Duct detectors are specialized housings installed on pieces of ductwork or air handling units that measure or sense that there is smoke in that air stream coming back to that unit. We use that signal from the smoke detector to shut down the air handler when it sees enough smoke in the air stream. We shut down air handlers so they don't spread smoke throughout the space. The code calls for duct detectors so we don't use the air handling system to spread smoke throughout the space and then slow down the egress of the building. Duct detectors are a major headache for false detection in buildings. It's a fairly poor way to detect a fire and it causes more harm than good, but nonetheless, it's part of the code and we have to do it.