Just a reminder for all of you noobs. Smoke detectors should be replaced at least once every 10 years and batteries should be replaced every 6 months or so. Check your local code for what you need.
I have a 4 unit building so mine need to be hardwired and interconnected within each unit. They just updated the code here a year or two ago to require CO detectors as well. I just upgraded my whole system to the combo smoke/CO detectors. They even talk to you and let you know if it is a fire/smoke or CO exposure.
Went into a friend's house last month and noticed the smoke detectors looked really old. Lit a match next to it and let it smoke... Not a peep. I took it down and saw that the date on the back was 1993!!! I was still in high school then! lol
Needless to say, that's what reminded me to replace mine.