Smoke Detector Woes


My neighbors are having a smoke detector problem. It recently started beeping every 30 seconds or so. The obvious solution is that the beep is a low battery indication and you should replace the battery, but that apparently is not the case. The facts as I know them are as follows.
    1. The building is a modular home of recent construction with     one floor and a basement.
    2. There are a total of 5 smoke detectors. There is one in each     of the bedrooms (3), one in the hallway, and one in the basement.
    3. The smoke detectors on the first floor are all BRK 2002RAC. I     forgot to check the model of the one in the basement, but it is     different. I cannot say for sure it is even a BRK unit, but based     on appearance and memory, it was probably BRK 9120B.
    4. According to the drawings, the four first floor detectors     were put in by the modular home manufacturer. The one in the     cellar appears to have put by the builder when they were putting     it up. They are all interconnected.
    5. The first thing I did was to turn of the breaker (green LED in     unit went out confirming loss of AC) and check the battery in each     of the five units. I replaced four marginal ones. Testing was     done with DVOM and a battery tester that added some load. The     one good battery tested 8.99 V under load. The rest, the were     replaced, were typically 8.5 V. The beeping remained on through     out the process and did not stop even after all units were back in     place with known good batteries and the power restored.
    6. The beeps did not always come from the same location, so I     could not tell which unit was the problem one. As a result, I     turned off the AC again, and took all five units down.
    7. This is where it gets strange. With all five units out and the     AC power off, the beeps continued from different locations at     different times, and they seemed to be coming from the original     locations in the ceiling, not the units.
    8. I then removed the battery from all five units, so none were     powered, just to be sure the units were not beeping. That did not     help as the beeping continued.
I know large value capacitors can can keep things running for a long time if the load is small, but there were four of us there, including my wife and the two neighbors, and we all confirmed that the beeping appeared to be coming from the ceiling mount point, and not the disconnected units on the floor that had no batteries in them.
Based on this I have a few questions, besides the obvious one of how to stop the beeping.
    1. I realize the smoke detectors are tied together so if one     detects smoke and alarms, they all alarm. Does the same hold true     for the low battery beep, and does it round robin through the     other units?
    2. Is it possible that there are transducers or other audio     transmitters near the mount point as well as on the unit?
I have never heard of such a thing, but I could see where someone might of thought it was great idea so that you would still know a remote sensor was alarming, even if the local unit was completely dead. The mounting bracket for the detector covered the electrical box behind it preventing a thorough inspection of what else might might be in the box besides the wiring.
    3. If the sound is coming from the ceiling location, and not the     units themselves, what is the source of the beep and where is the     power for it coming from?
All the units are on the floor, with no batteries in them, and the AC power has been turned off. The only thing I can think of is that there is a detector that we have not located that is still connected and it has a low battery, but if that is the case, it is not shown on the electrical drawing for the house as constructed, and I can not locate it. You would think it would not be buried out of sight.
I have looked around the archives for other postings that might be similar in scope as mine, but could not find any. If I missed one, my apologies, and please feel free to send me a URL rather than compose a similar response.
Thanks for your help,
RPN
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RPN wrote:

You might want to consider that smoke detectors become less sensitive with age. How old are these detectors? Maybe it might be good just to replace them all. I just replaced all mine about six months ago. They were all about 8 years old and I believe the recommendation was like 5-6 years.
--
Joseph Meehan

Dia \'s Muire duit
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I've had customers with similar situations. First of all I find it impossible to detect which unit is beeping, so I remove them all to an outside location. Typically the beeping continues and I have found it to be: Units mounted in the attic that no one knew about. Units in boiler rooms that no one knew about, and carbon monoxide detectors that people had plugged into wall outlets and never suspected

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Second that. The frequency of the beep disguises its location.
We had a similar situation. After disarming the smoke detectors - and continuing to hear the beep - we were convinced it was the burglar/fire alarm system. The alarm company mechanic found it: a carbon monoxide detector two rooms down the hall (it was plugged into a switched outlet and its backup batteries had failed)!
Just make a list of things that go beep. It's got to be one. Don't forget cell phones.
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if you want to try it.
Don Young
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says...

Thanks to all who replied. All of you provided useful advice. It turns out it was the carbon monoxide detector within 5 feet of the smoke detector. A steady tone instead of a chirp every 30 seconds or so would have made it easier to track down, but such is life. All is well that ends well.
Thanks again.
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