[Hugh R]

Island, gotta disagree.

1)They all wear out at the same rate, i.e., one doesn't work harder than another.

2)Some rooms I may not go in for weeks (daughter at college's bedroom for example), so won't hear the chirp.

3)When I didn't change them annually, I'd start getting one or two chirping in 12-14 months.

Better to change them all annually.