Where to Put a Carbon Monoxide Detector

This guide is for anyone installing carbon monoxide alarms for the first time or checking that a home’s existing alarms are in the right places.

Carbon monoxide (CO) has no color and no smell, so you can’t tell it’s building up until people start to feel sick, and a sleeping person may never notice. An alarm is the only practical early warning, and it only works if it’s where the gas will reach it while you can still hear it. Here’s where to put CO detectors, how many a home needs, and how to keep them working.

The quick answer

The U.S. Consumer Product Safety Commission (CPSC) says to install CO alarms on each level of the home and outside sleeping areas.² The CDC says the same thing in plain terms: put a battery-operated or battery-backup CO detector near every sleeping area in your home.³

So the short version of where to put a carbon monoxide detector is:

In the hallway or area just outside the bedrooms.
At least one on every level of the home.
In a basement or attic only if someone sleeps there.²

Why the bedrooms come first

CO poisoning is most dangerous at night. The CDC warns that people who are sleeping can die from CO poisoning before they have symptoms.³ An alarm in the hallway outside the bedrooms is loud enough to wake people behind closed doors in most homes, and that’s the whole point of the placement rule.

CPSC estimates that more than 200 people a year die from CO poisoning linked to consumer products, and more than 100 of those deaths involve portable generators.¹ The CDC puts the total from all accidental, non-fire CO poisoning at more than 400 deaths a year in the United States.³

How many CO detectors you need

Count the levels of your home, then count the separate sleeping areas. Each level needs at least one alarm, and each sleeping area needs one nearby.²

Home	Minimum CO alarms (our count, based on CPSC’s rule)
One-level apartment, bedrooms in one hallway	1, in the hallway outside the bedrooms
One-level home, bedrooms at opposite ends	2, one outside each sleeping area
Two-story house, all bedrooms upstairs	2, one upstairs outside the bedrooms and one on the main floor
Two stories plus a basement bedroom	3 or more: one per level, plus one outside each sleeping area

CPSC also says interconnected alarms are best, so that when one sounds, they all do.¹ Our advice: in a larger home or one where bedrooms are far apart, interconnection matters more than adding one extra alarm.

Where not to put a CO detector

CPSC says CO alarms shouldn’t go in attics or basements unless they include a sleeping area.² Beyond that, the alarm’s own manual sets the details, and these vary by model. Our advice on spots to avoid:

Right next to a furnace, water heater, stove or fireplace. Fuel-burning appliances can cause nuisance alarms when they start up. Follow the maker’s minimum distance.
In a bathroom or right outside a shower. Steam and humidity can affect the sensor.
Behind curtains, furniture or a door. Air needs to reach the alarm.
In direct sun, near a heating vent, or in a drafty spot by a window or fan, where airflow can keep the gas from reaching the sensor.
In a garage. It’s often too cold or hot and too dusty. Protect the rooms next to it instead.

Wall or ceiling? How high?

Unlike smoke, CO mixes with the air in a room rather than rising to the ceiling, so manufacturers differ on height. Our advice: mount the alarm at the height the instructions give, whether that’s on the wall at eye level, near the ceiling, or in a plug-in outlet, and don’t move it lower or higher on your own. A plug-in model is fine as long as nothing blocks it and the outlet isn’t controlled by a switch someone might turn off.

Choosing a CO alarm

Battery-operated or battery backup. CPSC recommends alarms that keep working in a power outage.¹ That matters, because outages are exactly when people run generators and fuel-burning heaters.
A digital readout. The CDC suggests choosing a detector with a digital readout.³
Combination smoke and CO alarms are available, according to CPSC.² Our advice: they save ceiling space and batteries, but check that the placement still works for both. Smoke alarms also belong inside each bedroom, which a CO-only rule doesn’t require. See our smoke detector placement guide.

Test monthly, replace on schedule

Test every alarm monthly and replace the batteries every year, as CPSC recommends.² The CDC suggests checking or changing the batteries when you change your clocks in spring and fall.³
Replace the detector itself following the maker’s instructions or every five years, according to the CDC.³

Our advice: write the install date on the back of each alarm with a marker, so you know when it’s due.

What the alarm can’t do: prevention

An alarm warns you; it doesn’t stop CO from being made. The CDC’s prevention steps:³

Have your heating system, water heater and any other gas, oil or coal-burning appliances serviced by a qualified technician every year.
Have your chimney checked or cleaned every year.
Never use a gas oven to heat your home.
Never burn charcoal indoors.
Never leave a car running in an attached garage, even with the door open.

CPSC adds that portable generators should run only outside, at least 20 feet from the house.¹ It also warns never to use fuel-burning heaters or lanterns while sleeping in tents, campers or vehicles.⁴

Know the symptoms

CPSC lists the symptoms of CO poisoning as headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion.¹ The CDC describes them as often “flu-like” and adds chest pain.³ CPSC notes that CO poisoning is easily mistaken for a cold or the flu.⁴ Our advice: be suspicious when several people in the house, or the pets, feel sick at the same time, or when symptoms ease after you leave home.

If the CO alarm goes off

CPSC’s advice is to get outside to fresh air immediately and then call 911.¹ Our advice:

Get everyone, including pets, out of the house. Don’t stop to open windows or look for the source.
Call 911 or the fire department from outside.
Don’t go back in until responders say it’s safe.
Have the appliance that caused it checked and repaired before you use it again.

If the alarm chirps instead of sounding a full alarm, that usually means a low battery or the end of its life. Check the manual.

Your CO detector checklist

One alarm outside each sleeping area.
At least one on every level of the home.
Not in attics or basements unless someone sleeps there.
Battery-operated or battery-backup, ideally interconnected.
Test monthly, change batteries yearly, replace the unit on schedule.
If it sounds: everyone out, then call 911.

Where to Put a Carbon Monoxide Detector (and How Many You Need)