Where smoke and CO detectors are legally required
Placement isn't optional — it's physics. Smoke rises. CO is roughly the same density as air but accumulates in certain patterns. Put a detector in the wrong spot and it either triggers too slowly to matter or triggers so often you disable it.
Smoke detector placement
Required locations (most state/local codes, based on NFPA 72):
- Inside each bedroom
- Outside each sleeping area (in the hallway serving the bedrooms)
- On every level of the home including basement
Placement specifics:
- Mount on the ceiling, ideally in the center of the room
- If ceiling mounting isn't possible, mount high on the wall — within 12 inches of the ceiling
- Keep at least 10 feet away from cooking appliances (nuisance trips)
- Keep at least 3 feet away from HVAC supply vents (airflow disrupts detection)
Why near the bedroom door: In a house fire at night, the smoke needs to reach the detector before it reaches you. A detector in the hallway outside bedrooms catches smoke from anywhere in the house before it fills the sleeping area.
Carbon monoxide detector placement
CO is a byproduct of incomplete combustion — gas stoves, gas furnaces, gas water heaters, fireplaces, attached garages, generators. It's colorless and odorless. It kills by displacing oxygen in your blood before you feel sick enough to wake up.
Required locations:
- Outside each sleeping area
- On every level
- Required by law in most states if you have any gas appliances, fuel-burning equipment, or an attached garage
Placement specifics:
- CO detectors can go at any height — CO distributes roughly evenly throughout a room
- Many combination smoke/CO detectors are available and code-compliant
- Keep away from garages (car exhaust would constantly trigger it) — mount just inside the door to the house
What triggers CO alarms:
- Blocked flue or chimney on gas furnace or water heater
- Cracked heat exchanger
- Running a car or generator in an attached garage
- Gas stove left on, pilot light issues
- Portable generators run indoors or too close to windows
Interconnected alarms
Modern code requires interconnected alarms — when one detects smoke, all alarms in the house sound. Hardwired systems are interconnected by wire. Battery systems can be interconnected wirelessly with compatible units.
The reason: a fire in the basement at 2am needs to wake you in a third-floor bedroom. A single alarm in the basement doesn't do that reliably.
Battery vs hardwired
Hardwired with battery backup: Required in new construction. The battery keeps it running during power outages.
Battery-only: Acceptable in existing homes where running wire is impractical. Use 10-year sealed lithium battery units — you install it once and replace the whole unit when it expires, instead of swapping batteries and forgetting.
The low-battery chirp: Single chirp every 30–60 seconds means the battery is dying. It's designed to be annoying. Don't remove the battery and leave the unit offline.
Testing and replacement
- Test every unit monthly using the TEST button
- Replace units every 10 years (smoke) and every 5–7 years (CO) — the sensors degrade
- The manufacture date is printed on the back; check it if you move into an existing home
If you move into a house or apartment and can't find smoke detectors meeting these requirements, you have grounds to require installation — this is a habitability issue in most states.
Quick reference
Smoke: Inside every bedroom + hallway outside sleeping areas + every floor + basement
CO: Outside sleeping areas + every floor + near any gas appliance or attached garage
Mount smoke on ceiling or high wall. CO can go anywhere.
Interconnected so all alarms sound when one triggers.
Replace smoke every 10 years. CO every 5–7.