2007 Lennox 3.5 ton.
Primary drain line runs into sink, no secondary - just a pan underneath unit with a float switch attached (which I tested and does not work).

Primary drain line, stating at the condenser, goes: capped vent stack/cleanout, p trap, open vent stack, and then a 4' run, right 90 deg turn to a 2' run, then a left 90 deg turn to a 10' run, then a right 90 deg turn to a 15' run, then finally vertically down to sink.

Chronic clogging problems with this line - had a few calls out to blow the line clear over the last 10 years. Never had issues with our downstairs unit clogging, and I am guessing it is because it's drain pipe length is about a 5' straight run to the vertical pipe to the sink (less chance to clog).

Early this week (thankfully after is was 100+ degrees outside), float switch at the condenser was cutting things off and sure enough, drain line backed up. Also noticed water in the pan. Shop vac really didn't help this time, so I bought one of these:



This unclogged the line, and there was a ton of crap that made it up to the sink, so success...trickle of water coming down to drain under sink, so I thought I was good.

yesterday, drain line backed up again and more water in the pan. More vinegar shop vac, and a blast of CO2 and the line cleared again. Last night, line backed up again, more water in the pan. Shop vac cleared.

Woke up this morning to check and a bit more water in the pan, but the drain line is clear/virtually empty.

1) What does it mean if the drain line is clear but water is ending up in the pan? Is this just that the condenser is draining faster than the drain line can?
2) Would most here agree that this much PVC with that many 90 deg bends in it is the primary problem here?
3) My thought is to just re-pvc everything, because new pvc is clean pvc and do that until the unit needs replacement, if it doesn't already. thoughts?

I'd guess you have 10 years of gunk still in the line that the air is blowing past. Little bit of condensate and you get a gunk mud dam.

The condenser is the outside unit. The air handler/fan coil/evaporator coil is the inside unit.

Clog might be in the p trap tie in. Line is clear, but gunk is jammed up in the little barb tie in. Use a small screwdriver and clean that out. Maybe.

And, why don't you have a secondary drain? That is very strange.

secondary - there are two PVC pipe coming out of the eaves under the roof, but I do not see any PVC coming out the side of the pan on this unit.

The hardest part about this unit is where it is located in the attic - there is very little space to get down and look underneath and such. Perhaps there is a secondary on the back side of it and I just haven't seen it yet.

In hindsight, the location of this unit in the attic is so problematic for a variety of reasons. It it were me, I would have placed in that nice open space 5' away from the wall where the drain line goes to the sink

I installed a PVC ball valve on the primary drain line right where it exits from the evaporator coil. That way the compressed air doesn't take the path of least resistance back up into the coil. Seems to work much better when I've needed it.

between the p-trap and the open vent stack there is a non-glued coupling I have been disassembling so I can shoot the Co2 one direction (down towards the sink).

When I pour hot water/vinegar into the line to flush things, I do so before the ptrap, so I don't think the ptrap is the blockage point. It's down the line, somewhere between 2' and 40'.

I even poured boiling water down it and felt how far it was going and it felt like right up until one of the 90 deg bends.

Got my AC friend coming out - via the phone he is suggesting there might be a blockage right at or just inside the unit to where it cannot drain enough and hence the overflow, but we will see.

yes we have discussed the ball valve idea - this to be specific: https://www.amazon.com/dp/B085FQ4QTB/ref=cm_sw_r_api_i_JD0EHQD21245677T3EYX_0

however as mentioned (maybe after you posted), I can separate the line at the non-glued coupling so there is only one direction of air flow

ok problem resolved. Massive clogs. Pumped some shots of pressurized liquid freon through both directions to finally get it.

We also put a ball valve on the down line side of the last vent, so I can close that off and shoot compressed air via the nozzle via my CO2 gun. Of course the line is 100% clear now, so likely won't have to touch it.



best part is my guy lets me pay him in golf lessons.

I've had issues in the wall beyond the sink in bathrooms that are rarely used. Now I run those sinks monthly and keep a close eye on them after flushing the attic lines.