Featuring America's Home Inspector: Nationally Syndicated Columnist, Barry Stone
Choosing the Best Type of Smoke Alarm

Choosing the Best Type of Smoke Alarm

The House Detective: by Barry Stone, Certified Home Inspector

Dear Barry:  I installed new smoke alarms in my home about a year ago. Now that I’m selling the property, the buyer’s home inspector says these are not the best kind of smoke alarms. He recommends replacing them with the “photoelectric” kind. As long as the smoke alarms work when tested, what difference does it make if they’re one kind or another?  Ben

Dear Ben:  The purpose of a smoke alarm is to detect minute particles in the air and to make a loud noise when particles are present. There are currently three kinds of detectors on the market: ionization detectors, photoelectric detectors, and combinations of both.

When smoke alarms came into common use in the 1970’s, the only kind available was the ionization type. Ionization smoke detectors contain a small amount of a radioactive substance known as americium-241. This material creates a small electrical current between two metal plates. When smoke particles pass between these plates, the current is interrupted, and the alarm is activated. The problem with ionization detectors is that they respond best when there is a flaming fire but are slow to respond when there is smoke from a smoldering fire.

Photoelectric smoke detectors were developed in more recent years and have a different means for sensing the presence of particles in the air. In a photoelectric detector, there is a light source and a light-sensitive electric sensor. The light, however, is not directed toward the sensor. When smoke particles are present, the light strikes these particles and is reflected against the sensor, and this activates the alarm. The advantage with this type of smoke alarm is that it senses fires that are in their early stages of smoldering, before they burst into flames. This can make a life-saving time difference in many fire situations.

For maximum fire protection, dual alarms are available. These contain ionization and photoelectric sensors in a single unit. Unfortunately, there are no industry standards for the sensitivity of sensors in dual sensor alarms. Because of this, the ionization sensors in some dual alarms are not reliable. However, as long as the photoelectric sensor is functional, the alarm meets national standards established by Underwriters Laboratories.

Photoelectric smoke alarms are recommended by the International Association of Fire Fighters (IAFF) because they are the most reliable and give the most advanced warnings when a fire begins to occur. The IAFF regards ionization alarms and dual alarms as unacceptable. This is why your buyer’s home inspector recommended that you upgrade to photoelectric smoke alarms.

Upgrading your smoke alarms may not be a legal requirement in your local area or in your purchase contract, but it is still advisable. It is also recommended that you get combination alarms: the kind that detect carbon monoxide (CO) as well as smoke because CO alarms can save lives and are now required near bedroom entrances and in other interior locations. If you do replace the current smoke alarms with combo alarms, find the ones that have a voice recording, indicating whether smoke or CO has been detected.

Where to Properly Place Carbon Monoxide Alarms

The House Detective: by Barry Stone, Certified Home Inspector

Dear Barry:  I keep hearing conflicting opinions about whether to install carbon monoxide alarms high or low. Some people say that CO is heavier than air and is more likely to set off an alarm near the floor. Others say it is lighter than air and advise installing alarms near the ceiling. What is the truth about this, and what is the best place to install a carbon monoxide alarm?  Jamie

Dear Jamie:  This question comes up frequently in the course of home inspections, and incorrect information about carbon monoxide has become commonplace. So here are the facts. At standard temperature and pressure, the weight of air is 0.0807 pounds per cubic foot, and the weight of carbon monoxide is 0.0780 pounds per cubic foot. Considering the positions of the decimal points in these numbers, these differences are miniscule, making the relative weights of air and carbon monoxide nearly equal, with carbon monoxide being very slightly lighter. So what’s the best position for alarms, high or low?

To answer this question, an experiment was conducted in May of 2011 at the Center for Hyperbaric Medicine at Virginia Mason Medical Center in Seattle, Washington. The purpose of the test was to observe the way that carbon monoxide mixes with air and thus to determine the safest placement for carbon monoxide alarms, to provide the earliest possible warning of CO contamination in a home.

An eight-foot-tall Plexiglas chamber was constructed and three carbon monoxide alarms were installed, one in the top portion, one at the bottom, and one in the middle section. Carbon monoxide was then injected into the chamber in a series of tests. Sometimes, the CO was injected at the top, sometimes at the bottom, and sometimes in the middle. In each case, the CO diffused so rapidly with the air that there was found to be no apparent advantage in placing a CO alarm high or low inside a home.

What matters when installing CO alarms is to place them close to all bedroom entrances and to have one on each level of a multi-level home. Although not required, it is also advisable to install a CO alarm in the garage, since an idling vehicle is a likely source of carbon monoxide. And be sure to test each alarm regularly to make sure it remains operable.