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Confounded smoke alarms

My electrician, who is safety-conscious above all else, has been bugging me for years now about smoke alarms. Sure, I have several battery-powered smoke alarms up, but from a safety improvement per dollar spent perspective, one really wants smoke alarms that are:

  • hard-wired, with
  • battery backup, and
  • interconnected

The batteries in battery-powered smoke alarms will run out. They do chirp to let us know it’s time to change the battery, but more often than not I won’t have a spare battery handy, or I won’t have a step stool nearby, or it will be the middle of the night, so instead of going back on the ceiling with a new battery like it’s supposed to, the alarm will sit around on a counter, battery-less, sometimes for weeks. Hard-wired smoke alarms solve the dead battery problem because they draw their power from the house electrical wiring. As it turns out, electrical fires that disrupt the power before smoke could be detected are really rare, and our power is pretty reliable, so the risk that the power’s off when the alarm needs to sound is really quite small, smaller than the risk that your battery-powered alarm will be sitting, battery-less, on the counter. And most hard-wired alarms also have battery backup, so you’re covered during power outages, too.

There are two smoke-detection technologies: ionization and photo-electric. Ionization sensors do well with small smoke particles, from fast-burning fires, while photoelectric sensors do better with large smoke particles from smoldering fires. Most safety recommendations (including Consumer Reports) are reluctant to specify one as being a better choice, and recommend both. So add to our wish-list:

  • dual-sensor

Interconnection of smoke alarms means that when one alarm goes off, all of them sound. So if there’s a fire in the basement while you’re asleep, the alarm in your second-floor bedroom will also go off, giving you much more time to escape than waiting either for enough smoke to set off a second-floor alarm or for you to hear the far-away alarm. The interconnection is conventionally done with three-conductor wiring: all the smoke alarms need to be installed on the same circuit and the third wire is used as the alarm interconnection signal wire. This is easy in new construction but really hard to retrofit: getting a new circuit to the ceiling of every location for a smoke alarm would mean lots and lots of holes in the walls and ceilings.

So an easier method for retrofit has emerged: wireless interconnection. The smoke alarms can still be hard-wired, but can be on different circuits, typically extended from existing ceiling fixtures. Sounds great! Kidde and First Alert both manufacture such systems. But each offers only one sensor type! Kidde’s hardwired with wireless interconnect detector uses an ionization sensor; First Alert’s uses photoelectric.

So which to choose? Compulsive complete-ist that I am, I scoured the internet looking for advice comparing the two detection methods, and eventually came across the NIST report which I believe all other advice is based upon. Like everything else I found, it, too, doesn’t recommend one technology over the other, but by sifting through its data, I’ve concluded that I definitely want both sensor types in each room, even if that means installing two smoke alarms in each room.

The NIST tests set fires of several types (flaming, smoldering, cooking) in two settings: a trailer home and a two-story home (although one without soffits between the rooms and the hallways). They measured the time it took for alarms of different types to sound, and also the time between the alarm sounding and the moment at which escape becomes “untenable,” meaning that the smoke density had reached some critical level.

For flaming fires, what struck me was that the total time between the start of the fire and the onset of untenable escape conditions was between 150 and 400 seconds. Ionization detectors sounded 24 to 64 seconds earlier than photoelectric, which is a significant amount of time on the time scale of flaming fires. Other research cited by NIST indicates that it can take the average person 50 seconds to actually exit a house, taking time to put on a bathrobe, grab wallet and keys, wake children up, and so forth. 

What isn’t said, but can be inferred, is that interconnected ionization detectors give the best and likely only chance to rescue, say, a child, from a bedroom in which the fire starts. The times to untenability only refer to the escape of people who aren’t in the same room as the fire. Fire codes “are not designed to protect people intimate with the initial fire;” survival in the same room as the fire is an entirely separate, more complicated, and controversial topic. But needless to say, every second would count in such a situation, thus the extra speed of an ionization detector in the afflicted room, plus interconnection to immediately let everyone else in the whole house know, would give as many extra seconds as possible.

So we know we want ionization detectors. What about photoelectric? Well, the total times to untenability for smoldering fires fell between an hour and an hour and twenty minutes. In many cases, photoelectric detectors responded 15 minutes to an hour faster, time scales which are significant fractions of the fire time. Catching such a fire 15 minutes sooner could significantly reduce the damage done by a fire–it might even give enough time for a homeowner to take care of the fire with a fire extinguisher. The only instances when photoelectric detectors didn’t outperform ionization detectors for smoldering fires was when the detector and fire were far apart: a fire in a bedroom with its door closed, with the smoke alarm in the nearby hallway, or a living room fire with the alarm in the hallway. But if there was an alarm in the room in which a smoldering fire started, the photoelectric detector sounded significantly sooner.

So, for now, two alarms per room.

1 comment

1 JWA { 08.24.09 at 11:07 am }

I was reading some of your old Bromtpon posts…

I too am a DC commuter, using a Brompton.

Has WMATA “forced” you to get a carry bag for your bike? I had an incident at Union Station where the station manager insisted a folder had to be inside a bag during ‘rush hour’… And then pulled out a WMATA brochure that said exactly that.

From 7 – 9:30 and 3-7, it must be in a bag…

So I went and bought the folding bag, which is a considerable expense, but also a nice piece of equipment.

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