For the life of me, I cannot figure this out. My air handler and heat pump were installed in 2006 and work great. For the first summer since I've owned the home, condensation started to not drain properly and is leaking onto the filter and onto the basement floor.

The inside of this handler is spotless, including the drain pan. I took apart the PVC pipe and cleaned it out well, although I don't think it needed it. I still have major dripping!

It does not look to me like it should be overflowing. It pours out of the hole circled below and then onto the floor and back into the unit and onto the filter. If it is coming out of this hole, why is it not going down the correct outlet to the pipe?!

enter image description here

The only change point I can think of is that I purchased a box of filters on the web that had a pretty high MERV rating of 12. The fan seems to push air through the filter pretty easily, but I wonder if the new filter is somehow producing a vacuum that is not letting the water drain correctly. I removed the plugs pointed out below by the arrows to try and relieve this vacuum, but it did not make a difference. Any thought or advice?

image of air handler drain connector with drain piping showing location of removed plugs

Last night, I cracked open a cold one and just watched in the basement. I can hear the water build up in the pan as it makes a gurgling sound like some force is preventing it from draining, but it is on the verge of overflowing. Once the blower stops, all the water drains quickly. Some down the pipe, some out of the hole. If the water is building up, I simply don't understand why it won't drain down the pipe.

The water also drained when I removed the filter and left the cover off while the blower was running.

  • Try using one of the older filters, and see if the problem goes away. If you've added restriction to the air flow, the coils could be icing up. This would release a larger volume of water at once (when the ice melts), instead of a steady stream (as the condensation drips). Could just be that the drain is being flooded.
    – Tester101
    Aug 26, 2014 at 14:09
  • I forgot to include this in my original post. I thought that removing the filter entirely would solve the issue and I tried that. It worked at first, but then yet again, water would still spill out with no filter. It wasn't as bad, but it still happened.
    – Evil Elf
    Aug 26, 2014 at 16:05
  • Have you checked the refrigerant level recently? Could be that you're under or over charged, and the filter magnifies the problem. Have you made any other changes to the system (closed registers, blocked returns, etc.)?
    – Tester101
    Aug 26, 2014 at 16:11
  • I've always had about half the registers on the first floor closed (two-story home) and it has never given me a problem before. I updated my original post as well.
    – Evil Elf
    Aug 27, 2014 at 12:32
  • Sounds like it's sucking air in through the drain, which I would think is because there's not enough return air provided through the return intake. If the system works fine without the new filters, but has problems with the filter. I say stop using those filters.
    – Tester101
    Aug 27, 2014 at 13:04

10 Answers 10


What you’re describing sounds like a combination of negative pressure in the compartment that the drain pan connected to and an incorrectly sized p-trap on the condensate drain.

The depth of the t-prap has to be greater than the negative pressure in inches of water column of the compartment where the drain is connected to. A t-prap with insufficient depth, will prevent the condensate from overcoming the negative pressure which prevents draining until the fan stops.

Further information on sizing of p-traps for condensate can be found in this pdf article: Condensate Traps by Ronald F. Brusha.

  • I don't have a P-trap. Could that T joint be an issue? The pipe is sloped every inch of the way to the basement floor drain. I have also tried to remove the entire pipe drain and just let that first 90 degree piece empty out onto the floor. Same results. Could it be that I NEED a P-trap?
    – Evil Elf
    Aug 27, 2014 at 17:37
  • If the compartment has a negative pressure, you should have a p-trap.
    – pdd
    Aug 27, 2014 at 19:20
  • I think this is the case. I will try to correct. How could this be installed incorrectly. Could the handler be more powerful than the return vents are originally designed for?
    – Evil Elf
    Aug 27, 2014 at 19:33
  • Your best bet is to look up your unit on the manufactures website and find the installation manual. It should include information on how to size the p-trap to account for the unit's operational pressures. If they don't explicitly detail the p-trap, you should be able to get the operational pressure and use the article I linked to to get the dimensions.
    – pdd
    Aug 27, 2014 at 20:01
  • 3
    @Tester101, The trap is not to prevent sewer gas in this application. It's purpose is to maintain separation between zones of different air pressure while allowing the condensate to drain.
    – pdd
    Aug 28, 2014 at 15:42

