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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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Programmer
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    Is it connected to anything? Are there wires or something on the other side? – The Evil Greebo Aug 20 '18 at 15:10
  • @TheEvilGreebo There are no wires on this side and I'm not sure if there are on the other side as I don't have access to that area. – Programmer Aug 20 '18 at 15:12
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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 '18 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 '18 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 '18 at 15:33
  • @Tyson that's definitely the one, but what is it for? And why would it be located where it is? – Programmer Aug 20 '18 at 15:39
  • Do you have an alarm system? Do you see signs that there was any such device installed previously? – Tyson Aug 20 '18 at 15:45
  • @Tyson we used to have an alarm system. The wiring has since been removed for the most part although there are still some wires in some of the walls. – Programmer Aug 20 '18 at 15:46
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    ..... and a siren in the return air duct – Tyson Aug 20 '18 at 15:47
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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 '18 at 15:57
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    @Programmer the siren is often tucked away in hard to access spots so that an intruder cannot easily find and disable it. The noise one of these makes is loud enough to cause disorientation to the intruder. Being unable to find it they will often be driven out of the house by the noise. – Kris Aug 20 '18 at 18:55

2 Answers2

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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.

Tyson
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  • why the "Unfortunately"? – dandavis Aug 20 '18 at 16:17
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    @dandavis The sentence before that.... – Tyson Aug 20 '18 at 16:35
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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 '18 at 16:58
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    What is a DSC surface siren? – user2357112 Aug 20 '18 at 18:33
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    It's actually in the duct? Won't that limit the distance that the siren can be heard? Or it only for in-house notification? – Arluin Aug 20 '18 at 18:48
  • @user2357112 added a sentence, end of first paragraph. – Tyson Aug 20 '18 at 18:52
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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 '18 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 '18 at 20:16
  • How does DSC recommend their system be installed? I can't imagine they're doing a *wink*wink*nudge*nudge* thing RE building code violations like so many installers allegedly are. – Dan Is Fiddling By Firelight Aug 20 '18 at 21:02
  • @nullability - if the fire is such that the device is melting and emitting toxic smoke - there are bigger problems ;) – NKCampbell Aug 20 '18 at 21:29
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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 '18 at 23:04
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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.

Steve Rominger
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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 '18 at 02:15