Carbon Monoxide Detectors: Placement and Types

Short answer: Put a carbon monoxide (CO) alarm outside each sleeping area and on every level of your home, and choose one listed to UL 2034, the current safety standard for CO alarms.^{Source [1]Source [2]} Battery, plug-in with battery backup, hardwired and combination smoke/CO alarms all work if they meet that standard; interconnected alarms are best because they all sound when one does.^{Source [1]} Test each alarm monthly and replace it on the manufacturer's schedule.^{Source [2]Source [3]}

CO is a gas you can't see or smell, and an alarm is the only practical way to know it's building up. This guide is the starting point for our indoor air pages. It covers why CO matters, where to put alarms, how the types differ, what the numbers on the box mean and how to keep them working.

Why a CO alarm matters

CDC estimates that every year more than 400 Americans die from unintentional CO poisoning not linked to fires, more than 100,000 visit an emergency department and more than 14,000 are hospitalized.^{Source [3]} The Consumer Product Safety Commission (CPSC) attributes about 170 deaths a year to CO from non-automotive consumer products such as furnaces, ranges, water heaters, room heaters and generators.^{Source [1]} In 2023, U.S. fire departments responded to 82,245 CO incidents, according to NFPA.^{Source [2]}

The danger is that early symptoms look like the flu without a fever: headache, tiredness, shortness of breath, nausea and dizziness.^{Source [1]} People who are asleep or drunk can die before they notice anything.^{Source [3]} Infants, pregnant people, older adults and people with heart or lung disease or anemia can be harmed at lower levels.^{Source [2]Source [3]}

Where CO comes from at home

Any appliance or engine that burns fuel can produce CO. EPA lists sources including gas stoves, unvented kerosene and gas space heaters, back-drafting furnaces, gas water heaters, wood stoves and fireplaces, generators and cars idling in attached garages.^{Source [4]} Background levels are low: EPA says homes without gas stoves average 0.5 to 5 parts per million (ppm). Levels near a properly adjusted gas stove are often 5 to 15 ppm, and near a poorly adjusted one they may be 30 ppm or higher.^{Source [4]}

Where to put carbon monoxide detectors

The three agencies we checked agree on the basics:

• Outside each sleeping area. NFPA says a central location outside each sleeping area; CDC says near every sleeping area.^{Source [2]Source [3]} This is so the alarm can wake you.
• On every level, including the basement.^{Source [1]Source [2]}
• Wherever local code requires. Many states and cities require CO alarms, so check with your local building code official.^{Source [1]Source [2]}

Mounting height

There isn't one correct height. CO mixes with air, and manufacturers specify where their sensor works best, so follow the instructions in the box.^{Source [2]} Indiana's state fire safety guidance notes that about 5 feet above the floor may be recommended for some models.^{Source [5]} CPSC says alarms can go in a plug-in outlet or high on a wall.^{Source [1]}

Where not to put one

• Not in the kitchen, or directly above a furnace, water heater or other fuel-burning appliance. CPSC does not recommend these spots.^{Source [1]}
• Not next to heating vents, and not behind furniture or curtains that block airflow.^{Source [1]}

Indiana's guidance puts it as near, but not right next to, fuel-burning appliances.^{Source [5]} If you rent, our renter checklist covers what to ask your landlord about alarms and appliance servicing.

Types of carbon monoxide detectors

Most home CO alarms use an electrochemical sensor, which reacts to CO in the air.^{Source [6]} Where they differ is how they get power and whether they also detect smoke.

CPSC says plug-in and hardwired alarms should have battery backup, so they still work in a power outage, which is when generators and space heaters come out.^{Source [1]} Combination alarms are recommended especially in homes with gas appliances, and some use a voice that says "Fire!" or "Warning: Carbon Monoxide!" so you know which emergency it is.^{Source [5]}

Features worth paying for

• Interconnection. When one alarm detects CO, all of them sound. CPSC and NFPA both recommend it.^{Source [1]Source [2]}
• Digital readout. CDC suggests considering a model with a display that shows the highest CO level it has recorded.^{Source [3]} That can help a technician or firefighter judge what happened.
• Testing-lab listing. Look for the label of a recognized testing laboratory on the package.^{Source [2]} For CO alarms, the standard to look for is UL 2034.^{Source [1]}

