Without knowing the height differences to the plumbing and seeing it in person were all guessing. Could install a condensate pump and just pump a high loop elsewhere in the stack

 

Need pics. Is the new vent you made after the trap and extended above drain pan?

 

The unit is on a shared drain line

Remove it from this shared line and see what happens. if others traps are dry in this shared line, you could be drawing in air preventing the other traps from draining properly. shut down the unit your having issues with, Place a piece of paper or business card on the end of condensate line see if its sucking in air. Note: if its not drawing on air when you check, it could be doing at other times. Is there a vent in the condensate line after the trap as it enters the common line?

 

We have an office building similar to this, except it has fan coil units (FCUs w/chilled water coils). The shared vertical drain pipes are our biggest problem. The problem with trying to blow them out is that the compressed air (or SWOOSH gun) will escape out the drains of the other units that are above the blockage. Sometimes we have success with a man at each of those units holding a rag over the drain to keep the pressure on the blockage. But often we have to cut open the shared vertical drain pipe and snake out the blockage. SWOOSH or air will work if the blockage is between one unit and the shared drain. It could be that the shared drain is backing up and overflowing only one unit because it is slightly lower than the others on that floor. Giving the appearance that only one is blocked. This is usually what happens to us, but if blowing that one unit doesn't work, we know to move on to the vertical drain. Past experience tells us that.

I should add that we cut the vertical pipe down low near the large horizontal pipe that carries all condensate from the building. Chose a strategic location that will make snaking both directions easier. All the way to the large drain and up towards the units. You won't know which side of the blockage you are on until you cut it open. Sometimes there will be two or more elbows in a short run, and we will cut that whole section out.

 

It could be that the shared drain is backing up and overflowing only one unit because it is slightly lower than the others on that floor. Giving the appearance that only one is blocked. This is usually what happens to us, but if blowing that one unit doesn't work, we know to move on to the vertical drain. Past experience tells us that.

I ran into a guy that would use garden hoses to clear out clogged drain lines, but we were talking about single families. If memory serves he said max 20PSI through the line. Have you ever used water to flush it out? Would that even work in this case with the shared drain line?

 

Let me post pics and perhaps that might help a bit.

 

I will post some pics shortly, but yes, I made sure to extend the vent after the trap above the drain pan.

 

Good call, I should have done that when I was out there the 2nd time...

 

That would be a relief if it was just a "simple" clog

 

Forgot to mention that customer said building maintenance hasn't kept up with regularly vacuuming out drain line like they used to due to staffing issues.

 

Sorry to spam the thread guys, I went to post those pics and it won't let me because I don't have enough posts. This one should push me over the min

 

Here are a couple of pics of the original install when I got there:

This was my 1st attempt:

Here are a few of the current install:

 

The 1st pic, the one w/ the clear tubing. I think that is referred to as a 'running trap' and it is not allowed on AC.
Then the pic of the re-done trap. Could a deeper trap have been installed by putting the trap to the left and down of the hot water tank

 

Looks like the trap is fine. The shared line could be partially clogged. Cold condensate can grow slime, pouring some bleach will help clear the lines.

 

The bottom of the trap looks lower than the wall opening.

 

That evap sure looks dirty.

 

Trap maybe too shallow like mentioned earlier

 

Would a dirty evap cause a diff air press across the drain line pulling air backwards thru the trap?

 

It has been said in another post, start with the basics. when pouring water in the drain pan, does the water flow out? if so its not the trap or drain.

A dirty coil can cause water to not drain off properly and allow water to be pulled into the unit. dirty filters or coil could cause a greater negative inside the unit, making it hard for the condensate to flow. Need to know your static pressure, you might need a deeper trap.

https://engineeredairsolutions.com/...hite-papers/p-trap-design/#:~:text=Do you know the proper,the drain of the unit.