Have an odd problem I temporarily solved with a hacksaw this week.

Several weeks ago, my wife called mid-afternoon from our house (in
Houston) and water was dripping from a downstairs A/C vent and quite a
bit. I rushed home from work and swapped out the bowls with a 5-gal
bucket and went upstairs and the upstairs master bath was in worse
shape. Water was dripping down the wall behind the sink.

Water was cool & clean and slowed when the A/C turned off. So, the A/C
folks I called said to run vinegar through the drain line to clean out
and they'd see how soon they could come up. I did this and traced the
drain line in the attic as far as I could go with it, but couldn't see
where it connected to our drain system, but it headed towards our
master bath sinks.

I couldn't see anything unusual under the sinks and the sinks haven't
plugged in awhile so not sure how this was an issue, but just for fun I
climbed on the roof and snaked the vent I suspected was causing the
trouble and sure enough I started smelling vinegar on the debris. This
fixed it for a few weeks, then while on vacation it started up again
(naturally) so this time I cut the drain line and am letting it fall
into the pan under the A/C unit and drip from the emergency drain at
the eave.

So, I suspect the A/C installer or plumber did a lousy job of sealing
this all up or they should have routed the drain into a trap instead.

FYI, with the A/C running, I could fill up a 5-gal bucket overnight
from the dripping we get.

So, any suggestions on the following courses of action?

1. Reroute the drain line to another sink vent that sees much less use
(hair, etc..)
2. Route the drain to the eave and down the side of the house
3. Route the drain to an actual sink drain, not an air vent.
4. Call a plumber