What the numbers mean

A CO alarm isn't a meter that sounds the moment it detects any CO. Under UL 2034, alarms respond to a combination of concentration and time.^{Source [1]} One manufacturer's spec sheet lists the standard's response windows this way:^{Source [6]}

CPSC explains the reasoning. At 1 to 70 ppm, most people have no symptoms, though some people with heart disease may have chest pain. Above 70 ppm, symptoms become noticeable, and sustained levels above 150 to 200 ppm can cause disorientation, unconsciousness and death.^{Source [1]} Alarms are designed to sound before levels become life-threatening, and newer alarms are less prone to nuisance alarms than older ones.^{Source [1]}

The trade-off is that a standard alarm will usually stay quiet at low, long-term levels. If someone in the home is especially vulnerable, a model with a digital readout gives you more information.^{Source [3]}

Testing, batteries and replacement

• Test monthly. Press the test button at least once a month.^{Source [2]} CPSC notes that the button checks the alarm's circuitry, not how accurately the sensor reads CO.^{Source [1]}
• Check batteries twice a year. CDC suggests checking batteries when you change your clocks in spring and fall.^{Source [3]}
• Know the chirps. A single chirp every 30 to 60 seconds usually means a low battery. If it keeps chirping after you put in a new battery, the alarm has reached the end of its life.^{Source [5]}
• Replace on schedule. CDC says to replace detectors according to the manufacturer's instructions or every five years.^{Source [3]} Sealed 10-year models give an end-of-life alert.^{Source [6]} Write the install date on the back.

What to do if the alarm sounds

CPSC and NFPA give the same order of steps:^{Source [1]Source [2]}

1. Don't try to find the source.
2. Get everyone outside to fresh air, or to an open window or door if you can't leave.
3. Count heads to make sure everyone is out.
4. Call 911 or the fire department from outside.
5. Don't go back in until responders say it's safe.

If the alarm goes off again within 24 hours, have a qualified technician inspect your fuel-burning appliances.^{Source [1]} If the alarm gives a trouble signal instead, check the battery; if it still sounds, call the fire department.^{Source [2]}

An alarm is a backup, not prevention

CPSC is clear that an alarm doesn't replace proper use and upkeep of appliances.^{Source [1]} The prevention steps:

• Have your heating system, water heater and other gas, oil or coal appliances serviced every year, and have the chimney checked and cleaned yearly.^{Source [3]}
• Run portable generators outside only, more than 20 feet from windows, doors and vents.^{Source [3]}
• Never heat your home with a gas range or oven.^{Source [1]}
• Use an exhaust fan vented outdoors over a gas stove.^{Source [4]}
• After snow, clear dryer, furnace, stove and fireplace vents, and don't warm up a car in the garage.^{Source [2]}

CO is one part of a home air audit. Our guides to radon testing, mold testing, humidity and moisture and air purifier sizing cover the others. If your basement feels damp, our guide to sizing and running a basement dehumidifier explains how to keep it dry.

Frequently asked questions

Where should a carbon monoxide detector be placed?

Put one outside each sleeping area and on every level of your home, including the basement. Follow the manufacturer's instructions for mounting height, and keep it away from heating vents and out of kitchens and the space directly above fuel-burning appliances.

Do I need a carbon monoxide detector if I have no gas appliances?

If nothing in or attached to your home burns fuel, your risk is lower. But fireplaces, attached garages, portable generators and neighboring units can all be sources, and many states and cities require CO alarms. Check your local code.

How long do carbon monoxide detectors last?

Follow the replacement date from the manufacturer. CDC says to replace a detector on that schedule or every five years, and many sealed-battery models give an end-of-life signal. If an alarm keeps chirping after you replace the battery, replace the alarm.

What level of carbon monoxide sets off an alarm?

Alarms listed to UL 2034 respond based on both concentration and time. One manufacturer lists the standard's windows as 60 to 240 minutes at 70 ppm, 10 to 50 minutes at 150 ppm and 4 to 15 minutes at 400 ppm. That's why a detector may not sound at low levels.

What should I do if my carbon monoxide alarm goes off?

Don't search for the source. Get everyone outside to fresh air, call 911 or the fire department from outside, and don't go back in until responders say it's safe.