My house, built in 1996, has hard-wired Firex 120-538B smoke alarm. Since buying uit in Sept 2008, I had to replace batteries once. Today the 3 upstairs detectors started to beep, indicating low battery. Replaced all 3 with new Rayovac. 2 of the alarms started to beep again after about 10 mins, with a yellow light flashing next to the permanent green. Had to remove these alarms to be able to live. Please advise what is wrong.