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  1. #1
    Join Date
    Jul 2008
    Posts
    33

    Loud gurgling or knocking noise coming from drain pipe after AC has been running

    We recently bought a 1960's house in Houston, TX. 2 stories, the 2nd story is half bedrooms and half attic. The main air handler and furnace is in the attic. After the AC has been running a while, a loud noise like a gurgling or knocking, somewhat randomly but about 1-3 seconds apart starts coming from the drain pipe in the mud room (where the washing machine drain hooks into the wall). It also feels like there's a little positive air pressure coming out of the drain pipe. Sometimes the AC can run for a while before this noise starts, but once it starts, it won't stop until the AC turns off, and then it stops instantly. The noise is loud enough that I can hear it in the living room over the TV so need to stop it. When the AC is on but before it starts gurgling, I can hear a little drip of water down the drain pipe but you have to be right next to the drain pipe to hear it.

    There's a white PVC pipe running out of the AC which I assume is the condensate line. It runs in the direction of the drain pipe but because of the slope of the roof I can't access the area where they might meet. There is no trap in the condensate line that I can see. My theory is that condensation is building up in the line that runs to the drain pipe and after a while the air pressure and water are interacting, somehow sending vibrations into the drain pipe which is echoing out the mud room and through the house. Am I off base? Is the fix for this as simple as installing a trap at the condensate line?

    Also, does anyone know what kind of ancient flex duct this is? Would new flex duct have better insulation and be more efficient?

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  2. #2
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,052
    You need to get a pro in there. If the condensate drainage goes directly into the Drain, Waste, and Vent or DWV soil pipe, it is a plumbing code violation and a problem. Now, if your AC is before the blower, it will tend to suck air up through the condensate line, which must be trapped properly. Otherwise, you risk pulling sewer gases into the AC including explosive methane and deadly sulfur dioxide. Moreover, if it is not properly trapped, water, trying to drain by gravity will compete with the air rushing into the unit thereby causing a gurgling sound. Regardless, the whole thing needs to be inspected by a qualified pro.

    Typically, the AC condensate needs to discharge through an open air gap into a properly trapped standpipe that drains into the DWV, open sink, sump pump, directly outside or other approved means.
    HTH

  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    5,659
    By the looks of that ductwork you're wasting a lot of energy through the leaky ducts into the attic.

  4. #4
    Join Date
    Jul 2008
    Posts
    33
    Thanks I am just looking for something to tell the tech so he doesn't waste a lot of time in case the noise is not occurring when he comes out.

    Sewer gas not an issue with the AC - if gas were going to come out, the washer drain hookup is open to the air in the house regardless of the AC's condition. The drain pipe must be trapped further down. Also, the blower/furnace is before the coil, so it is positive pressure, not going to suck up sewer gas.

  5. #5
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 07-07-2013 at 07:18 PM.

  6. #6
    Join Date
    Jul 2008
    Posts
    33
    Yes my noise is very similar to yours, although not as rapid as yours, on average 3 seconds apart.

    Quote Originally Posted by WhatDayIsItMike View Post
    I think this is similar to what I've been experiencing. I made a thread about the noise that comes after my AC has been running here: http://hvac-talk.com/vbb/showthread....is-this-noise-!

    I also uploaded a sound bite of it; perhaps you can compare? http://picosong.com/RCkW/

  7. #7
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,512
    probably just needs a ptrap would install one with a safety switch on it .also the big duct looks to be wrapped hard pipe.
    We really need change now

  8. #8
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    24,947
    WhatDayIsItMike

    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.

  9. #9
    Join Date
    Jul 2008
    Posts
    33
    Quote Originally Posted by catmanacman View Post
    also the big duct looks to be wrapped hard pipe.
    You are correct. Is it more energy efficient to have the tech replace the insulation on the hard pipe or replace the entire section with flex duct?

  10. #10
    Join Date
    Jul 2008
    Posts
    33
    The noise was eliminated by installing a u-trap. Thanks all.

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