Reference

Modern UL 217 added three tests no residential smoke alarm had to pass before: smouldering and flaming polyurethane foam fires, and a cooking nuisance test the alarm passes by staying silent. This guide covers how the 8th, 9th and 10th Editions differ, what NIST measured on real alarms, and whether yours must be replaced.

A smoke alarm that goes off with no fire is nearly always detecting something real that is not smoke: cooking aerosols, shower steam, dust, an insect in the chamber, or a sensor that has drifted with age. This guide works the causes by symptom, gives the durable fix for each, and covers what NIST found about new alarms.

Ionization and photoelectric smoke detectors sense different fires at different speeds, and NIST's full-scale testing shows the gap can be a survival margin. This guide covers how each sensor works, the UL 217 8th Edition tests that reshaped the market in 2024, NFPA 72 placement rules, and a worked five-alarm plan for a two-level home.