Where to Place a Carbon Monoxide Detector in Your Home
Placement decides whether an alarm reaches you in time. The rules are simpler than the internet makes them look, and most of the confusion comes from mixing up carbon monoxide with two other gases that behave nothing like it.
Carbon monoxide does not sit at the ceiling or the floor
Natural gas rises, propane pools low, and carbon monoxide does neither for long. It has almost the same density as air and it travels with the warm air from whatever is producing it, so a room mixes rather than stratifies. This is why manufacturers do not publish a ceiling-or-floor rule for CO alarms the way they do for combustible gas alarms.
What follows from that: height matters far less than which room and how far from the bedroom door. An alarm that wakes a sleeping household is doing the job the product exists for.
Start with sleeping areas
Put an alarm where it will wake people. In practice:
- Outside each sleeping area, in the hallway or landing the bedroom doors open onto. A closed bedroom door is a barrier, so one alarm covering four bedrooms along a corridor is the usual arrangement.
- On every level with fuel-burning appliances or a habitable room, including a finished basement.
- In the room where people sleep if someone sleeps in a room with a fuel-burning appliance, for example a bedroom with a gas fire or a boiler cupboard.
Follow the mounting instruction in your model's manual for the exact height it was tested at. Where a manual gives a range, the practical answer is at the height where you will hear it and can reach the test button without a ladder.
Where not to put one
These are the placements that produce either a silent alarm or a nuisance one:
- Directly above or beside a fuel-burning appliance. Normal start-up can produce a brief spike. Manufacturers typically ask for a clearance around the boiler or stove, and the manual gives the distance.
- In a bathroom or anywhere with steam and humidity. Sensors and condensation are a bad combination.
- In a garage, a loft or an unheated porch. Temperature swings sit outside the operating range printed on the box, and a cold garage with a car in it is a false-alarm generator.
- Behind furniture, curtains or a door that swings over it. Dead air reaches a sensor last.
- Right next to a window, an extractor or a ceiling fan. Moving air pushes the sample away from the unit.
One alarm per level is a floor, not a target
Building requirements differ by state and country, and your local rule is what governs. Treat any single number you read online as a minimum rather than a recommendation: the number of units that actually protects a house depends on its layout, where people sleep and where the fuel-burning appliances are.
If you rent, the alarm is usually the landlord's obligation and the batteries are usually yours. Check the manufacture date on the back of whatever is already on the wall before you assume the flat is covered: an alarm past its service life is a wall decoration, and what the chirping means covers how a unit announces that.
Combination alarms change the height question
If you are buying a unit that detects both carbon monoxide and combustible gas, mounting height stops being a detail. Those units have to be placed for the gas, not for the CO: high for natural gas, which rises, and low for propane, which sinks. Our gas leak detector collection covers that split and which products do both jobs.
Ready to choose a unit? The carbon monoxide alarm collection compares a battery-only alarm against a plug-in model with a parts-per-million display.