CO Detection Chart
Where carbon monoxide alarms are required and how they alarm, per NFPA 72 (2022 edition), which took over the CO provisions when NFPA 720 was withdrawn. CO detection is required where fuel-fired appliances or an attached garage can produce it, placed near sleeping areas and on every level. Because CO mixes evenly with air, mounting height is not restricted.
Where CO detection is required
- The dwelling unit contains a fuel-fired (fuel-burning) appliance
- The dwelling unit has an attached garage with an opening into the dwelling
Placement
| Placement | Requirement |
|---|---|
| Outside sleeping areas | Outside each separate sleeping area, within 21 ft of any bedroom door along the path of travel |
| Each level | On every habitable level, including basements, centrally located |
| Bedrooms with a CO source | Inside a bedroom (or its bathroom) that contains a fuel-burning appliance |
| Appliance rooms | On the ceiling in the same room as a permanently installed fuel-burning appliance |
UL 2034 alarm response
| CO concentration | Must NOT alarm before | Must alarm before |
|---|---|---|
| 30 ppm | 8 hours | — (no alarm required) |
| 70 ppm | 60 minutes | 240 minutes |
| 150 ppm | 10 minutes | 50 minutes |
| 400 ppm | 4 minutes | 15 minutes |
Why CO detection is unlike smoke detection
A CO alarm is placed and mounted differently from a smoke detector for one physical reason: carbon monoxide is almost exactly as dense as air and mixes through a room evenly, instead of collecting at the ceiling the way smoke and hot gas do. That is why there is no 4-to-12-inch rule or ceiling requirement for CO — any height works, and the placement is about being near the people who need warning (outside bedrooms, on every level) rather than in the path of a rising plume.
The alarm logic is different too. Smoke detection responds as fast as it can; CO detection is deliberately time-weighted, because CO poisoning depends on dose — concentration multiplied by time. A brief whiff of 70 ppm is harmless, so the alarm holds off, but a sustained 400 ppm is an emergency, so it sounds within minutes. That is the entire shape of the UL 2034 table.
Common questions
Where are carbon monoxide detectors required?
Under NFPA 72 (2022 edition, which absorbed the former NFPA 720), CO detection is required in a dwelling unit that has a fuel-fired appliance or an attached garage. Alarms go outside each separate sleeping area, on every level including the basement, inside any bedroom that contains a fuel-burning appliance, and on the ceiling of rooms with permanently installed fuel-fired equipment.
What height should a CO detector be mounted at?
Any height — NFPA 72 sets no general mounting-height requirement for CO alarms, because carbon monoxide mixes readily with room air rather than rising or sinking. Wall, ceiling, and outlet-height mounting all satisfy the code. The one exception is an appliance room, where the alarm is mounted on the ceiling.
At what CO level does an alarm go off?
CO alarms follow UL 2034, which sets a concentration-versus-time response: at 400 ppm the alarm must sound within 15 minutes, at 150 ppm within 50 minutes, and at 70 ppm within 240 minutes — while at 30 ppm it must not alarm for at least 8 hours. The staged response reflects that CO harm depends on both concentration and exposure time.
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