How to Use a Carbon Monoxide Detector

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Where to put it so it actually wakes you

Carbon monoxide mixes freely with air, so it does not stratify the way propane or petrol vapour does. That means a CO alarm can be mounted at any height, and the manufacturer’s instructions usually allow wall or ceiling placement. What matters far more is the path between the alarm and your ears. The NFPA and the manufacturers’ instructions generally call for an alarm outside each sleeping area and on every level of the home, including the basement. If you sleep with the bedroom door closed, an alarm in the hallway may not wake you; a second alarm inside the bedroom is the safer arrangement.

Keep the alarm away from the immediate vicinity of fuel-burning appliances. A detector sitting on top of a gas cooker will nuisance-alarm every time you cook, and repeated nuisance alarms train people to ignore the device. The listing for the Kidde COBD battery model states that it can be wall-mounted or set on a table, which is useful for travel, but a table placement in a hotel room should still be near the bed, not next to the kettle.

Do not put a CO alarm in a closed cupboard, behind furniture, or in a room that is normally sealed and unheated. The sensor needs to see the room air. Plug-in units need an outlet that is not controlled by a wall switch; the First Alert plug-in alarm and the Kidde plug-in with 9V backup both rely on continuous mains power, and a switched outlet defeats the purpose.

The test button does not test the sensor

Every consumer CO alarm has a test button, and every manufacturer tells you to press it weekly. What the button actually does is check the battery, the electronics and the sounder. It does not expose the sensor to carbon monoxide, so it cannot tell you whether the sensor has drifted or died. The only way to test the sensing element is with a known concentration of CO gas, which is a job for a calibrated instrument and a technician.

That is why the end-of-life signal matters. Electrochemical CO sensors consume themselves slowly, and their output falls over time. The listing for the Kidde smart CO and air quality monitor states that the voice alarm announces “Replace Alarm” when the unit has reached its end of life. When an alarm starts chirping in a pattern that is not the low-battery chirp, read the label. Most units have a service life of five to ten years, and the listing for each product here does not publish that figure, so check the back of the unit or the manual. Write the installation date on the back with a marker.

Batteries are a separate schedule. The Kidde AA battery model ships with two AA batteries and runs on them alone, so a weak battery is a dead alarm. The plug-in units with battery backup, such as the Kidde 9V backup model, will keep working during an outage, but the backup battery still needs replacing on the interval the manual states. Not stated in the listing bullets for any of the four products is the exact replacement interval, so follow the printed instructions.

What the number on the display means, and what it does not

Digital-display alarms show a CO concentration in parts per million (ppm). That number is a snapshot of the sensor’s current reading, not a dose. Carbon monoxide harm depends on concentration and time together: 100 ppm for an hour is not the same exposure as 100 ppm for a minute. Consumer alarms are designed around that principle. They do not sound at the first ppm they see; they use internal thresholds and time-weighted logic to avoid nuisance alarms from cooking or a passing puff of exhaust. The listing for the First Alert plug-in alarm states that it has a peak level feature, which displays the highest concentration measured since the last reset. That is useful for a technician or a homeowner trying to work out whether an alarm was a one-off or a trend, but it is not a substitute for a calibrated meter.

If the display shows a number and the alarm has not sounded, do not assume the air is safe. It means the concentration has not yet met the alarm’s internal criteria. Open windows, turn off fuel-burning appliances, and investigate. If the display shows a number and the alarm is sounding, leave.

One common confusion: a CO alarm is not a combustible gas detector. It will not warn you about a natural gas leak, and a natural gas alarm will not warn you about carbon monoxide. The two hazards are different and require different sensors. For the distinction, see gas leak detector versus carbon monoxide detector. Likewise, a CO alarm does not measure oxygen, and it does not tell you whether a room is safe for entry after a fire; that is a job for a confined-space gas detector with an oxygen cell and a CO cell, used by a trained person.