I had the exact problem with my condensate drain not draining while running. My unit design is such that the blower pulls air through the A-Coil rather than pushing it through. This design pulls air through the drain pipes and won't allow water to drain. Typical drain configuration utilizes a Tee with a stand pipe so the drain is open above the trap. This is fine if your system pushes air into the A-Coil. However the negative pressure or vacuum of my system requires that you cap off the stand pipe so that air cannot flow into the drain pan housing disrupting the gravity flow of the water. Instead, allow air to enter the drain pan through the secondary drain port. If this scenario pulls water out of the trap, consider installing a larger trap. Worked like a charm, high and dry now!

  • Much after the fact, but I had this problem as well. I rebuilt my p-trap because it had become too clogged to clear because I wasn't good enough about bleaching it regularly. After the rebuild, which was sized correctly, I ran into the water/leak problem the OP had. Exact same setup and opening on the unit and everything. As soon as I capped the opening on the drain, I heard the pan drain into the p-trap. Guess I must have not put the cap back on while trying to clear it or after adding bleach, not thinning about the pressure problem.
    – asp8811
    Jul 25, 2020 at 1:57

I know its a little late but I had a similar problem and my coils were stopped up with dirt. I cleaned my coils with coil cleaner and a tooth brush and rinsed with water. Make sure you brush with the grain on the coil so it doesn't get damaged. Then rinse with water. I repeated this process 3 times because it was so dirty. I used a pump up sprayer to rinse. Lol. After cleaning the coils everything is running smoothly.

  • If he doesn't have a p-trap, he does need one. But I had a p-trap and was having this problem (after about 12 years of operation without problem). So I think the problem is what you describe (which a deeper p-trap would maybe solve it... but cleaning it would help efficiency and solve it too)
    – Esteban
    Mar 18, 2020 at 15:29

I can assume that this has been fixed by now being as that I am 2 years late, but I'll answer this anyway, that pipe on the left doesn't need to be there at all it serves no purpose (normally it is used on units that sit on their sides and have an external secondary drain pan) just put a cap on it and forget about it. The one on the right is built wrong. The vent that you see on your drain (the part sticking up that is open to the air) should be on the other side of the trap not between the trap and the unit. Easiest way to fix is just call a local HVAC tech and have him fix it we will have those parts on our trucks if not we know where to easily get them. If you don't want to go that route just go by home Depot and buy some 3/4 PVC, a male adapter, some elbows, one tee and some PVC glue. You will have to cut out all of that p-trap and rebuild it with the vent on the other side on the trap don't forget that the vent needs to end higher than the hole leaving the unit.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Aug 21, 2019 at 10:21

Not sure where to put update mods...

Here is my setup after correcting. I started getting overflow again and I reread the thread. I had the clean out above the trap open. I capped it with the threaded fitting and it immediately started to drain again. I'll post back if this is not a permanent solution. I made the trap one inch longer than the installation manual stated.

enter image description here


I had a problem last year starting in May, 2016. The Goodman AVPTC4260 air handler had been installed new in the summer of 2013, along with my heat pump. We did unit installs, with an HVAC friend, who installs and maintains Goodmans for a living, handling that end of the job.

Starting in May 2016, I went through 3 motors in 3 months, not including the original one. The moisture was not draining properly even though the filter was cleaned according to the same schedule I had maintained for the 3 previous years. The drainage pipe was never clogged.

My air handler is mounted horizontally in my basement. This unit is designed where the drain pan is directly below the motor and it seems, the "pull" on the motor was so great that it was pulling the water back into the motor and flooding it out. These motors cost $450 each and each lasted a month. I replaced in May, and then surprise - the same problem occurred again in June; at that time, we had a larger pipe installed. Then this happened again in July. With the 3rd motor, I purchased, we had a second drain installed. This seems to have solved the issue.

