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

When you say "wired into the house wiring" I'm assuming that a) they
are 120VAC units and b) they are also "tandem" detectors, i.e. there's
an extra wire or pair of wires between all of the detectors so that
when one detects smoke it causes the sounder in all of them to sound.
Kind of a fire alarm system without a panel if you will. If that is
the case I would believe your source that anything 22 years old may not
be compatible with current equipment. If nothing else a 22 year old
smoke detector probably has drifted significantly in sensitivity and I
wouldn't really trust it to protect me. I'd sleep better at night
knowing that they were all new. If you have an ADI nearby they
probably have something you can use with your current wiring, just take
one of the existing ones in with you, but buy three of them.

good luck,

nate