"nr" wrote in message
...

I recently removed a smoke detector to patch a hole and paint the
small area that needed painting. When I reinstalled the detector (120
VAC, battery backup) it started chirping. I opened the battery
compartment to find no battery. I have been in this house since March
and this detector has never made a sound before this.

Only the manufacturer can tell you. Since this is a mains
SD with battery backup, that feature may have been designed
to cope with power outages: so if there was no power outage
there was no occasion for it to send the Dead Battery signal.

I then checked the AC connections (advice from a friend) and found the
detector's wires needed more insulation stripped, which I did, and
reconnected. Still the chirping. I put a battery in and the chirping
stopped.

The manufacturer can also tell you the probable service life
of the unit. This is determined by the radioactive elements
it uses, which last for 3 to 10 years. In Canadian units,
SDs chirp when the radioactive detectors become weak
as well as when batteries are about to expire.

--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)