Smoke Alarms

My brother visited recently, and a couple of the alarms chose that time to begin that awful chirping. The units are hard-wired into the electrical grid with battery back ups in case of power outages. Changing the batteries shut one of them up but not the other. I was under the impression that the basic hard-wired portions were good indefinitely, but apparently not so, according to this handyman article:

http://www.naturalhandyman.com/iip/inffire/infsmokealarm.shtm

These units are approaching nine years of age, so there's another item (sigh!) added to the To Do list. Likely to be expensive, but less costly/traumatic than a fire........

Yuska