HS-100 Series Plenum Application Duct Smoke Detectors Spaces, Ultra-Low Flow, In-Duct Mounting, Smoke Dampers is designed specifically for applications where our standard external mount duct detectors cannot be utilized. Examples in addition to commercial rooftop package unit applications include air shafts, between?ceiling plenum spaces, dampers and applications requiring extremely low or no air velocity (? 100 ft/min).