What is this? Phone Jack in the ceiling

[matthewk]

So I found this over the weekend when cleaning the AC vents. It is a female to female RJ-11 adapter connected to a telephone line. Any ideas?

[overlander63]

That's a connector to the a/c communication wiring. As long as the air conditioning works, don't mess with it.

[tvketchum]

It’s how the thermostat talks to the unit.....

[BayouBiker]

It's a spare com port to allow one AC unit to daisy chain to another so you can control multiple zones from the same thermostatic controller.