Smoking out the differences in smoke detectors

It’s time to pull out the ionization detectors and go photoelectric.

These kinds of newspaper headlines are depressingly common. The copy in the article says “there were no working smoke alarms.” This is a significant difference in nuance; smoke detectors have been legally required in all housing for decades. But they are often not maintained, and in fact they are often disabled because of false alarms, usually from cooking.

This is obviously a serious problem for everyone, but also for those interested in green and healthy building as we try and get rid of flame retardants. And like everything I complain about, it is also a design problem; people don’t do these things randomly but respond to their conditions, their environment. If people are disabling their smoke detectors then there is something fundamentally wrong with their design.

Photoelectric vs ionization

NFPA Public service download/Public Domain

90 percent of installed smoke detectors are ionization detectors, which run on batteries, which is why they are so common; they are easier to install. There is a tiny bit of Americium 241 which emits alpha radiation, which ionizes the air between two plates connected to the battery. When a particle of smoke enters the detector it disrupts the ionization and sets off the alarm. It doesn’t take a lot of electricity to keep this going so it can run a long time on batteries. Some, like the ones I have in my cabin, have lithium batteries that can last as long as the smoke detector; most cheaper ones need batteries to be changed.

Ionization alarms are supposed to be better for “fast flame” fires that come from something like cooking oil or grease, which is why they tend to go off when people cook. And no doubt, the ongoing trend to open kitchens just exacerbates the problem with them.

photoelectric detectorsNFPA Public service download/Public Domain

Photoelectric smoke detectors have a light emitting diode that shines a light into a photocell; the smoke particles physically block the beam of light. Keeping that light going takes more energy, so they are usually hard-wired (although there are battery powered units available). But photoelectric detectors are better with smouldering fires, which often produce smoke before there is fire. In a provocative post by Skip Walker of Walker Property Evaluation Services, he notes:

The smoldering fire tests standards were developed when most home furnishings were natural materials, cotton, wool, etc. Today, virtually all furnishings and a large percentage of the building materials are synthetic and engineered materials. The behavior of natural and synthetic materials in a fire is radically different. Yet the UL standards have not been adjusted to account for this shift.

Green and healthy homes need photoelectric smoke detectors.

Of course, TreeHugger promotes the return to natural materials made without synthetic foam and without halogenated flame retardants. So in a green and healthy home where stuff smoulders instead of flaming up, you want photoelectric detectors. Skip Walker says they are also far more effective.

In 2007, UL published the “Smoke Characterization Study”. This study tested both types of smoke alarms using current UL testing standards and materials; they also tested the alarms using UL test criteria integrating a variety of synthetic materials and current tests such as smoldering toast. The results are frightening. Ionization alarms failed the UL 217 test 20% of the time using the current standard test materials. This is the test that the alarms must pass 100% of the time to be offered for sale and installed in US homes. When tested using synthetic materials, ionization alarms DID NOT TRIGGER (DNT) in 7 out of 8 synthetic test scenarios.

He also notes, as I have, that ionization smoke detectors are “notorious for nuisance tripping” and are eight times more likely to be disabled by occupants. “An Alaskan Public Housing Study shows that about 20% of ionization alarms will be disabled within the first year of installation; other studies indicate that this percentage may be higher.”

After reading all of this, I was curious about what the Nest people used in developing their thermostat. They write:

We ruled out making an ionization alarm early on. The radiation in them isn’t considered dangerous, but we don’t like that they use radioactive particles – what a weird thing to put in your home, when you think about it. France and the Netherlands have even banned them. But more importantly, ionization sensors are notorious for false alarms. They’re more likely to go off when nothing’s wrong – when you’re taking a shower or are just lighting a candle. They’re good at catching fast-flaming fires, but they’re great at driving you insane.

I do not particularly worry about alpha particles like those from Americium 241; they are stopped by a piece of paper and unless you eat your smoke detector it is not a problem. But Nest also claims to have improved the photoelectric detector with two light sensors, one red and one blue, that provides more information to their computer chips and is more sensitive to fast fires.

Of course, the smoke detector manufacturers response to all of this is to say BUY TWO! One of each! There is some logic to this, in that if the power goes out, you are still protected by the battery unit. But not much; if people are protected by the photoelectric they are going to be even less concerned about maintaining the ionization one or putting up with its false alarms.

Skip Walker concludes:

With everything we know, all the facts tell us that photoelectric alarms provide superior protection in real-world fatal fires… This year, don’t just replace your smoke alarm batteries – replace your alarms with photoelectric alarms and recommend that everyone you know do the same!

I think he is right, not because they are more effective at detecting fires, but because a disabled detector is totally useless. In four years with a hard wired photoelectric detector I have never once had a nuisance alarm.

If you are renovating or building new, make sure they are hard wired instead of battery powered. If you are going multi-family, make sure they are interconnected. And if you do change your ionization detectors to photoelectric, check before you toss the old one in the garbage; some municipalities consider them to be hazardous radioactive waste. That should tell you something.

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2018-07-13 20:14:42 – Source: