Domestic Smoke Alarms

Smoke Alarms are primarily used in domestic residential situations. They are different from the Smoke Detectors used in Fire Detection and Alarm Systems because they have a built-in alerting device, and are designed, tested, and manufactured to different standards. The most common examples are the battery-operated units available in many hardware stores. 

Early warning of the presence of smoke in a building is critical to life safety. Some fires can grow rapidly making escape very difficult, especially if the occupants are asleep, intoxicated, and/or the escape routes are smoke-logged. The time difference between escaping from a burning building or dying in a fire can often be measured in seconds. Sometimes fires can smoulder for hours, filling the premises with toxic fumes. Without the early warning provided by a smoke alarm, occupants can perish as they sleep without ever waking. 

“Type 1” Smoke Alarms are mandatory under the New Zealand Building Code Compliance Documents for new installations in domestic residential situations, including detached dwellings. Stand-alone battery-powered units are the minimum, however for optimal life-safety benefits, or larger dwellings, interconnected units are necessary to provide adequate sounder audibility to wake all occupants. Permanently-wired mains power, with long-life battery backup, is the most reliable configuration.

Because they have none of the inherent self-supervision or maintenance regimes of commercial Fire Detection and Alarm Systems, and they are not listed on a building’s compliance schedule for occupancy warrant of fitness, it is imperative that dwelling owners conduct their own regular maintenance and testing of their smoke alarms. 

These procedures include: annual cleaning with a vacuum cleaner (no disassembly), monthly testing with the alarm’s “test” button, and regular battery replacement (interval depends on type – annual for ordinary dry-cell batteries). 

A New Zealand Standard NZS 4514:2002 Interconnected Smoke Alarms for Single Household Units exists, however it is not mandatory. Under the current “Type 1” Compliance Document regime, the requirements are contained in section F7/AS1, and installation is required to be to AS 1670.6 and the manufacturer’s instructions.

Ionisation or Photoelectric? 

Commercially-available domestic smoke alarms are usually either ionisation chamber or photoelectric (light scattering) types. Considerable debate has taken place in the media as to which technology gives better performance, especially considering the price difference between the two is minimal (historically, photoelectric was considerably more expensive and drained batteries much faster). 

Ionisation smoke alarms respond most readily to the invisible products of combustion typical of fast flaming fires, but have a much slower response to smouldering fires. Escape routes can therefore become more smoke-logged before a warning is given. Ionisation detectors are prone to nuisance alarms from cooking activities, so must be sited well away from kitchens. They also contain a (very small) radioactive source, which in some countries presents environmental issues (not in New Zealand, where, under current legislation, they are permitted to be disposed of in the landfill with normal household rubbish). 

Photoelectric smoke alarms respond most readily to visible smoke, so can give a slower response to fast flaming fires with invisible products of combustion. They are also prone to nuisance alarms from steam, so must be sited well away from bathrooms and saunas. 

Photoelectric is the preferred smoke alarm technology for life safety in new installations, as it offers the most consistent early detection performance across all the likely fire scenarios in a residential environment.