"Buoyant flows dominate where active ventilation is not present. Mixing
between connected rooms on a floor occurred with a time scale of an hour or
two with doors open. Mixing between floors was very small in a downward
direction and significant in an upward direction. Closed doors were
effective in limiting CO dispersion in the absence of active ventilation. "

PDF file
http://www.nfpa.org/assets/files//PD...torSpacing.pdf

Bottom line: If there is a mechanical ventilation system, it doesn't matter
where you place the detector(s). If the doors are open in the building,
place the detector on the top floor. If the doors are closed, place the
detector high in the room that contains the CO generating device.