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.