The D355PL non-relay photoelectric duct smoke detector features a pivoting housing that fits both square and rectangular footprints capable of mounting to a round or rectangular duct.Includes a SD355R factory installed detector. Backward compatibility with System Sensor duct smoke detectors and remote accessories.RECOMMENDED SAMPLING TUBES:— DST1 Up to 1 ft.— DST1.5 1 to 2 ft.— DST3 2 to 4 ft.— DST5 4 to 8 ft.— DST10 8 to 12 ft.(2-piece)