Good question and don't mind answering it. I am interfacing 3 AC head units where I was able to get a open collector output. I was going to use that voltage variant between when the unit is on and off approx 4V verses 12V to determining the status of the AC using the analog 34 mode.
When I connected the relays the 4V went to zero in the off state because of the relay coil load and when it is on the 12V drives the relay on. I then connected the relay to the respective zone inputs. The relays btw are seperate 12VDC relays not on an ELK expander board.
Little more work but all in all a better more robust out come. Something about a relay contact that can not be misinterpreted.
Regards,
Fleetz