Beeping smoke detector will not shut UP

Low battery.

Your unit has no backup battery. Because of this, a power failure would leave the unit without power to notify you that it had no power. So, the unit has a capacitor that holds a certain amount of power, enough to keep the power failure beeping going for a finite amount of time. Once that amount of time has passed, the chirping stops. However, the chirping is also used to notify you that the detector (from age or defect) needs repair or replacement. So if it needs attention, the chirping starts. So, here's what happened: the unit started chirping to tell you it needed attention. When you finally disconnected the power, it continued to chirp, but this time to tell you the power was disconnected. When the capacitor ran out, so did the chirp. Chuck it and get a new one.

I had this same problem. I also thought it was the smoke detector, which is wired into the ceiling. It was actually my carbon monoxide detector that was chirping, which had low batteries. So maybe it's your CO detector!

Just to bring closure--closure of a profoundly embarrassing nature, I must add--to this saga: the chirping noise started again the next evening, originating (as far as I could tell) in the ceiling area from which I had removed the detector; I called the fire dept.; they sent out a very, very tall young man who looked the situation over, stuck his head into the adjacent bedroom, and discovered .... Yeah. There was another smoke detector, stuck up on the wall right above the door, half-hidden by the door molding, which I had somehow completely overlooked. A battery-operated smoke detector. Whose battery was going bad. And which was close enough to the site of the original smoke detector that it was hard to tell the noise was coming from a different location. So, problem solved, and the takeaway for those of you reading along at home: in 99.5% of hardware/tech glitches where you're waving your arms and shrieking "I tried EVERYTHING!!" the problem is with the diagnosis, not the solution. Thanks again to everyone who commented and advised!

I have had ones that chirp even when disconnected with the backup battery DISCONNECTED. They do have a capacitor in there somewhere. Hit the Test button and that should drain the cap pretty quickly.

FYI, we've been replacing all the detectors in our (new to us) house a couple at at time. For like $4 more than a regular detector, you can get the ones w/ the 10-year battery (normally you change the 9v annually) so basically after 10 years you just chuck the device. No need to hard wire, although networked ones are nice for early detection. For like an extra $8 you can get wireless ones that talk, but I have not used those. I think we're paying like $12 for a 10-year batteried-hushable detector. Soooo much better than what was in place when we got the house.

Smoke detectors use radioactive (alpha) decay to detect the presence of smoke. The radioactive source inside will gradually decay, and after approximately 10 years will need to be replaced. Most smoke detectors use http://en.wikipedia.org/wiki/Americium-241 as a radiation source, which has a half-life of 432 years. So, it's not likely that the radiation source is the problem. It's more likely that one or more of the electronic components have failed.

Assuming you do get a new one, you need to either exchange the old one if the store will take it back, or take it to a hazardous materials pickup site. Do not throw it in the trash; it has a small amount of radioactive element in it.

Smoke detectors use radioactive (alpha) decay to detect the presence of smoke. The radioactive source inside will gradually decay, and after approximately 10 years will need to be replaced. Because radioactive decay is random what you may have been hearing was a warning that will come and go with the random nature of the decay as the source "fails".