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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).