I have a unit that is installed in the attic and is about 2 years old. For the first year everything was fine and then one day it wouldn’t run. So I hooked up the shopvac and sucked the condensation line outside and it started up. This worked for about a week and then I had to redo, then it became a daily occurrence.
I have cleaned out the drain pipe, poured bleach in it and still no luck. I put some anti-algae tablets a few inches away from the primary condensation opening. I have checked the outside drain and nothing. It seems to be just draining in the secondary overflow pan under the unit which is a pain because it fills up and hits the overflow switch and shuts off.

  • Hello, and welcome to Home Improvement. How long is the condensate drain? Are you sure it's becoming blocked? Could you replace it with something larger? (A picture and/or diagram would help us help you.) Commented May 11, 2019 at 21:41
  • Air filter being dirty or using a fancy high filtration type can cause suction inside air handler that won’t allow drain to function properly
    – Kris
    Commented May 12, 2019 at 3:04
  • Daniel, the drain line is 3/4 PVC, it goes across the attic about 10 feet exits the house and goes down the wall to a trap.
    – garygolfgh
    Commented May 13, 2019 at 0:06
  • Kris, the filter is about 2 weeks old and is mid grade. I believe you are on to something about a suction problem. Every time I shopvac from the outside water pours out. But when the shopvac is turned off, it drips for a while and then stops even though the unit is running. If I turn the shopvac back on, then more water pours out. So the shopvac is overcoming the suction problem.
    – garygolfgh
    Commented May 13, 2019 at 0:20
  • It's been awhile since I've been up there but I recall there is not much space between the exit hole where the line attaches and possibly the coil. Could this cause the condensate to freeze blocking the hole? Going up tomorrow to take some pics
    – garygolfgh
    Commented May 13, 2019 at 0:21

1 Answer 1


Given that it worked fine for a year, I wonder if something came loose and produced a dip in the line. I used to have to maintain a few dehumidifiers that were (supposedly) drained by garden hoses connected to the tanks. The garden hoses went down to a drain 12 feet below, so you'd think there would be plenty of incentive for the water to drain, and yet fairly slight bends in the hoses (not pinching them) that were below the tank would still manage to hang up the drains to the point that they would fill the tank and shut off. We'd go rearrange them and the tank would drain, but some time later things would shift enough that the problem recurred. It was not a good system.

Not sure if I'd go for trying to solve what's there, or putting in a larger drain line on general principles, but then, I don't know what size yours actually is, just that it seems common for A/C installers to use tiny pipes (and sometimes hoses, or sections of hose) for the condensate drains.

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