[Refer to your installation instructions with regard to mounting, because there is a section on motor orientation, water collection and possible motor damage. This was not our issue,]

As a side note, we generally vacuum out the filter every month. I have 2 filters, one gets installed, one gets vacuumed and washed in warm weather. I think you should find your own perfect duration for filter maintenance, as it really does depend on what you do in your house. Pet fur is a great "binder" for dust and sawdust clogs filters fast. Moisture can sit in the dust then and not drain. We were turning off the HVAC when my floors were being redone last summer, so that the inordinate amount of dust did not cause a problem. It was August, so I got the house nice and cool, then turned everything off. The filter was vacuumed more often during this home project, as we never put our finger on what caused the May - July problems. The only thing I could think of was, it was humid and the water was accumulating faster. We were generating more dust, but we were cleaning the filter more often. Anyway, everything has been fine since the second drain was installed.


I just had a similar problem. Unit worked well for two years but then low pressure fault and water leakage when the fan shut off. Turned out there was negative pressure that prevented draining. Sucked air in through the drain hole flooded cabinet and water leaked when fan shut off. Cut in additional cold air return and solved problem. Unit must have been right on border of low pressure fault and pressure sensor weakened over time. Unit was a geocool geothermal 5 ton. I have had good luck having it 5 years.


In my view, the suction in the air handler becomes excessive when there is an insufficient volume of return air. Enlarging the return, or adding a second return with filter grille is the best option. Manual J specifies that you need more return air than supply air in the design. This problem seems to be most prevalent in single ceiling mounted air returns in a cheap design that only gets return air from this one return and has no unit mounted filter.


I have the same problem of condensate dripping from the coil and in looking for a solution, which I don't yet have, I have come across the following suggestions. All of these assume everything was working fine and then began to fail.

The suggestions seem to fall into two main areas: (1) the evaporator coil condition has changed so that water droplets are no longer flowing down the coil to the drip pan and (2) air flow in the system has dropped due to some cause resulting in the coils becoming too cold. The coils may also be too cold due to a low charge.

The suggestions I've found, in no particular order:

  • clean the coils to remove oil, dust, or any other coating that reduces the slickness of the coils and impedes water droplets flowing down the coil to drip into the drip pan (my understanding is that commercial cleaners leave a slippery conditioning film on the coils which improves water droplet flow)
  • check the charge to see if it is low which can cause freezing of water drops which then unfreeze and drip as well as interfere with the normal water droplet flow down the coil
  • check that the coil has the proper degree of pitch to encourage water droplets to move towards the drip pan without so much pitch that water blows off the coil rather than flows along it.
  • check the seals on the air handler for degraded, leaking seals that need to be replaced
  • clean the blower as dirty fan blades can reduce the amount of air being pushed through the system
  • the coil fins may be corroded or otherwise less smooth causing water droplets to catch in pits and crevices rather than flow down the coil to drip into the drip pan (a suggestion I've seen and am unwilling to try is to very carefully and gently slightly sand the edges of the fins with very fine sandpaper however just know that the fins are easy to bend with very light pressure and if they bend, the coil becomes less efficient at transferring heat between the air stream and refrigerant flowing through the coil)
  • filters that have a higher MERV/MPR level than the system was designed for may be reducing the air flow especially with smaller sized ducts and smaller returns and longer ducts or operating in high particulate count (dusty or smokey) conditions (using a thicker filter, say 5" rather than 1" thick filter, provides more filter face surface area but the return must be designed for the thicker filter)
  • check the ductwork for obstructions or changes that impede or modify air flow

In addition to the above there may be a problem with the drip pan and the pipe for drainage. Check the drip pan and pipe for leaks or obstructions. Cleaning may also be needed.


So one more tidbit related to the comments above...

I had a clogged line and decided, while unclogging, to install a hose connection for easier flushing next time around. My horizontal pipes were 2/3 full of semi-solid gunk and after flushing all of it out I was convinced that I had fixed the problem while also at the same time making the cleaning easier the next time around.

I turned on the ac, let it run and dripping into the pan below continued.

I read all the posts here and it became obvious that the ac was sucking air (and water) up the drain pipe, there by not allowing the water to drain down it. The problem was that I had the hose connection not fully sealed and above the p-trap. So, although I fixed the original root cause, I introduced another problem.

I ended up putting a rubber balloon over the hose connector and that solved the issue.

  • 1
    This could use some formatting to make it more readable, otherwise, thanks for sharing.
    – FreeMan
    May 17, 2022 at 17:52

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