The alarm windows
Summaries of UL 2034 from alarm makers give three time bands. At 70 parts per million, the alarm sounds within 60 to 240 minutes. At 150 ppm it is 10 to 50 minutes, and at 400 ppm it is 4 to 15 minutes. The bands are about the dose over time, not a single reading.
Why it stays quiet
The standard also says an alarm must not sound below 30 ppm, to avoid false alarms from brief, low exposures. For a wood stove owner, that means slow spillage from a poorly drafting stove may never set it off. A display model at least shows you a number, and that is the reason to pay extra for one.
What the sensor is
Home alarms generally use an electrochemical sensor with a finite life, which is why every unit has a replace-by date. Write the install date on the alarm. Battery, plug-in and hardwired models all answer to the same standard.
What to do if it sounds
Get everyone outside and into fresh air, then call 911 or the fire department from outside. Do not reset it and stay indoors. Have the stove and chimney checked before you light another fire.
Questions that keep arriving by post
Why did my alarm not sound with a low CO reading on the display?
Because UL 2034 alarms are meant to hold off at low levels. A display model shows a low reading without sounding. That is by design.
How often do I replace a CO alarm?
Follow the date on the unit. Most makers state about 10 years, and some models end sooner, around 6 or 7 years. Many alarms sound an end-of-life signal.
Where should it go near a wood stove?
Follow the alarm’s manual and your local code. NFPA 72 and NFPA 720 deal with installation. The usual advice is one on each level and near sleeping areas.
Last reviewed November 1, 2026