Carbon Monoxide (CO) has approximately the same vapor density as room air, meaning that it will hang around at the vertical mid point of a given space. The CO detector should ideally be installed at the four foot level, assuming the ceiling height is eight feet. If you only have one detector, I would put it in close proximity to the bedrooms. If your residence is multilevel, I'd put one on each level. Additionally, keep in mind that the CO sensors are usually good for only about 18 months. If the detector is located in an area where it would receive above average amounts of CO, such as garages, fire place or wood burning stove areas, etc. the sensor lifespan could be shorter. Check the manufacturers information for additional details and specific mounting requirements.