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?

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    Is it connected to anything? Are there wires or something on the other side? – The Evil Greebo Aug 20 at 15:10
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    It’s an alarm speaker or speaker with self contained siren driver (there are several models with the same housing). Also private labeled by several national alarm companies. Having trouble finding a current link, it seems the current model has rounded edges. I think DSC is the OEM. – Tyson Aug 20 at 15:24
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    It’s not rated nor allowed in return air vents, but unfortunately there are many out there just like yours. – Tyson Aug 20 at 15:28
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    Found a good search: put “DSC surface siren” in a google IMAGE search... in addition to that model it’s also sold “speaker only”. – Tyson Aug 20 at 15:33
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    I didn’t mention but it’s also not necessarily obvious either, you asked “why it would be there?” Homeowners don’t like ugly things on walls, alarm installers conveniently hide them in [return] air ducts. Home Alarms are never inspected for code violations unless installed during original construction, and even then “speakers in return air ducts” may not be on the inspectors radar. – Tyson Aug 20 at 15:57
up vote 13 down vote accepted

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.

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    Well, is it "unfortunate" as in a potential fiery virtual Sword of Damocles dangling over thousand's heads, or unfortunate as in "sloppy worker, tisk tisk, this will get dusty, nbd"? – dandavis Aug 20 at 16:58
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    What is a DSC surface siren? – user2357112 Aug 20 at 18:33
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    @Arluin it’s rather creative, the ducts actually carry sound rather well throughout the home. And yes it’s for indoor notification and loud. Another argument installers use for “hiding” it the duct is that it would be harder to find to silence by the accused. – Tyson Aug 20 at 18:55
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    @dandavis It's unfortunate because it's potentially a source of toxic smoke during a fire that would spread quickly throughout the home. The device is not plenum-rated and as such is not allowed in an air duct in any building code. – nullability Aug 20 at 20:16
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    @NKCampbell, the siren changes the airflow around it. This can create a "dead area" that encourages dust build-up. If this dust ignites, it can cause the siren to emit toxic smoke that would kill everyone in the house while the fire is still small enough to put out with a single extinguisher. This isn't a theoretical concern -- the reason for "plenum rating" and similar fire-safety rules is the sheer number of people killed by toxic fumes from otherwise-survivable fires. – Mark Aug 20 at 23:04

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.

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    It’s not a smoke detector. It’s a simple speaker, it may have an internal siren driver circuit board or it may not. – Tyson Aug 21 at 2:15

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