Tested carefully, and proven effective.
First? If you have gas appliances of any kind, you have simply gotta have certainty that there are no accumulations of carbon monoxide, or of the gas itself ... either one can kill, and does regularly: There is NO EXCUSE for having no warning.
I researched carefully, but quickly, when one of our CO detectors failed my regular tests (something else folks oughta do a few times a year). The technology this model is based upon is reportedly the best available to consumers, and the fact it uses a 9V battery only as backup, being primarily powered by an outlet, makes it nearly perfect by my measure (it ould be made to operate much more efficiently w/o adding much to the costs of production, but I've lost very little sleep over the small amount of energy it wastes ~;-)
Although I didn't evaluate the accuracy of their PPM count, response of the unit to a controlled release of propane was w/in just a few seconds; much more immediate than expected, or required, w/o any doubt. Likewise, an incomplete burn produced an alarm before I could notice any type of odor* produced by the yellow flame.
*IMPORTANT* to know that carbon monoxide (CO) is completely odorless ... it's the partially burned fuel that I was tryin' to sniff out.
As to placement? I have a light switch w/ a receptacle, just as you enter a large dining room from the kitchen end ... carbon monoxide mixes readily w/ the air in your home, as does the propane and natural gas (we have both in our home), so the ideal location is all about air flow. Logically, you wanna have 'em downwind of potential sources (i.e. where the air from that area goes to). Be sure it can be heard from sleeping areas, and use that test button regularly to wake folks up, most esp. young children (never ever allow kids to sleep through any alarm -- wake 'em up, and make 'em head for the exit, every single time one goes off).
All that said? I'm gonna buy at least one more of these right now, and a few fresh smoke detectors, since there's no such thing as TOO MANY in any home. And, if you' ve got one w/o a battery, just 'cause you're too cheap to buy one? Somebody oughta kill ya, right now.
Verified purchase: YesCondition: Pre-owned