This link
Carbon Monoxide Detector Placement: Where to Place CO Alarms | Safety.com gives you all the information you need to know on Carbon Monoxide detectors and where they should be placed in the home.
The CM detector was in place when we purchased our home and it was just about as high on the Cathedral ceiling as you could get .... WRONG. CM detectors need to be around knee height. Ours are on the night stands, on end tables, all at knee height.