I noticed some water dripping from my A/C's secondary condensate drain. The primary condensate drain line still has what seems like a normal flow of water coming from it, so I don't think it is clogged (may be partially clogged?). I went up in the attic and, sure enough, the overflow pan had a small amount of water in it. This water is coming from a fitting in the evaporator coil that looks like a PVC cap, but with a small hole in it: enter image description here And an overview image: enter image description here Notice in the bottom picture that the the primary drain has a float valve; I inspected this and it appears to be in working order, and it has not tripped (A/C running and cooling normally).

The question I have is what is the purpose of the fitting with the hole in it, and why might water be dripping from it?

  • Water is dripping from it because there's a hole in the cap. Now the question is why is there a hole in the cap?
    – FreeMan
    Commented May 17, 2022 at 11:39
  • 1
    Your edit would better serve the community as a whole if it were added as an Answer, down below. Self-answered questions are acceptable around here and are actually encouraged.
    – FreeMan
    Commented May 17, 2022 at 17:21

2 Answers 2


(From OP) I found this image online, which I believe is what I have in my coil box:

enter image description here

If the primary drain (right outlet) gets clogged or partially clogged, the condensate inside the evaporator will rise up above the "dam" built in to the left outlet. The condensate water then starts to exit this secondary drain. One could choose to install a sensor (ie, condensate overflow switch) on this outlet which would stop the AC from working, thus alerting the owner that something is wrong with the primary drain.

In my case, the installer put the cap-with-small-hole on the secondary drain knowing that the water would be captured by my pan, and then drain out the secondary drain and I would see the drips from that drain and know something was up (which is exactly what happened). Since the primary is still draining, I suspect it has a partial clog (enough to back up into the coil box, but not enough to trip the primary drain float valve.

  • You should also accept your answer so others know that there is a "correct" answer to the question. It should help discoverability of this so you can also help others.
    – FreeMan
    Commented May 18, 2022 at 14:16
  • 1
    I will, once SO lets me. You have to wait 24 hours after posting to accept your own answer when you have almost 0 reputation. Thanks.
    – TeeRaff
    Commented May 18, 2022 at 15:55
  • It says "answered yesterday", so I though enough time had passed. Didn't mean to harass you about it. ;)
    – FreeMan
    Commented May 18, 2022 at 21:59

It sounds like you need to treat your condensation drip pan with a algae blocker, I call them black gunkies they build up and restrict the flow then at high flow times some of the condensate water ends up in the over flow.

Drip pans are usually ignored until a failure occurs in my area a season worth the cleansing tablets are between 10-20$ big box stores do carry the tablets or they can be ordered online a tablet in the pan helps to kill the gunkies and prevents them from growing and plugging the drain completely.

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