How many carbon monoxide detectors are needed in a house and where is it best to place it? Is it better to place it close to the ground or close to the ceiling?


The International Association of Fire Chiefs recommend a carbon monoxide detector on every floor of your home, including the basement. A detector should be located within 10 feet of each bedroom door and there should be one near or over any attached garage. Each detector should be replaced every five to six years.

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Positioning them at head height is recommended, and Be-Alarmed suggests:

  • Do not place the alarm in a cupboard, behind furniture, near an outside door or ventilation ( e.g. extractor fans or cooker hoods)
  • Do not place alarms directly next to fires, boilers, cookers or heaters – the alarm should be at least 1 meter away from any of these appliances
  • Do not place an alarm in an area of high condensation and steam such as a bathroom or kitchen or sources of steam and condensation
  • The alarm should not be placed on a ceiling like a smoke alarm
  • I don't understand why it's not good to place the CO detector close to the furnace and in the ceiling. Is it just for limit the call to the emergency service? – ucsky Mar 3 '13 at 17:45
  • Too many false positives if you place them too close to the furnace. And the ceiling may not pick up CO, as it is slightly heavier than air. – Rory Alsop Mar 3 '13 at 18:30
  • I agree that in that case if your put the CO detector near the furnace or in the ceiling, if the CO detector go off that not mean you have 30 ppm every where in the place (or whatever is the threshold value) but I suspect that mean that there is a problem. I don't think anybody want to breathe 25 ppm of CO every day. That will probably give you some serious injuries. Most of cheap CO detector will not detect 25 ppm – ucsky Mar 3 '13 at 18:40
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    Hmmm - the page I was reading said avoid the ceiling and floor, as the CO was only slightly different so it would move with airflow. I'm guessing I misread lighter/heavier - either way, it is very similar, so ceiling is not good, unlike heat detectors etc. – Rory Alsop Mar 3 '13 at 19:03
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    Whatever you do, make sure you have one on the same floor as where you sleep – Steven Mar 4 '13 at 1:46

I work in EHS with a mobile fleet that people sit and work in, so this is a very common question I get asked. It is a common misconception that CO is heavier than air so the detectors should be near the floor. In fact, they are slightly lighter than air, but not significant enough that it will rise upward. The molar mass of air is 29g/mol, O2 (oxygen we breathe) is 32g/mol, and CO 28g/mol. As such, CO will move with air flow. The optimal placement is at head height in the room. If placing in your living room where you mostly sit on the couch and watch TV, optimal placement would be at the average head height sitting on your couch; Pillow level in bedrooms; etc.

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