I was looking through my HVAC ducts with a camera and found this device attached to the side of a floor joist. It is about 2-3 feet from the air return vent. What is it?
It’s some form of the DSC surface siren. It may have a built in siren driver or it may be “speaker only”. They are typically used with home alarm systems.
These have been private labeled over time by various national alarm companies.
They are not rated for use in a return air duct, nor allowed by any code. Unfortunately there are many in ducts just like yours is.
I suspect it is the SD model which would make it a smoke detector. Smoke detectors are often installed in return air ducts in the commercial world, however, this is probably not meant to be installed in a duct. That buildup of dust will likely lead to false alarms.