So my AC unit is draining water from the overflow drain again. I did my usual dance of cleaning out the main drain line - I made sure the point where it drains into the sewer was clear, put some drain cleaner, waited, poured a gallon of water down it, and then finally blew the line out. But water was still coming out the overflow.

So I disconnected the drains from the AC unit, and water preferred to come out of the overflow, even tho it was higher than the main line. I can see nothing that is preventing water from reaching the main line. If I block the overflow, water comes out the main, but if I put the pipes back on, it goes back to the overflow.

By pipes, I mean the connecting stubs that are a few inches long, so I know they are not blocked.

Why is the system draining this way? What can I do about it?

My theories: - I put in higher "quality" filters recently and somehow that has changed the airflow - the house has shifted and the overflow is no longer high enough to be an overflow - the coils are dirty and so the water is coming from a different place than it used to

What say you?

EDIT: It is a Carrier mounted horizontally in the attic. I couldn't find a model number, but it is about 12-16 years old?

Drain arrangement

Here's the drain hole arrangement. Air is blowing out of the rightmost hole pretty hard, and seems to be blowing the water out that way, instead of through the main drain line. The extra fittings are so I didn't have to cut and re-glue the pipes when the drain clogged. They've worked fine for 6+ years. I generally have had to clear the drain every two or 3 years.

UPDATE 2: I have now changed the air filters back to my usual, and waited a few days. The draining out the overflow continued. This morning, I opened up the evaporator compartment. I sprayed some coil cleaner, and the fins are shiny now, where they used to be dull. Also, I poured water in the pan and watched it run out the primary drain, so pitch isn't the issue. I think it could be an airflow thing. Hopefully the coil cleaner will change the dynamics enough to fix the issue, and the problem isn't with the upstairs/downstairs diverter thing. (we have one unit, but two zones). Evaporator coils post cleaning

  • 1
    Some photos might help. Make and model of the equipment would also help. Or at least tell is if it's upflow, downflow, horizontal, and where the evaporator is with relation to the blower. Is the unit level? Do you feel any suction on the main line?
    – Tester101
    Jun 17, 2017 at 15:30
  • It's horizontally mounted in the attic. The drains are all gravity powered. Not sure what you mean by upflow/downflow. @Tester101 I will edit question when I make back to get photos.
    – MrPhilTX
    Jun 18, 2017 at 11:40
  • Is there a trap on the overflow too, or just the main drain? Is there are blockage in the pan inside the unit, that would prevent water from going out the main drain?
    – Tester101
    Jun 18, 2017 at 13:13
  • @tester101 The overflow is the pipe on the right, it vents straight down into the drain pan. I can see into the system when I take the pipes off, and I could not see any blockages. When I blocked the overflow pipe with my finger, water would come out the main line.
    – MrPhilTX
    Jun 18, 2017 at 22:33
  • Has this setup worked correctly before? Does water only come from the secondary drain when the humidity is high, or when the unit is run for a long time? When the water is coming out the secondary drain, is it also coming out the primary drain?
    – Tester101
    Jun 19, 2017 at 12:54

1 Answer 1


So this morning I opened up the evaporator compartment and sprayed coil cleaner around. This evening, my patio is dry (where the overflow drain comes out) - I don't have water coming out anymore. It seems that the coils were either not draining properly or the airflow was misdirected by the dirty coils enough to disrupt the normal draining. Thanks @Tester101 for your efforts to help me.

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