When it sounds: the sequence that keeps people alive

The alarm is loud by design. The listing for the Kidde COBD states an 85-decibel alarm, and the Kidde plug-in model states the same. That is roughly the level of a petrol-powered lawnmower at close range. It is meant to wake you, not to be pleasant. The correct response is not to press the hush button and go back to sleep. The correct sequence is: get everyone out of the building, including pets; call the fire service from outside; do not re-enter until they have measured the atmosphere and told you it is clear. If anyone has a headache, dizziness, nausea or confusion, tell the emergency operator, because those are symptoms of CO exposure and the person may need oxygen.

The hush button exists for nuisance alarms, not for real ones. If you are certain the alarm is false (for example, you have just used a gas hob with the extractor off and the alarm is in the kitchen), ventilate the room and press hush. But if you are not certain, treat it as real. The cost of a false alarm is a wasted call; the cost of ignoring a real one is much higher.

After a real alarm, do not simply reset the unit and carry on. The source needs to be found. A qualified technician should inspect and test every fuel-burning appliance, flue and chimney. Consumer alarms are not diagnostic tools, and the listing for none of the four products here publishes a measurement accuracy figure, because they are not intended to be used as meters.

The maintenance nobody does, and the reason it matters

Most people press the test button once and then forget the device for a decade. That is the failure mode the industry worries about most. Electrochemical sensors drift, dust blocks the inlet, and batteries leak. A CO alarm that is ten years old may still chirp when you press the button and still be useless. The fix is a written schedule: test weekly, vacuum the vents monthly with a brush attachment, replace batteries on the interval the manual states, and replace the entire unit at the end of its stated service life.

The listing for the Kidde smart CO and indoor air quality monitor adds a second maintenance burden: the indoor air quality sensor. That sensor measures something other than CO, and its output is not a substitute for a CO reading. If you buy a combination unit, read which sensor drives which alarm. The voice announcement on that model distinguishes “Warning Carbon Monoxide” from other alerts, which is a genuine usability improvement, but it does not remove the need to test and replace on schedule.

Travel and temporary placement introduce their own errors. The Kidde battery model is described as travel-ready, and using it in a hotel or holiday rental is a reasonable precaution. But a detector rattling around in a suitcase can be damaged, and a unit placed on a bedside table may be knocked or covered. Check it before each trip, and do not rely on a single travel alarm as your only protection at home.

Where a consumer CO alarm stops being the right answer

A consumer CO alarm is a life-safety device. It is not a leak detector, not a diagnostic meter, and not a compliance tool for a workplace. If you are a landlord or a tradesperson who needs to prove that an appliance is safe, you need a calibrated flue gas analyser or a certified CO meter with a known accuracy and a calibration certificate. The listing for none of the four products here publishes an accuracy figure or a calibration interval, because they are not that class of instrument. For the selection criteria that matter in a meter, see how to choose a carbon monoxide meter.

Similarly, a CO alarm does not detect natural gas, propane or refrigerant. If you smell gas, that is a different problem and needs a combustible gas detector used by a competent person. The safe use of those instruments is covered in how to use a combustible gas detector safely. And if you are checking whether a CO alarm responds to natural gas, the answer is no, and the reason is explained in does a carbon monoxide detector detect natural gas.

Finally, a CO alarm is not a substitute for maintaining your appliances. The most effective CO protection is an annual service of every fuel-burning appliance by a qualified person, plus adequate ventilation, plus an alarm as a backstop. The alarm is the last line, not the first.

The habit that makes the difference

A carbon monoxide detector is one of the few instruments where the correct use is mostly about routine: place it near where you sleep, test it weekly, replace the batteries and the unit on schedule, and treat every alarm as real until proven otherwise. The technology is mature and the devices are inexpensive relative to the risk. The failure is almost always human, not electronic. If you want a number rather than an alarm, buy a meter and have it calibrated; if you want to wake up in time, buy an alarm and look after it.

Frequently Asked Questions