In general, when only one detector is required in a room or

space, the detector should be placed as close to the center of

the ceiling as possible. Central location of the detector is best

for sensing fires in any part of the room. If a center location

is not possible, the detector may be placed no closer than 4

inches from the wall, or if listed for wall mounting, it may

be mounted on the wall. Wall-mounted detectors should be

located not less than 4 inches nor more than 12 inches from

the ceiling to the top of the detector, and at least 4 inches (10

cm) from any corner wall junction.  (NFPA

72-2007)

When air supply and/or air return ducts are present in a room

or space, the detector(s) should not be placed in the path of

the air flow supply or return (NFPA 72-2007).