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Thread: Positive airflow out of condensate drain into living space

  1. #1

    Default Positive airflow out of condensate drain into living space

    When one or both of my HVAC blowers are on, I get air through the condensate drain line. This goes up through the bath sink drain, which makes the bath smell funky. Sort of like the Pirates of the Caribbean ride, a musty smell. The vanity sink drain is not the cleanest place in the world, nor is the pan in the attic. Air running across these two places is smelly.

    I'm pretty sure the hvac contractor hooked up the condensate drain right. It is teed in before the P trap. The last 16 inches of the condensate line is a flexible plastic or rubber hose that hooks to the tee. It is definitely not on the sewer side of the trap. The humidity is pretty dry these days at around 25%, so the trap in the attic is sure to be dry. There is not "a lot" of airflow, but certainly enough to make the bath smell funny after awhile. It took years to figure out the smell, but when both hvac units are on the smell gets funky. With one unit on, not too bad.

    Are there any plumbing solutions I can try on the condensate line to fix this? I was considering maybe adding a loop in the hose to make another p trap, but in dry winters I am not sure how this trap would get filled with water.

    Makes me wonder why condensate drains aren't run to the outside of the house. Basically, I have a 5/8" air leak between my attic and living space...

  2. #2
    Master Plumber Redwood's Avatar
    Join Date
    Dec 2007


    I believe the condensate line itself should have a trap to keep the airflow inside the unit.
    Wait for an HVAC guy to verify this though.

  3. #3
    Master Plumber nhmaster's Avatar
    Join Date
    Jul 2008
    S. Maine


    Mr. Redwood has indeed served the correct answer to your dilemma. A trap must be installed on the condensate line.

  4. #4
    DIY Senior Member burleymike's Avatar
    Join Date
    Nov 2008


    All the air handlers I have installed say in the manual not to connect the condensate drain into a sewer line. It is supposed to go into a floor drain or outside. Of course if that is not possible into a condensate pump.

    The condensate line should also have a p-trap in it. It is not uncommon during the heating season for the trap to dry out and you will have some conditioned air blowing out the condensate line.

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