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Carbon Monoxide Detectors

A listed CO alarm is legally not allowed to alarm below 30 ppm. Once you know that, most of the confusion in this category resolves itself.

Plug-in carbon monoxide detector with a digital display on a hallway wall

Buy a UL 2034 alarm with a digital display, put one on every level and within earshot of every sleeping area, and know the symptoms. The thing almost nobody explains is that UL 2034 sets response times — 70 ppm within 60 to 240 minutes, 150 ppm within 10 to 50 minutes, 400 ppm within 4 to 15 minutes — and forbids alarming below 30 ppm at all. A standard alarm is designed to stop you dying, not to tell you the air is bad. Those are different jobs, and the second one needs a different instrument.

By Ryder M.Last reviewed

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What to buy first

For most homes: a UL 2034 listed alarm with a digital display, one per level, positioned within earshot of the bedrooms. The display is not a gimmick — it is the difference between knowing an alarm went off and knowing what concentration triggered it, which is the first thing a responding technician will ask.

The peak-level memory matters at least as much. A chronic low-level leak from a cracked heat exchanger or a blocked flue may never reach the alarm threshold while you are standing there, but it will be recorded. Some units log concentrations well below the alarm point for exactly that reason. Our picks are in the best carbon monoxide detectors.

If your alarms and your CO detection can share a ceiling position, a combination unit is the tidier answer — see combo vs separate alarms for where that is a good idea and where it is not.

The 30 ppm floor, and why low-level monitors exist

UL 2034 is written to prevent acute poisoning in a healthy adult. Its response requirements are time-and-concentration pairs — the alarm must sound within 60 to 240 minutes at 70 ppm, within 10 to 50 minutes at 150 ppm, and within 4 to 15 minutes at 400 ppm — and it must not alarm below 30 ppm. That last requirement exists to stop nuisance alarms, and it is deliberate.

For most households that is the right design. For some it is not: infants, people who are pregnant, older adults and anyone with a cardiac or respiratory condition can be affected by sustained exposure well below the level a listed alarm may legally report. That is the entire reason low-level monitors exist, and it is covered in the best low-level CO monitors.

A low-level monitor is a supplement, not a replacement. It is not a UL 2034 residential alarm and it does not satisfy a code requirement for one. Buy the listed alarm first; add the monitor if your household is in one of those groups.

Where they go — and the 'high or low' question

CO has almost the same density as air. It does not pool at the floor like propane and it does not rise to the ceiling like smoke; it mixes and moves with air currents and with the warm gas it arrives in. That is why the 'high or low' question has no single satisfying answer and why manufacturers' instructions differ.

The practical guidance: follow the manufacturer's instructions for the specific unit, put alarms where you will hear them from bed, and put at least one on every level. Avoid dead air within about 6 in of a corner or the ceiling-wall junction, avoid directly above a fuel-burning appliance where it will nuisance-trip on start-up, and avoid bathrooms and unheated spaces. Where to install CO detectors works through each room.

The one placement people get badly wrong is the basement. A furnace or water heater in a basement is the most common source in a house, and a basement with no alarm is the most common gap. Put one down there, not directly over the appliance.

The mistake buyers make

Assuming an all-electric house does not need CO detection. It very often does: an attached garage, a gas or wood fireplace, a portable generator used during an outage, a charcoal grill used too close to the house, or a neighboring unit in a duplex can all put CO into a home with no gas appliances at all. Do I need a CO detector works through the cases honestly, including the ones where the answer is genuinely no.

The second mistake is treating a CO alarm as permanent. The electrochemical sensor has a finite life — commonly 7 to 10 years depending on the model — and the unit signals end of life with a distinct pattern that people mistake for a low battery. CO detector lifespan and replacement and CO detector beeping codes cover the difference, which genuinely matters: one means change a battery, the other means the device has stopped protecting you.

The third is generators. CPSC attributes more than 200 US deaths a year to CO from non-automotive consumer products, and portable generators are a large part of that. A generator in a garage with the door open is still indoors. Generator CO safety has the distances.

Everything in carbon monoxide detectors

All 11 pages in this section. Roundup tiles carry their top pick so you do not have to dig for it.

Carbon Monoxide Detectors: common questions

Should a carbon monoxide detector be high or low on the wall?
Follow the specific manufacturer's instructions, because CO mixes with air rather than rising or sinking reliably. In practice, put alarms where you will hear them from bed, at least one per level, avoiding dead air within about 6 in of a ceiling-wall corner and avoiding positions directly above a fuel-burning appliance.
Do I need a CO detector if I don't have gas?
Often yes. An attached garage, a fireplace, a portable generator during an outage or a shared wall with another dwelling can all introduce CO into an all-electric home. A genuinely detached, all-electric home with no garage, no fireplace and no generator is the one case where the answer may be no.
What does it mean when a CO detector beeps 3 times?
Patterns vary by manufacturer, so check the manual for your model — but on many units four rapid beeps is the CO alarm itself, one chirp a minute is low battery, and a distinct repeated pattern with an END or ERR display is end of life. If there is any doubt at all, treat it as a CO alarm, get everyone outside and call 911.
How long do carbon monoxide detectors last?
The electrochemical sensor has a finite life, commonly 7 to 10 years depending on the model, and the whole unit is replaced at end of life. The date is on the back. A unit past it should be replaced even if it still responds to the test button, which tests the horn and not the cell.

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