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.


2 Answers 2


All of the Mini split systems I have Installed specify no loop of any kind even after a 8’ drop to the ground there needs to be a consistent down slope. On split with an air handler a loop is still the wrong thing a “p” trap or “u” bend are the proper method to allow water out and no sewer gasses in a loop traps air and will not drain. Since you said draining on the wall if the drain exits the house and drains on the ground no loop is needed. If it drains into a sewer drain then a trap is needed but this may be problematic see the manufacturers install instructions for proper setup.

  • Hi Ed, thanks for that info... sounds like a different type of trap is what will be needed, as it is draining into the septic vent stack. I could extend the tube to the exterior but didn't want dripping down the side of the house / onto patio, etc. May 23, 2020 at 17:49

I've seen this problem with hoses draining dehumidifers (not even trying for a trap as they drained into a channel drain.) They would back up 10 feet above the dip in the line if the line got a dip in it.

I think it comes to lack of venting and the small diameter of the hose. If you put an air gap/standpipe and P-trap (of some normal diameter for drain plumbing) for the straight sloped line to drain into I think that will solve both sides of your problem.

Depending what you have available, draining into a sump pit is another approach to not have it dripping down the side of the house.

  • Good tip. There is room to extend the PVC vertically quite a bit, replacing a lot of the smaller hose. That would be a logical place to add a trap as well. TY May 23, 2020 at 17:54

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