"Myrna" wrote in message
oups.com...
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?

The alarms must be able to "talk" to each other. When one alarm trip, the
others will sound. Different brands and different models may not work
together.

At 22 years, they should be replace anyway. For the sake of $20, do you want
to take chances of the second alarm now working properly? Chances are, the
old one does not have battery backup, but the new ones will. That alone is a
good reason to replace them.