I noticed the drywall on my bathroom ceiling was starting to bubble and discolor in the corners like it was soaking wet. I have an CAC/BDP Evap coil mounted in my attic right above the same bathroom so went up to see if the condensation pvc pipe was leaking and I found this one port that does not have a drain connected. Instead there is a half milk container to catch the water that fell over and spilling the water onto the drywall. Can I just tee this port into drain line that is already there? The drain goes down under the house, out a vent and exits out onto the grass.
no. that is the backup/secondary drain port which is to allow the coil to drain if the first one plugs up or the drain line plugs up. it should be drained outside separately. then the primary/first one needs to be dismantled and cleaned and the drain line flushed/blown thru. usually the installer puts a larger metal drain pain under the entire air handler with a drain on it or alarm so that if water leaks out from any area of the airhandler from a plugged drain you don't get ceiling water damage and it notifies you. you can have a safety device called a wet switch in that pan and wired so it shuts off the ac.