Ionization vs Photoelectric Detectors

…or why I have two smoke detectors next to each other in the basement. 

One of the biggest safety issues I see in my home inspections is also one that is easy to remedy: the lack of smoke detectors or (more commonly) the prevalence of smoke detectors that are past the 10-year life expectancy and should be replaced. It’s an easy fix: almost every hardware store sells them for a very affordable price. They are not only an invaluable life safety device that can save lives but, along with fire extinguishers, can help stop a fire from becoming out of control and destroying a home.  As mentioned, I have two smoke detectors in very close in proximity in the basement. The obvious reason for this is so that there is no question we will hear the alarm upstairs while sleeping and as a backup if one fails, but it is a little more complicated than that. The main reason is that the detector present when we moved in was ionization and I wanted an photoelectric alarm, especially near the dryer. Does this mean a photoelectric detector is better? The truth is more complicated than that. 

Ionization Advantages
The biggest advantage of this type of detector is that it is superior in detecting flaming fires. In other words, fast moving fires that can quickly overwhelm occupants and block escape routes. This would be fires as the result of paper, wood or flammable liquids to give a few examples. These detectors are, believe it or not, filled with a radioactive substance called Americium. As scary as that sounds, the amount is astronomically tiny and poses no threat as long as they are not deliberately dismantled. In my experience, most smoke detectors utilize ionization technology. 
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Why Use a Photoelectric Detector?
Photoelectric smoke detectors are better at detecting smoldering fires, which are fires that are barely visible with a lazy haze of smoke, often for hours. A common example would be a burning cigarette, and a photoelectric detector can alert occupants to a situation that can lead to a flaming fire and give time to act (either fight or flee) accordingly.

Which is Better?
I will again use my most common stock phrase and say “it depends” but the most important fact is that placement is far more important than what type you have. The simple facts are: 

• Both types of detectors detect both types of fires, just at different timescales
• There is no way to predict if a house fire will be a smoldering or a flaming fire
• Ionization alarms are much slower at detecting smoldering fires compared to photoelectric alarms sounding in flaming fires, but this can be offset somewhat by how rapidly flaming fires spread. 
• It is FAR more important to actually have working smoke alarms than worrying about whether you have the right one.

While it is a great idea to know the difference between the two types of residential smoke detectors, this is not generally something that home inspectors note. What’s important is that smoke detectors are installed, are working, and are well placed to maximize safety and minimize nuisance alarms. It is concerning that few homes have proper smoke detectors installed and this is one of the easiest DIY jobs. I hope that one day I will never again hear about a fatality caused by occupants not being aware of a house fire.