It cannot be stated definitively which is more effective because each is better at detecting distinctly different, yet potentially fatal fires. Because no one can predict what type of fire might start in a home, the USFA, NFPA and Underwriter Labs recommend that every residence and place where people sleep be equipped with both ionization AND photoelectric smoke alarms, OR dual sensor smoke alarms, which contain both ionization and photoelectric smoke sensors. 

AC/DC and interconnected smoke alarms were introduced into the building codes starting in 1989.  AC/DC alarms still need their batteries changed on an annual basis but with lithium batteries being approved for use, we are now seeing smoke alarms marketed with lifetime batteries. Lithium batteries installed in a smoke alarm will last over 10 years.

Inspecting Smoke Alarms
First and foremost, look at the alarm to judge its age.  The NFPA and most manufacturers recommend that any alarm over 10 years old should be replaced.  Most alarms start out white/beige in color but as they age, they take on a yellow appearance.  If you suspect it’s greater than 10 years old, then it should be replaced. The sensors on smoke alarms are easily contaminated by paint, grease, etc.  If the alarm has any paint or grease on the cover I assume the sensors are compromised and I recommend replacement.

Most AC/DC alarms have a light that indicates it is operating. I look for the light before I hit the test button.  Most “battery only” alarms do not have indicator lights so the only way to know if it’s operational is to hit the test button.  Most homes built after 2000 have interconnected smoke alarms. As part of the test you should hold the test button long enough to trigger all of the other alarms.

The current NFPA requirements call for smoke alarms in each bedroom and on each level (including the basement) and in the hallways. Smoke alarms should not be installed directly outside of bathrooms or within 20 feet of the kitchens to reduce false alarms.  In homes with vaulted ceilings the smoke alarm should be installed near the top of the ceiling.

Carbon Monoxide Alarms
Carbon Monoxide (CO) is odorless, tasteless and invisible, and a leading cause of accidental poisoning deaths in North America –  it’s a silent killer. Anything that burns fossil fuels is a potential source of CO.  CO can drift in from the outside and sicken the occupants.  Cars warming up on a cold day in a garage are a major source of CO.

Twenty-five states now have statutes that require carbon monoxide detectors in certain residential buildings. A listing by state can be found here http://www.ncsl.org/issues-research/env-res/carbon-monoxide-detectors-state-statutes.aspx. In addition, Virginia allows tenants to install carbon monoxide detectors in rental properties if they believe it is necessary to ensure their safety. Texas requires carbon monoxide detectors in certain day-care centers, group day-care homes, and family homes. Connecticut and Maryland require installation of carbon monoxide detectors in schools.

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In some states the requirement is only if there are fossil-fueled devices in the home or an attached garage. Other states, such as Washington, require it regardless.

Inspecting CO Alarms
Most states will defer to the manufacturer’s installation instructions but generally CO alarms can be mounted low or high and can be plug-in or battery.  They should be installed near the bedrooms in a hallway but away from the bathrooms.  You should have one on each level of the home.

CO alarms will have a test button and it should alarm when pressed. All CO Alarms are dated and should be replaced after five to seven years or according to manufacturer’s schedule. Generally speaking if you see an alarm that is older than eight years, it should be replaced.

Are Ionization Smoke Alarm’s Faulty?
Every few years an alarm is sounded that ionization detectors are “bad.” Approximately 98 percent of detectors installed today are ionization. This assertion has been extensively studied and isn’t supported by any of the research on smoke alarms.  This issue started back in the 90s when there were some smoldering fires that ionization detectors were slow to detect. Texas A+M did a study and identified some scenarios where a photoelectric alarm would go off quickly and an ionization alarm could take as much as 200 percent longer to sound the alarm.  If you look at the single scenario there would be no argument, however, homes are made of many different materials, some that smolder and some that will burn very quickly. In fact, most synthetic materials will burn quickly.

The NFPA, CPSC and NIST went back and studied this issue. To paraphrase their results: “different technology ‘alarms’ at different rates, depending on the material burning, but both would alarm and allow occupants time to escape.”  The state of Ohio, Maryland, and California State Fire Marshals all formed independent task forces to study this issue and came to a similar conclusion.  I have consolidated links to all the reports on my website.

NIST did note that lightweight construction and modern interior furnishing tend to burn faster (which favors the ionization alarm technology) and allow less time for occupants to escape.  To ensure the occupants have the greatest amount of time to exit the structure, each home should be equipped with a mix of smoke alarm technology installed to the NFPA specifications.  With the costs of smoke alarms dropping it now makes sense to purchase dual sensor detectors.

About the Author
Rick Bunzel is the Principle Inspector at Pacific Crest Inspections in Anacortes, WA. He has 38 year’s experience as a firefighter in New Jersey, California, Colorado and Washington State. He is currently a Lieutenant with the Mt. Erie Fire Department. If you have questions or comments, Rick Bunzel can be contacted at Pacific Crest Inspections (360) 588-9956 or www.paccrestinspections.com.