Where should carbon monoxide detectors be placed?

Carbon monoxide is colourless, odourless, and its early symptoms — headache, nausea, fatigue — read exactly like the flu. That is what makes detection non-negotiable in any home with fuel-burning appliances or an attached garage. Placement is simple, but there is one persistent myth worth clearing up.

The height myth

People are often told CO rises, or that it sinks. Neither is quite right: carbon monoxide has almost the same density as air and mixes throughout a room rather than stratifying. Manufacturers therefore allow a wide range of mounting heights. Follow the specific unit’s instructions, but do not agonise — being in the right room matters far more than being at the right height in it.

Where they go

• One on every level of the home, including a finished basement.
• One outside each separate sleeping area — close enough that the alarm will wake someone asleep behind a closed door.
• Near an attached garage, since a car warming up is a common source.
• Follow the manufacturer’s clearance from fuel-burning appliances — usually at least 15 feet — so normal startup does not cause nuisance alarms.

Where they should not go

Avoid bathrooms and very humid spots, direct sunlight, and anywhere within reach of a draft — beside a vent, fan, or open window — which can blow CO away from the sensor. Also skip dead-air corners and the peak of a vaulted ceiling.

Replacement

CO sensors have a shorter life than smoke sensors: typically 5–7 years, sometimes 10 on newer units, and the unit will usually have an end-of-life chirp distinct from the low-battery chirp. Check the manufacture date on the back, the same as with smoke detectors. Test monthly, replace batteries annually where applicable.

If the alarm goes off

Get everyone outside into fresh air first, then call the fire department or gas utility from outside. Do not spend time hunting for the source, and do not assume it is a false alarm because nobody feels unwell — symptoms lag exposure. Only re-enter once someone with a meter says it is clear.

The most common sources are a cracked furnace heat exchanger, a blocked flue, and a car running in an attached garage. An annual furnace inspection covers the first two.