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       As electronic devices, detectors are subject to random failures. Product, installation, and maintenance standards are used to assure products work as designed despite this. Part of the technical basis for the first detector product standard was an assessment of expected failure rate, estimated at four per million hours of operation or one every 30 years. Early field studies of detector reliability, notably by Canada's Ontario Housing Corporation, confirmed the essential accuracy of this estimate, restated as a 3% failure rate per year. This means a very small fraction of home smoke detectors will fail almost immediately, and 3% will fail by the end of the first year. After 30 years, nearly all the detectors will have failed, most years earlier.

       How soon should you replace your detector? This is a value judgment. Only 3% of detectors are likely to fail in the first year, and annual replacement would be very expensive, so that doesn't make sense. At 15 years, the chances are better than 50/50 that your detector has failed, and that seems too big a risk to take. Manufacturers' warranties for the early detectors typically ran out in 3-5 years. So, in ten years there is roughly a 30% probability of failure before replacement. This seemed to balance safety and cost in a way that made sense to the responsible technical committees.

       If a 30% failure probability still seems too high, remember that replacement on a schedule is only a backup for replacement based on testing. A national study found home smoke detectors, when they fail, tend to fail totally, as opposed to hard-to-detect creeping failure, such as a loss of sensitivity.¹ Regular monthly testing will help discover detector failure as well as a dead or missing battery. You can replace your detector when it needs replacing.

       The same study showed all the inoperable detectors tested in 1992 were at least 5 years old and predated a 1987 change in product standards that reduced sensitivity to reduce nuisance alarms. Changes in detector chip design, among other improvements, make it likely that electronic failure now occurs at a rate much less than 4 times per million hours of operation.

       Replacing detectors after 10 years protects against the accumulated chance of failure, but monthly testing is still your first, best means of making sure detectors work. Today's detectors are even less vulnerable than the original detectors. Regular maintenance of the more sophisticated systems used in larger buildings can keep them working very reliably for many decades.