My AC Unit in my attic is roughly 10 years old, still heats/cools exactly as it should, but I noticed yesterday that the drip pan is overflowing.

What do I need to do to get the drip pan to drain? And what is causing the drip pan to fill?

  • Is drip pan not currently connected to a drain? Has it been unusually humid in your area recently? Jun 28, 2017 at 14:14
  • the drip pan is connected to a drain, if I check the end of the drain there is a slow steady drip coming out of it and the ground around the pipe is wet. The past 72 hours has been extremely humid! Jun 28, 2017 at 14:16
  • So the drain is working but very slowly, is that right? Jun 28, 2017 at 14:20
  • Yes, that seems to be the issue. Jun 28, 2017 at 14:20
  • 2
    When you say "drip pan", do you mean the pan under the unit, or the pan inside the unit? Does the unit have an overflow drain, or just a primary drain? Are there traps on the drains? Could you include some photos of the drains at the unit? Has the unit been running longer than usual?
    – Tester101
    Jun 28, 2017 at 15:00

3 Answers 3


I had a problem like this many years ago. I did some experimenting:

  • I found if I disconnected the condensate drain pipe, the water would flow out of the A/C without restriction.
  • I could pour water into the disconnected pipe and it would flow easily.
  • My condensate drain pipe ran to the drain pipe of a nearby bathroom sink, which was draining just fine.

I concluded there was a pressure problem and the condensate drain pipe would work correctly if I added a vent.

Here is a picture of one person's solution. It doesn't look too elegant, but it illustrates the method.

My drain pipe ran from the condenser to an elbow, and then vertically downward, IIRC. I just replaced the elbow with a tee and added a short length of pipe at the top of the tee, leaving it open at the top end.

Problem solved, in my case.

Now, it had been working fine before all of this happened. But the problem showed up just like you described it. I don't know why or how it was working before I did this, but I never had trouble again.

enter image description here

  • This solution worked! The issue that I had was that the area where my unit resides is in such a tight enclosed area, I had to remove part of a wall to get to where the pipe actually was. Jun 29, 2017 at 12:25

First - why is your drip pan filling?

Because the process of cooling air requires the use of a condenser which gets very cold. Humid air when it contacts cold surfaces causes moisture in the air to condense (think how a glass with cold liquid gets condensation on it).

This water falls off the condenser and into the drip pan.

Second - Why is your drip pan over-flowing?

Sounds like your drip drain is clogged. You should try to run a thin snake through the entire drain. Hopefully the drain pipe is big enough to allow this.

enter image description here

Do NOT put drain cleaner in the pipe as, as you've seen, it will drain into your yard and cause damage to plants.

  • 1
    I can get to the drain pipe outside, but I am not sure I could get to the drain pipe where it is at the unit.....If I use this tool from the outside pipe would it cause problems by "pushing" the blockage into the unit? Jun 28, 2017 at 14:25
  • 1
    You could get immediate relief by sucking on the line with a shop vac. Once you have pulled out the water that is filling the pan you could clean it with a a pint of diluted bleach. A less smelly alternative that might work is hydrogen peroxide. There are tablets or liquid specifically designed for cleaning the drain line. Jun 28, 2017 at 21:25

Often this happens because your drain line goes outside. You've got a cool, moist and dark pipe so algae can grow there. More modern installs tend to try and put it into a vent stack inside the attic, where it's not as likely going to be a problem.

There's a couple of ways to deal with this

  1. Pour some bleach down the drain line once every few months. Not a lot of bleach, mind you, but enough to kill anything growing in the pipe (maybe 1/4 cup bleach with 3/4 cup water). My mother didn't know this and hers backed up into the house. In her case, the drain was inside the HVAC closet by the blower with a cap she could have accessed.
  2. Get an HVAC guy to install a condensate pump. He can then put the condensate there and the pump can push it into your vent stack, wherever that is. Normally your pan shouldn't be catching the water directly, as it's more of a "limit the damage" measure. The condensate pump will sit inside the pan so if it fails, the pan catches it.

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