Using our split air conditioner for the first time, the wall unit started dripping water. Its condensate tube runs through the wall into an adjacent attic and then into the DWV vent stack.
The tubing had a trap formed as a simple loop with a radius of about 4". When I inspected the tube I could see that some water had accumulated in the bottom of the loop but hadn't reached the top where it would actually drain out into the DWV. Strangely it was not backed up vertically above that point into the pan - there was actually air in that section also.
I took the loop out making it a straight run and the whole thing started draining properly after that. The loop was ~12" below the drip pan height, and everything slopes downwards or almost vertically.
The tubing itself is about 3/4", its a lightweight plastic translucent corrugated type. There was no blockage, the tube was not kinked, etc. No obvious visible reason why it wouldn't drain. The DWV vent stack is about 2' away. Could there be some kind of air lock?
Obviously I want to get it back to having a trap, but need to understand the problem first.