"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. "
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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.