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Put a carbon monoxide alarm on every level of your home, including the basement, and outside each separate sleeping area. That is the CPSC recommendation, and a typical two-story house needs at least two alarms. Height on the wall is not critical — CO mixes with air rather than rising or sinking — so follow the height range in your unit's manual. What genuinely matters is distance: keep alarms at least 15 feet from furnaces, water heaters, gas ranges and fireplaces, and out of bathrooms.
Carbon monoxide is colorless, odorless and tasteless. The CPSC calls it the Invisible Killer because there is no sensory warning at all — people become confused and drowsy before they understand what is happening, and most fatal exposures happen while the household is asleep. That is the entire reason placement is a mechanical problem with a right answer. Our device rankings are in best carbon monoxide detector; we do not test first-hand, and the method is explained in how we rank.
How many carbon monoxide detectors do I need?
One per level plus one outside each sleeping area. In practice that means two alarms in a single-story home with one bedroom hallway, two to three in a two-story home, and three to four once you add a basement and a second sleeping area. Count sleeping areas, not bedrooms: three bedrooms off one hallway is one sleeping area and needs one alarm covering that hall. A bedroom over an attached garage, or a converted attic bedroom, is a separate sleeping area even if it is the only room up there.
| Home | Minimum CO alarms | Where |
|---|---|---|
| 1-story, one hallway of bedrooms | 2 | Bedroom hallway; main living area |
| 2-story, bedrooms upstairs | 2–3 | Upstairs hall; main floor; basement if the furnace is there |
| 2-story + finished basement | 3–4 | Add one on the basement level, 15+ ft from the furnace |
| Home with attached garage | +1 | In the room directly adjoining the garage, on the living-space side of the door |
| Apartment or condo | 1–2 | Outside the bedroom door; a second in the main room if there is gas |
How high should a carbon monoxide detector be mounted?
Anywhere in the range your manual allows, because the gas does not stratify the way most people expect. Carbon monoxide has a molecular weight near 28 against air's 29 — effectively neutral buoyancy — so it disperses through the room volume instead of pooling at the floor or banking against the ceiling. Warm combustion gases do rise initially, which is why some manufacturers prefer a high mount, but within a normal room the difference between knee height and ceiling height is small and slow.
This is why manufacturers give contradictory advice and all of it is defensible. Kidde permits wall or ceiling; plug-in units sit at outlet height by design; some brands specify at least 5 feet up so a crawling child cannot reach it. Follow the unit in front of you, then optimize for the human factors: a plug-in with a digital readout is worth putting where you will glance at it daily, and any alarm needs to be audible through a closed bedroom door.
Where should you never put a CO detector?
The exclusions do more work than the inclusions. Each of these causes either a false alarm or a missed one:
- Within 15 feet of a fuel-burning appliance — furnace, water heater, gas range, fireplace, boiler, gas dryer. Startup produces a short CO spike that is harmless in the room but enormous at 3 feet.
- Bathrooms, or the direct path of a bathroom door. Sustained humidity and steam degrade electrochemical sensors and cause spurious readings.
- Unconditioned garages, attics and crawl spaces. Most units are rated roughly 40–100°F; outside that range they misread or stop working.
- Beside a window, exterior door or supply vent. Fresh air dilutes the sample the alarm is measuring, so a genuine buildup registers late.
- Directly under a ceiling fan. Same problem: the airflow keeps the sensor in a stirred pocket instead of representative room air.
- Behind curtains, furniture or inside a cabinet. The sensor needs open air, and a muffled 85 dB alarm may not wake a sleeper.
Do you need a CO detector if you have no gas appliances?
Yes, in almost every case. An all-electric home still has three common CO pathways: an attached garage, where a car warmed up with the door open still pushes exhaust into the house; a wood-burning fireplace or stove, where a closed or blocked damper backdrafts combustion gas into the room; and a portable generator during a power outage. That third one is the killer. The CPSC attributes more than 200 accidental non-fire CO deaths a year to consumer products, and portable generators account for the largest share of them — concentrated in the days after storms, when people run a generator in a garage, on a porch, or under a window because it is raining.
The generator rule is unambiguous: never indoors, never in a garage even with the door open, and at least 20 feet from any window, door or vent, with the exhaust pointed away from the house. If you size or schedule a generator run during an outage, our generator runtime calculator carries the same warning alongside the fuel math. A CO alarm on the level nearest the generator is not optional during an outage — and it needs a battery, since the grid is down.
We cover the electric-heat case in more detail in do you need a carbon monoxide detector with electric heat.
Where do CO alarms go in an RV, a rental or a cabin?
In an RV or camper, one alarm in the main living space is standard and one near the sleeping area is better; the risks there are the propane furnace, the onboard generator and the vehicle parked next door running its own generator. In a rental, most states now require the landlord to provide CO alarms, but requirements and enforcement vary widely by state and city, so check your local code and add your own battery unit if you are unsure. In a cabin or vacation home, the alarm matters most in the first 24 hours after the heating is switched on for the season, which is precisely when nobody is thinking about it — a portable battery alarm in your bag solves the problem for hotel rooms too.
How do you know it is working?
Press the test button monthly. That confirms the horn, the battery and the electronics, but not the sensor — there is no home method to verify the electrochemical cell, which is why expiry dates exist. CO sensors typically last 5 to 7 years, some sealed units 10, and the alarm chirps at end of life in a pattern people routinely mistake for a low battery. Write the installation date on the housing with a marker the day you mount it, and check the manufacture date stamped on the back before you install a unit that has been sitting in a warehouse. See do carbon monoxide detectors expire for the chirp patterns and the replacement math.
If an alarm sounds and anyone has a headache, nausea, dizziness or confusion, get everyone outside into fresh air and call 911 or the fire department from outside. Do not stay in the building to investigate. This page describes device placement and public-safety guidance; it is not medical advice.
Where to go next
Model shortlists are in best carbon monoxide detector and, if you would rather run one device instead of two, best combination smoke and carbon monoxide detector. Smoke alarm placement follows completely different rules — see how many smoke detectors do I need. Everything else in the category sits in the air quality hub, and side-by-side spec tables are in compare.