[Porsche-O-Phile]

I have Firex hard-wired in our place (came with the house) - they're about 7 years old and still test okay but do give annoying false alarms once every couple of months or so. I haven't bothered to replace them yet because I'm still not sure we're staying there and I can deal with an occasional false positive, a failure to test would warrant immediate replacement.

In my experiences, residential-grade smoke/CO detectors aren't all that good and you'll be lucky to get five years out of them reliably. You really should test them every half year when you change the batteries (yes, this goes for hard-wired devices too - I normally do it when you change the clocks for Daylight time - obviously don't rely on this in AZ, but I digress...)

Anyway test them by getting a can of test spray (most better contractor's supply stores have it).

Commercial grade detectors (EST, Simplex, etc.) are far better in terms of their construction and reliability, but most residential owners don't want to invest $5,000 in the associated panels and other hardware. Unfortunately residential-grade stuff is pretty poor these days - and that's not just limited to smoke/CO detectors, it's a function of demand created during the bubble days ("give me the cheapest one you've got!"). I would suspect this will change as market pressures shift in the coming months/years, but it hasn't happened yet that I can see.