On Aug 11, 9:34 am, wrote:
Recently moved into a 2 year old townhouse and the smoke detectors are
hardwired all on one circuit.
If one goes off, do they all go off?

Probably: That now seems to be standard here if there is more than one
inhabited floor.
And some 30 years ago was the rule in all townhouses owned and
operated by our Public Housing Commission.
Mandatory AC operation instead of the usual 9 volt battery operation
was used because children might steal the batteries to operate the
then popular transistor radios!
In a friends extensively and recently renovated (to code) house the
new smoke alarms on each of 3 floors are linked. They appear to be AC
operated with apparently a battery backup?