room with a fuel-burning appliance.
How many carbon monoxide detectors do I need?
At least one per level plus one outside each separate sleeping area. A two-story house with a basement and one bedroom hallway usually needs at least three.
What does it mean when a carbon monoxide detector beeps 4 times?
Four beeps and a pause, repeating, is the standard CO alarm. Get everyone outside and call 911 from the fresh air.
Why is my carbon monoxide detector chirping?
A single chirp every 30 to 60 seconds usually means a low battery. If it keeps chirping after a new battery, the alarm has reached its end of life.
How long do carbon monoxide detectors last?
Check the date and manual for your model. CDC advises replacing them according to the manufacturer's instructions or every 5 years.
What should I do if my CO alarm goes off?
Get everyone out, call 911 from outside and stay out until responders say it is safe. Have the appliance checked before using it again.
Can a smoke detector detect carbon monoxide?
Only a combination smoke and CO alarm can. A standard smoke alarm reacts to smoke, so check the label for both functions.
What are the first signs of carbon monoxide poisoning?
Headache, dizziness, weakness, nausea and confusion, often mistaken for flu. Get to fresh air and call 911 if you suspect CO.
Sources
Official safety and health guidance, checked on 27 October 2026.
- CDC · Carbon monoxide poisoning basics ↗Symptoms, higher-risk groups, risk during sleep, yearly deaths and hospital figures, detectors near sleeping areas, battery checks and replacement every 5 years.
- CPSC and NFPA · Carbon monoxide alarms talking points ↗Every level and outside each separate sleeping area, UL 2034, battery backup for plug-in alarms, monthly tests, end-of-life signals, calling 911, annual appliance inspection and the American Housing Survey figure.
- NC State Extension · Are CO alarms in a home a good idea? ↗Alarm types, battery backup for wired units, UL 2034 end-of-life warning and leaving the home when the alarm sounds.
- Purdue Extension · Learn the sounds of fire safety ↗Three beeps for smoke, four beeps for CO, low-battery chirps and end-of-life chirping, based on NFPA material.
- NHS · Carbon monoxide poisoning ↗UK symptoms, when to call 999 or 111, alarms in rooms with fuel-burning appliances and the gas emergency number.
- GOV.UK · Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022: guidance ↗Landlord duties in England from 1 October 2022, fixed combustion appliances, placement at head height 1 to 3 meters from the source, sealed batteries.
Check your CO alarms today
Press the test button on each one, read the date on the back and note when it needs replacing.