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Recently, four of the six battery-operated smoke alarms in our house have started going off every few days, for no obvious reason. The four that have been going off are of three different models, including both ionization and optical types. They are not wired together. We have a CO alarm which has not been going off.

This happens in the absence of any obvious source of smoke, dust, water vapour etc.

So the question is: what, apart from the obvious, would cause multiple types of alarms to go off?

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    I had a brand new house with wired smoke detectors. About once a year I'd have them go off, often in the middle of the night. One pulse that would set all of them of. I never figured it out either. Am curious if anyone sheds light here.
    – Machavity
    Commented Dec 19, 2016 at 21:18
  • Wired smoke alarms are supposed to go off together. Low batteries? Legalized weed in your state?
    – Harper - Reinstate Monica
    Commented Dec 19, 2016 at 21:21
  • I recently (October 2020) installed 5 interconnected by wire smoke alarms. All brand new. I have had 2 false alarms in the middle of the night also. NO sign of what triggered the voltage in the interconnect wire to set them ALL off.
    – Sean
    Commented Nov 21, 2020 at 13:15

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Some battery-operated alarms use a radio signal to interconnect so they're all set off together. You should be able to verify/test this using the test button: press it on one, and if the others go off then you know they're interlinked.

As far as what's triggering them, there's a few possibilities:

  • Dust in in the sensors. Recent renovations especially can cause dust to get on the sensors, making them very sensitive to even a slight change in the air. Using a vacuum to clean them out can often remedy this.
  • Signal from other nearby units. If you have wireless interconnect, they may be on the same frequency as neighbour's alarms. There should be a method to change the code they use (often some dip switches on the back).
  • Bad location. Smoke alarms should be located a minimum of 4" away from a edge of the wall/ceiling, inside corners, and the peak of a vaulted ceiling to avoid the dead air space. Good airflow is important.
  • Wireless interference. Apparently some brands can be sensitive to interference from other electronics, and it's best to relocate the alarm or electronics.
  • Defective alarm. It's possible one or more alarm is simply defective. Most smoke alarms have a 10-year age limit.
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    Strong point on the detector age limit. The manufacturers recommend replacement of the detectors according to an expected lifetime.
    – Michael Karas ♦
    Commented Dec 20, 2016 at 1:04
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Some of our alarms go off as the batteries are close to dying. Yes, the alarm goes off, not just the beep that tells you to check the battery. If you replaced the batteries in the effected units at the same time, perhaps that might be the cause.

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