Why does the humidity set off our smoke alarms?

Smoke alarm

We have hard-wired smoke detectors at our cottage. Every summer, particularly on humid nights, all three units will go off simultaneously. Replacing them, and vacuuming away the dust and bugs, has had no impact. Could the wiring be the problem?
—Desperately Seeking Silence

It’s possible, but Robert Redford of Robert Redford Electrical Services in New Westminster, BC, doesn’t think so. “In my experience, it’s very seldom faulty wiring.” So, first rule out the other common reasons for nuisance alarms.

Since the smoke detectors are going off on humid nights…maybe it’s the humidity. If they’re ionization models, try replacing them with photoelectric alarms (they’re less susceptible to moisture in the air). 

Another possibility is that the locations of the alarms are causing them to malfunction. Ideally, you should place them on the ceiling, in the centre of the room, away from doors, windows, corners, lights, fans, vents, air registers, or other sources of heat, dust, or steam. Unfortunately, “without knowing the layout of the cottage, it’s hard to say what the problem might be,” says Ryan Betts of the Ontario Office of the Fire Marshal and Emergency Management. With the units interconnected, if even one is located improperly—too close to a bathroom, for example—they’re all affected. “If someone has a shower on one floor, that sets off the alarm, and then the rest go off,” explains Redford.

But say these smoke detectors are new, humidity-resistant, clean, properly located and maintained, with fresh batteries, and they’re still screeching for no good reason? Call in an electrician or a fire-safety technician.