Short answer
Any home with a fuel-burning appliance, fireplace, or attached garage needs carbon monoxide alarms. Place them outside each sleeping area and on every level, and near the basement equipment at the distance the manufacturer allows. Keep them out of garages, bathrooms, and direct sunlight, and away from appliances. Local code sets the specific requirements.
In this article
How does carbon monoxide spread through a home?
Carbon monoxide is produced when fuel burns incompletely in furnaces, water heaters, ranges, fireplaces, generators, and vehicles. It is invisible, odorless, and close to the weight of air, so it mixes through rooms rather than collecting at the floor or ceiling. It travels with airflow, which is why placement follows sleeping areas and sources rather than height.
That mixing behavior is the main difference from smoke. Smoke rises with heat and gathers at the ceiling, so smoke alarms go high. Carbon monoxide spreads with the air moving through the house, carried by forced-air heating, stairwells, and doors left open between an attached garage and the living space. A CO alarm can therefore be mounted on a wall, on the ceiling, or plugged into an outlet, as long as the manufacturer allows that position.
The risk is greatest during sleep, because early symptoms are easy to mistake for the flu and a sleeping person may not notice them at all. Placement is built around waking people.
Do I need a carbon monoxide detector?
A home needs carbon monoxide alarms if it has any fuel-burning appliance, a fireplace, or an attached garage, and many state and local codes require them in those homes, often at sale or rental. An all-electric home with no attached garage has lower risk, but a portable generator or an adjoining unit can still bring CO inside.
Fuel-burning equipment includes gas or oil furnaces, gas water heaters, gas ranges and dryers, wood or gas fireplaces, and wood stoves. An attached garage counts because a vehicle or generator running inside it, even briefly with the door open, can push CO into the house. Requirements differ by jurisdiction in where alarms go and which types are accepted, so the local fire official or building department is the reliable source for a particular home. Where no law requires an alarm, the risk from these sources is the same.
Where should carbon monoxide detectors be placed?
Carbon monoxide alarms belong outside each sleeping area, on every level of the home, and wherever local code requires them. Many households also add one inside any bedroom that has a fuel-burning appliance nearby, and near the door to an attached garage. The manufacturer's instructions set the mounting height and minimum distance from appliances for each model.
Outside the sleeping areas comes first because the alarm has to wake people. A unit in the hallway outside the bedrooms catches CO moving through the house before it reaches the people asleep there. Each additional level matters because a furnace or water heater in the basement may produce CO far from the bedrooms, and an alarm on that level detects it closer to the source. Combination smoke and CO alarms can cover both hazards from one ceiling location if the spot suits both; our guide to smoke and carbon monoxide detectors explains how the two plans overlap.
Where should carbon monoxide detectors not go?
Avoid placing CO alarms directly beside or above fuel-burning appliances, inside garages, in bathrooms or other humid rooms, in direct sunlight, near windows and vents that move outside air across the sensor, and behind furniture or curtains. Those positions can cause nuisance alarms from brief, normal emissions or keep the sensor from reading the air people actually breathe.
Distance from appliances is the detail most often guessed. Furnaces and water heaters can release a small amount of CO as they start, and an alarm mounted right next to one may react to that and end up taken down. Manufacturers specify a minimum distance for each model, and it should be checked rather than assumed. Plug-in units placed low behind a couch, or wall units covered by a curtain, are easy to forget and slow to read the room.
What does good CO placement mean in practice?
In practice, good CO placement means an alarm outside every sleeping area, one on each level, one near the basement equipment at the distance the manufacturer allows, and attention to the door from an attached garage. Every unit should be within its replacement date and, where possible, interconnected or monitored so one alarm is heard throughout the house.
A CO alarm that sounds in a basement while the household sleeps two floors up gives little warning on its own. Interconnection or monitoring is what turns that alarm into a signal that reaches people. On a monitored system, CO alarms are handled as life-safety signals according to the procedure agreed for the property, which is explained on our page about carbon monoxide and fire alarm monitoring. When an existing alarm starts to chirp or sound, our guide to what CO detector beeps mean explains each pattern.