I was getting tired of my old central air conditioning's condensation flowing into the drainpipe that goes out to the street. Problem was, this drainpipe got stopped up frequently, would back up at any notice and soak my hall carpet unless I kept a watchful eye open. Finally one maintenance man who knows his way around AC told me to put about a half cup of Clorox bleach down this pvc drain pipe say once a month and voila - no more backup! Cleaned out!
Now I was no longer afraid to go on vacation. I also learned this must be done with caution because if the drain still backs up and the bleach is in that water, it may cause fabric discoloration on carpet, rugs etc. near the unit. Dilute the bleach if you feel better before trying this.