Notes on Smoke Detectors

Russell Bateman
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Randomly firing smoke detectors

This has been happening to me. I don't mean the beep every couple of minutes indicating the need for battery replacement, but the whole (linked) system going off. I did some research and found some interesting points that likely explain my problem.

How do smoke detectors work?

There is a source of beta particle radiation (yes, smoke detectors are weakly radioactive) that continually releases particles across a small space to be picked up by a detector. As soon as the stream of particles or even part of the stream is interrupted, the alarm goes off.

Systems that meet modern codes are interconnected both to 120v power and also linked by an additional electrical wire such that when once detector finds reason to go off, every detector in the system goes off in sympathy. This results in a whole house protection that is very effective.

Even though a modern smoke-detection system is wired to sector current and powered by it, each detector has a back-up battery much as radio alarm clocks and other devices for the "convenience" that this provides during power outages. The convenience of this in the case of a smoke detector is simply that if the power's off or out and your house begins to burn, one or more detectors will presumably sense it and sound the alarm.

What can set off smoke detectors?

  1. Low battery.
  2. Condensation.
  3. Change in temperature, especially from warm to cold, probably because of condensation.
  4. Cooking smoke or vapors.
  5. Dust particles.
  6. Tiny insects and arachnids.*

* This appears to have been my case because of the randomness of occurrence coupled with the random length, but short duration of the alarm. My guess is that the vibration of the alarm is annoying enough to chase the little beasties out of the hole in the detector sensor mechanism.

Is it possible to diagnose the exact cause?

If the alarm duration permits it, running around examining each detector will reveal, for most brands, that the offending detector is showing a red LED light instead of the usual, continual green one. This is the detector causing the rest to fire off.

When should I change smoke detector batteries?

Other than the obvious need to do this when the detector begins to complain by beeping every few minutes, many advise spending an hour each New Year's Day or the evening before your locale goes on or off Daylight Savings Time. Detector batteries usually last a good year making this a good policy.

I personally do not replace the batteries until I hear the detector beep. I find that my batteries are lasting several years.

Do smoke detectors go bad?

Yes. However, people frequently replace them unnecessarily with new ones after 1) a family member or acquaintance experiences a catastrophic fire, 2) unexplained failure creates mistrust. Still, better safe than sorry.

Why is the need to replace batteries only detected at night?

This probably lies in the realm of urban lore. No one seems to know why, but it is widely reported. Many also report that it drives them nuts while other inhabitants of the household seem to be able to sleep through the beeping.