if they are 2-wire, replace the noisy one and test the others. if they
are 3-wire units, replace them all: that way when one sounds it tells
its other two to sound also. more at:
http://home.howstuffworks.com/question576.htm
and:
http://en.wikipedia.org/wiki/Smoke_alarm


Myrna wrote:
In our home there are 3 smoke alarms, all wired into the house wiring.
All are the same age (about 22 yrs). There's a high pitched noise
coming from one of them.

In a recent discussion, one guy told me to replace all of them, not
just the one. He said new ones aren't compatible with the new ones. I
didn't ask him what he meant about being compatible.

So, can anyone tell me what this means? Do I have to replace all 3?

Thanks,
Myrna