Here's from a question and answer about CO Detectors:
Where should I install my Carbon Monoxide Alarm?
Install at least one CO Alarm near or in each separate sleeping area. For added protection, install an additional CO Alarm at least 15-20 feet away from the furnace or fuel burning heat sources. Also, locate CO Alarms at least 10 feet from sources of humidity like bathrooms and showers. In two story houses, install one CO Alarm on each level of the home. If you have a basement, install that CO Alarm at the top of the basement stairs.
Is there anywhere I shouldn't install my Carbon Monoxide Alarm?
DO NOT locate a CO Alarm in garages, kitchens, furnace rooms, or in any extremely dusty, dirty, humid, or greasy areas. Do not place units in direct sunlight, or areas subjected to temperature extremes. These include unconditioned crawl spaces, unfinished attics, uninsulated or poorly insulated ceilings, and porches. CO Alarms should not be located in outlets covered by curtains or other obstructions. Do not place in turbulent air-near ceiling fans, heat vents, air conditioners, fresh air returns, or open windows. Blowing air may prevent CO from reaching the CO sensors.
Is one CO Alarm enough for my home? If not, how many should I have?
Install a CO Alarm on every level of your home. If you install only one CO Alarm in your home, locate it near or in your bedroom.
How high should I install my CO Alarm?
For ease of viewing you can locate the alarm about 5 feet off the floor. Carbon monoxide weighs about the same as air and distributes evenly throughout the room/house. Choose a location where the CO Alarm will stay clean and out of the way of children or pets. See your User's Manual for specific installation requirements.