Carbon monoxide is the one home hazard you get zero warning about from your own senses — no color, no smell, no irritation until symptoms start. Detectors are cheap and the placement rules are short. The mistakes are almost all about location: one lonely unit in the basement furnace room protects nobody sleeping two floors up.
| Home | Typical count | Placement |
|---|---|---|
| Apartment, bedrooms together | 1 | Hallway outside the bedrooms |
| Single-story house | 1–2 | Outside sleeping area; add one near the garage entry if attached |
| Two-story house | 3 | One per level, with the upstairs unit outside the bedrooms |
| Two-story + finished basement | 3–4 | One per level; add one outside a basement sleeping area if there is one |
Height surprises people: unlike smoke, which rises, CO mixes evenly with air, so detectors work at almost any height — which is why plug-in units near the floor are acceptable if the maker says so. Wall-mounted units usually go around head height; ceiling-mounted ones at least a few inches out from the wall. The maker's sheet wins any argument, so follow it over generic advice, this page included.
CO sensors wear out. Every detector has a manufacture or replace-by date printed on it — most expire 7–10 years out — and an expired unit can sit there looking fine while sensing nothing. Test monthly with the button (that checks the alarm, not fully the sensor), replace at the date regardless, and vacuum the vents when you do your monthly home safety check. Interconnected models, where one alarm sounds all of them, are worth the small premium in any house with a floor between the furnace and the bedrooms. Note that a CO detector isn't a smoke alarm — they're different sensors with different placement logic, so if you're sorting out both, pair this page with the smoke detector placement and replacement guide and walk the house once with the full home safety inspection checklist. If that walk-through is happening because a baby is on the way, detectors are one line item on a longer list — our guide to baby-proofing your house covers the outlet covers, gates, and furniture anchoring that belong on the same pass.
One detector for the whole house, placed by the furnace — the single most common setup, and it fails exactly where it matters, outside the bedrooms. Mounting within a few feet of a gas appliance and then unplugging the "defective" unit after false alarms. And treating a combo smoke/CO unit on the ceiling as full coverage for a multi-story house: levels matter more than device cleverness.
Outside every sleeping area, every level, garage-adjacent but not in it, 15 feet clear of the appliances themselves. Three or four units cover most houses, they cost less than a pizza night each, and the only ongoing job is respecting the replace-by date.
If you have no fuel-burning appliances, no fireplace, and no attached garage, most codes don't require one and the risk is minimal. Add any of those three — a gas range counts, and so does a wood stove — and the answer flips to yes. Many people install one anyway for the attached-garage scenario alone.
Either works, because CO mixes evenly with room air rather than rising or sinking. Follow the manufacturer's instruction for your model: many wall units specify roughly head height, and plug-in models are designed to work at outlet height. Avoid placing any of them where furniture blocks airflow.
Typically 7–10 years from the manufacture date, depending on the model — the sensor degrades even if the alarm still beeps on test. Check the date printed on the back; many newer units chirp an end-of-life warning when they expire. Replace the whole unit, not just the battery.
Different sensors watching for different things: smoke alarms detect particles from a fire, CO detectors detect an invisible gas produced by incomplete combustion. A house fire can produce both, which is why combo units exist — but placement guidance differs, and having one type doesn't cover you for the other unless the unit is genuinely both.
True random alarms are rare. Nuisance alarms usually trace back to placement (too close to an appliance or bathroom), a dying sensor past its date, or very high humidity. But treat every alarm as real until a responder with a professional meter says otherwise — detectors occasionally catch slow, low-level leaks that come and go with appliance use.