CO Detector Placement Check

A carbon monoxide detector that is installed in the wrong location may not alert you in time. CO is colorless and odorless, and it mixes with air at roughly the same density, which means it does not reliably rise or sink. A detector placed too far from sleeping areas, too close to a fuel-burning appliance, or in a dead-air zone may detect CO too late or trigger nuisance alarms that lead to removal.

This 2-minute placement check asks six questions about your home layout, the number of levels, where you sleep, what fuel sources are present, how your detectors are mounted, and where they are positioned relative to key rooms. Based on your answers, you will get a clear assessment of whether your current placement likely meets best practices and code requirements, with specific guidance on what to move or add.

The 21-foot figure this check works from is NFPA 72's, and it is the number your city is most likely to overwrite. Seven states write a distance from the sleeping area into a state instrument, and every one is tighter than NFPA's: ten feet in Florida, Massachusetts, Minnesota and Tennessee, fifteen in Colorado, Illinois and Oregon. Massachusetts sets it at 527 CMR 1.00 section 13.7.6.5(3), no more than ten feet from each bedroom door measured in any direction, and Oregon at OAR 837-047-0130, within each bedroom or within fifteen feet outside each bedroom door. An eighth state writes a distance and does not tighten it: Wisconsin's ATCP 72.18(12)(e) puts the alarm within 21 feet of any door to a sleeping room, measured along a path of travel, which is NFPA's own figure. Then we read 93 municipal codes on top of those, and the two cities that go furthest from each other are eight-fold apart. Box Elder, South Dakota requires the alarm within five feet of the entrance to the sleeping area; Wilmington, Delaware allows forty; Covington, Kentucky sits between them at fifteen for every rental in the city. Box Elder measures to a bedroom that does not exist yet, since its rule reaches a future sleeping area drawn on the approved plans, so an unfinished basement gets its alarm when the house is built rather than when somebody frames the room. Wilmington leans on audibility instead of distance, requiring the alarm to be easily audible in all sleeping rooms whatever the forty feet allows.

Note: Placement rules come from NFPA 72, manufacturer guidelines (Kidde, First Alert), and CPSC recommendations. Your city may set a tighter or looser distance than either, as the two above show. This check covers the most widely accepted rules, always verify against your local code for the final answer.

In 2 Minutes

• Every level of your home needs at least one CO detector, including the basement and any level with a fuel-burning appliance.
• Outside each sleeping area is the highest-priority placement, NFPA 72 specifies within 21 feet of each bedroom door, measured along the path of travel rather than straight through the wall.
• Mounting height does not significantly affect detection speed, CO mixes evenly with air. Wall or ceiling mounting both work.
• Too close to appliances causes false alarms; too far delays detection. The sweet spot is 5-20 feet from fuel-burning sources.
• Dead-air zones, corners, behind furniture, near windows or HVAC vents, reduce detector effectiveness.
• The distance is the part your city is most likely to change. Across the 93 municipal codes we read, the tightest requirement is five feet from the sleeping-area entrance (Box Elder, South Dakota) and the loosest is forty (Wilmington, Delaware). NFPA 72 sits at 21.

What This Check Covers

• Whether you have enough CO detectors for your home's layout.
• Whether detectors are positioned near sleeping areas as recommended by NFPA 72.
• Whether any detectors are in locations that cause nuisance alarms or delayed detection.
• Mounting height and method, wall vs. ceiling vs. plug-in placement.
• Basement, garage-adjacent, and multi-story coverage gaps.
• Specific guidance on what to move, add, or relocate.

Start the Check

How to Interpret Your Result

This check evaluates your detector placement against NFPA 72 recommendations and common manufacturer guidelines.

• Placement Looks Good: Your CO detector placement appears to align with NFPA 72 recommendations and common manufacturer guidelines. You have adequate coverage across levels, detectors near sleeping areas, appropriate proximity to fuel-burning sources, and no obvious problem placements.
• Adjustments Recommended: Your CO detectors cover most of your home, but one or two placement issues were identified. These are common problems that are easy to fix, usually by relocating a detector a few feet or adding one in a specific location.
• Significant Gaps Found: Your home has meaningful CO detector coverage or placement gaps. Key areas, sleeping zones, fuel-burning appliance levels, or multiple floors, lack adequate detection. Addressing these gaps should be a priority.

Sources & References

1. NFPA, NFPA 72: National Fire Alarm and Signaling Code . Section 29.7.1.1 is the placement rule: outside each separate sleeping area, within 21 ft (6.4 m) of any door to a sleeping room measured along a path of travel, and on every occupiable level including basements. The code text is not on this page and NFPA sells the standard; the wording here was read from NFPA's own free Second Draft Report for the 2022 edition.
2. U.S. EPA, Carbon Monoxide's Impact on Indoor Air Quality . Health effects and indoor sources. Its alarm section reproduces CPSC's recommendation, "CO alarms on each level outside each sleeping area"; it carries no mounting height.
3. CPSC, Carbon Monoxide Questions and Answers . "Install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas", the 1 to 70 / above 70 / 150 to 200 ppm bands, and "CPSC does not recommend installing CO alarms in kitchens or above fuel-burning appliances."
4. Kidde, Placement of Carbon Monoxide Alarms . 5-20 ft from CO sources, every level + outside sleeping areas.
5. Google Nest, Why CO Alarms Don't Need to Be Near the Floor . The clearest debunking of the CO-is-heavier-than-air myth we have found: carbon monoxide is slightly lighter than air and diffuses evenly. Note that it quotes NFPA 720, 2005 edition, and NFPA 720 was withdrawn in 2018 with its household material moved into NFPA 72; the equivalent current wording is 29.7.1.1 and 29.7.1.2.
6. First Alert, CO Legislation by State . A state selector covering smoke and CO legislation. A manufacturer's summary, not a statute; our own state pages cite the law itself.
7. NCSL, Carbon Monoxide Detector Installation Statutes . State-by-state table. It no longer carries a count: the page now says only that "most states have adopted requirements", from statute, regulation or code. Written for legislators and, in its own words, "may not be reflective of all requirements".
8. City of Box Elder, South Dakota, Code of Ordinances Section 6-240, amending IRC R315.3 . The five-foot rule, and the clause that measures to a future sleeping area shown on the approved plans. Ordinance 642, adopted 14 February 2020, predates SDCL 11-10-12, which later capped local residential requirements at the model code; several provisions read as more stringent than the model, so ask the city before relying on them. Read 11 August 2026 for our municipal survey.
9. City of Wilmington, Delaware, City Code Section 34-232.1, carbon monoxide alarm devices . The forty-foot rule, the loosest distance figure in our municipal survey. Read it with the audibility clause in the same sentence rather than as a standard on its own. Read 11 August 2026.
10. Massachusetts State Fire Marshal, Preparing for Smoke and CO Alarm Inspection . What Massachusetts inspects when a one- or two-family home is sold or transferred, and how to get the certificate of compliance. The requirements themselves are in the linked consumer guide, not on this page.