I apologize in advance for this long post. This is the first summer in this home that I bought. I'm a first time homeowner. One day I realized a big stain in my ceiling right under the attic and found out that the drain pan was overflowing. Called the AC guy and he came and blew air into the clean out of the drain line as well as from the other end outside of the house. He also attached a sensor on the side of the drain pan.

We thought that had solved the issue. A few days later I go into the attic to check the roof since we are going to get the roof done soon. I noticed that the pan was pretty full but the sensor did nothing to tell me this. Decided to do some YouTubing and used a wet/dry vac to clean up the water in the pan as well as suck it out of the drain line. Basically every time we leave the AC on for a day or so the drain line fills back up all the way to the top where the clean out is. Then the AC unit starts to drip underneath. I'm not sure if it ever drips out from the clean out at all but it could be very likely. Every few days we have to vacuum this drain line. We even did the whole vinegar thing for 30min and then flushed it with warm water which came out clean.

Did some more research and learned about P-Traps and realized that I do not have one of these connected to my AC unit. The area in my attic is basically just a crawl space and very tight. From what I can see, there is just one pipe that goes underneath the whole unit then takes a 90 degree turn to the left wall.

I've attached some pics of my unit and hoping someone can explain to me what could be wrong and what I should do. Wondering if I should cut the tape and open up the panel?enter image description here enter image description here

  • 1
    Is the line from the unit dumping into the pan? Then supposed to drain from the pan to the outside? Not supposed to be like that. The drain from the pan is emergency drain supposed to drip in an obvious place like from an eave in front of a window. The pan should not be wet at all except when primary drain line fails.
    – Kris
    Commented Aug 2, 2020 at 0:14

2 Answers 2


The laws of physics cannot be ignored, water doesn't flow up hill...

If the condensation is building up in the pipe to where is drips out of the top of that first Tee fitting, it means it is not flowing out to the outside. Either something is still blocking that pipe, or the pipe is going up hill. How long is that pipe going outside? If it's relatively short, try running a sink snake through it to see if mold or animal nests have blocked it.

  • After crawling as close as possible and basically putting my face on the floor I saw that the pipe is slightly going uphill before going into the wall. This is probably causing it to drain slower than normal. The inside of the unit is actually very clean after opening it up to look. I basically attached a pvc pipe to the top of the tee to extend it and hopefully it will allow the water to eventually flow downward as it fills up before it comes out the top.
    – LT724
    Commented Aug 4, 2020 at 19:00

The condensation exit pipe needs to have a trap to break the air pressure, resulting from the pull of the fan. That pull will prevent the water from flowing downhill. The air handler installation manual will specify how dimensions of the trap. The "professionals" that installed mine skipped the trap; this is how I know.

Is there an inline trap in the outlet pipe?

  • I don't think it's flowing out of the top of the T but from under the unit. The water just sits at the top of the Tee but never overflowing. I've poked a coaxial cable in towards the unit as well as downward and into the 90 degree angle which points under the unit. Not sure how long it is but it goes under then takes a left turn into the wall. From there I think it goes down and turns out towards the side of the house in-between the first floor and basement ceiling. Once clearing the drain line it flows out consistently but then after a few days of using it, it will just fill back up.
    – LT724
    Commented Aug 2, 2020 at 21:55
  • As Evil Elf said, I believe it could possibly be due to the fact that there's no trap. I've spent many nights reading about this and that traps need to come down a certain length and then up again. As you can see there isn't much space for a trap which is why the installer didn't put one in. Also I believe the blower is to the left of the coils. Reasoning for this is that the unit in my basement is the same thing just stood upright. So I think it's a blow through unit which I've read doesn't require a trap.
    – LT724
    Commented Aug 2, 2020 at 21:55
  • The static pressure can hold back the water flowing, then the drip pan can overflow, causing the water to pour on the filter and out the bottom of the air handler. Is the pan overflowing?
    – Evil Elf
    Commented Aug 3, 2020 at 1:05

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