False smoke alarms in AC ducts: troubleshooting tips?

July 16, 2014 11:32 AM

Our new commercial space is experiencing frequent false smoke alarms in one specific area of our AC ducts. The company that installed the AC system is stumped, and so is the electronic security company that installed the smoke detectors. We've just had our ducts cleaned top to bottom but the problem persists even though we're presumably dust-free. Any thoughts on what to try next? Engineers? Inspection services?
posted by ducky l'orange to Technology (8 answers total)

IMHO, whoever is responsible for the contract with the security company needs to put more pressure on them to figure out what the problem is. "We're stumped" isn't an acceptable answer. If that doesn't work, my next step would be to contact a lawyer to see if the security company can be held liable for the cost of any additional outside contractors that need to be brought in to get the system working properly.
posted by drlith at 12:01 PM on July 16, 2014 [1 favorite]

We just pulled the permit for rewiring our house, the city said "oh, by the way, you have to put in wired smoke detectors", so I just starting to learn about such things...

Any chance you could get the security company to put in ionizing detectors in that region of the ductwork rather than photoelectric ones? There's a chance that the photoelectric ones are getting false alarms because of dust that the ionizing ones won't trigger on. (I think usually you go the other way to avert false alarms, but in this case...?)
posted by straw at 12:23 PM on July 16, 2014

Random dumb thought from a stupid person: a lot of hvac systems now use electrostatic filtration. Like the sharper image tower filters, etc. these produce ozone and ionize the air. If you have the ionizing detector smoke alarms... Maybe those are playing fucky with each other? Sort of the reverse of what someone said above.

I've seen both the ionizing air cleaners and the ionizing water filters cause weird problems with stuff before and even break things in certain edge case odd situations. I definitely wouldn't rule that out.
posted by emptythought at 12:26 PM on July 16, 2014

From an anonymous commenter:

"tl;dr: if it's a single unit going into alarm, it's likely a bad unit. If it's multiple units going into alarm, it's probably something in the air handler. If it's any number of units going into trouble, it's probably an installation issue.

I am a controls contractor. I am not your controls contractor. This is not controls advice.

I integrate controls equipment with building automation and security systems on the institutional scale (prisons, hospitals, et cetera).I have sites with dozens of duct smokes and sites with just one or two.

First of all is it actually going into alarm or it it going into trouble? If a single unit goes into trouble on the regular then it's possibly a faulty unit, replace with another and test. If multiple units are going into trouble then it's probably an installation issue, see below.

If you're seeing multiple duct smokes shitting it up in a single zone then it's time to check out what's unique to that zone. Were the smokes installed by an apprentice? Don't laugh, I just had to have a tenant space's JAGA units rewired because the apprentice ran the analog outputs through a relay. Were other areas installed by this contractor and they are working fine? If they did the smokes in multiple zones and it's just the one zone that is fucking up, it's probably the air handler, maybe there's something stupid going on with a heating coil or a filter is shedding shit or something. Does the air handler have its own return system (mixed air)? If so is it tripping in the supply or return ducts, or both? If it is 100% OA, are there any associated exhaust systems that are tripping as well? Is the duct system confirmed to be sealed? Sometimes someone will pull a dumbass and run a fan when the supply ducting is closed off, and pop a seal or even split a duct, and then you get all kinds of fancy attic shit getting sucked or blow around or whatever. One time I had a guy wire up the SA and RA dampers the same direction so both of them closed on the same voltage and he crushed the shit out of a lot of ducting, then didn't tell anyone and reported it good.

If it's not any of those things then it could be the actual smokes I guess, but it's pretty unlikely that it is the actual units. 90% of the time if multiple sensors of any type fail in a common system, it's an installation issue common to all the affected sensors. Basically someone figured they were smart enough that they didn't need to read the little sheet of paper that comes in the box and explains how to put the thing in. The two most common duct smoke fuckups I see are installations too close to turbulence (elbows, balancing dampers, stuff that makes the air flow less steady), and improper sampling tube/high side installation relative to airflow and exhaust tube. It's really unlikely that airspeed is the issue but just in case you will wanna confirm that you are getting some decent FPM at the sampling tube. Your balancer or commissioning agent can confirm this but really it's unlikely to be the issue.

posted by restless_nomad at 12:46 PM on July 16, 2014 [2 favorites]

Water mist can set off some smoke detectors.

My guess is that in the problem area, cold air in the AC is meeting warmer air with enough moisture in it to generate fog, and the fog is setting off the detector.
posted by jamjam at 1:16 PM on July 16, 2014

First we need to know the type of detection. What type of detection do the devices use ?
posted by The Violet Cypher at 11:46 AM on July 17, 2014

Note to AskMefi question submitters: after submitting your question, come back after a couple hours and see if we are asking you to clarify something! 24 hours, nothing further from OP ...
posted by intermod at 12:57 PM on July 17, 2014

Response by poster: All fantastic responses! Thank you everyone, and apologies for not checking back sooner!
posted by ducky l'orange at 10:01 PM on July 20, 2014