I can't figure out where is the best place to install a carbon monoxide 
detecter. Some you can plug into a wall plug, some say mount near 
bedrooms on the wall, and they have combination detecters for both 
carbon monoxide and smoke detectors which you install high on a wall 
or ceiling.

So where is the best place to install a carbon monoxide detector 
according to the National Center for Bitoechnology information (NCBI) 
in conjunction with the Center for Hyperbaric Medicine, Virginia 
Mason Medical Center, Seattle WA 980101 did a test by introducing 
carbon monoxide at the top, middle, and bottom of a Plexiglass 
Chamber and found and over time the gas spread equally throughout the 
Plexiglass chamber. This web address provides information on the 
test: http://www.ncbi.nlm.nih.gov/pubmed/

So it doesn't matter where you place the detecter just make sure it 
is not obstructed by curtains, furniture, etc., and outside of sleeping 
areas at any level on the wall.

Best rule, read the directions for the carbon monoxide detector you 
purchased. If doesn't state what level on the wall to install you 
can choose the best location on the wall for you. If you have small children you 
might want to consider placing the detector on the middle of the wall 
or higher to keep away from little fingers.