OK, the plot thickens.
We found an electrician who will provide us with an interface device (for ~ $100) to connect our line-voltage smoke detectors to a low-voltage alarm panel (and thus include it in our 24 hour monitoring service). But he doesn't want to come to our house for what he says is a '5 minute job'. They're all exceptionally busy right now, and can be very picky about what they do. I don't blame him -- nobody is offering to supplement his income when times are slow, so he's making hay while the sun shines.
We have three smoke detectors installed, interfaced. There are 3 wires to each one -- a black, white and yellow. The yellow is the wire through which the three detectors communicate with each other when smoke is detected by any one of them.
It looks to me as though all I have to do is wire the interface device to the yellow wire in one of the smoke detectors. See page 5 of the Firex Smoke Detector manual at the link below.
If that is the correct procedure, I am sure I can do that myself. I'd simply shut off the breaker to that circuit, check to be sure it is shut off, cut the yellow wire, and splice both ends plus an extension wire with a wire nut. The extension wire can then go to the alarm panel, where the alarm company will connect it to the panel.
Does that make sense?
The alternative to installing the line voltage interface myself is to have the alarm company install 3 wireless smoke detectors at an estimated price of $775.
Java
Here is a link that might be useful: Firex Smoke Detector Manual
Q