Slow drain actually might be clogged air vent

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by reybo, Mar 14, 2014.

  1. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    May sound familiar but it's not the usual.
    4400 sq.ft. 62-year-old house. Dual sink in the kitchen with plastic drain pipes. Drains 16" apart. One enters the wall 8" below the other.

    Left sink (lower drain) will back up if a pot of water is emptied in or after a heavy stream from the faucet. It drains, but slowly. A plunger is mildly effective in speeding the flow. Problem has become more noticeable over the last 3 months.

    Right sink almost never backs up, but when a pot of water is emptied in, while it flows out a gurgling noise comes from the left sink.

    Drains have been flushed with boiling water, treated several times over the last 5 weeks with Drano crystals following the instructions, and treated 12 or so times in the last 3 months with a product "Drain Care" (Zep) claiming to be an enzyme conditioner that eats pipe grease.

    All ineffective, though these treatments may have prevented this from growing worse (or possibly made it worse.)

    Why might this be a vent issue? I'll describe the vent, and try to attach a drawing.

    One iron vent pipe on the roof enters the attic and via a 3-way connector, makes 90-degree turns into 3 pipes sloping downhill at a slight grade. The right one becomes the vent for the clothes washer. The center one leads to the cellar where it goes untapped straight through the concrete floor. I have no idea where it goes. The left one is the vent for the kitchen sinks.

    There is a plugged floor drain in the utility room in the cellar. If you pull either of the 2 plugs in that drain while the clothes washer is emptying, water will back up through the unplugged hole, then drain out. The washer is on the floor above. There is no drainage issue at the washer. It works fine.

    Sad to say, this is not a town with abundant talented plumbers. 90 miles away they can earn twice as much, and many of the good ones commute. The best of the rest are hired by the local university and two hospitals, leaving slim pickings for the homeowner. You can pay though the nose for the firm with six trucks and an office staff, or two-thirds less for the home-based journeyman whose wife handles the logistics. The quality of the plumber who comes to the door will be the same, more likely than not.

    The builder plans and engineering drawings for the house disappeared around 1958, so determining the drainage paths is a challenge. We have good drain rooters in town but before I call one, I want to be sure that's the best first thing to do.

    Any guidance is much appreciated.

    Attached Files:

  2. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    Have the drain rodded, plugged vents don't make slow drains.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,026
    Location:
    Cave Creek, Arizona
    quote; plugged vents don't make slow drains.

    But they may contribute to it if there is also a problem with the drain line itself. Your description is enigmatic, since if the lower sink drain backs up, the upper one should also since they both use the same drain line and vent.
  4. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    Fun, educational experiment. Place a drinking straw in a glass of water, cover the top hole with your thumb, then lift the straw. See where the water in the straw goes.
  5. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,259
    Location:
    Land of Cheese
    What you are saying is true, but a drain system is designed in such a way that if the drain is clear of obstructions, the volume of water in the pipe never fills the pipe completely. If the drain is clear and properly pitched, it will siphon the water from the trap if the vent is not clear.
  6. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
  7. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    Which sounds like what is happening when the right sink is asked to drain a large pot of water. It does it, but with gurgling from the left drain, the lower one.

    But not the reverse. The lower drain does not make the upper one gurgle, which is why I'm seeking expertise.
  8. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,259
    Location:
    Land of Cheese
    If the left and right drains do not tie in at the same height, then the lower connection would have a separate vent. The vents could tie back together above the level of the countertop.
  9. bluebinky

    bluebinky Member

    Messages:
    408
    Location:
    Santa Clara, CA
    Then cut a hole in the middle of the straw and try again...
  10. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    Another point to support that: If I close the left (lower) drain, the right drain is much slower. Very much. So the gurgle is definitely the sound of air being sucked through the left trap when the right sink drains.

    Rooting the drain pipe does not sound like a cure for this issue, though it would do no harm.

    It seems to me that if there's a blockage in the vent, it is above the point where the upper branch of that vent goes to the right sink. It could be something is plugging the 3-way connector, which is the first obstruction in the air-way. The garden hose is nearby, and years ago I bought a Drain King unclogger. It's for drains 3" to 6" and may be a size too large.

    That avenue of attack has become appealing.
  11. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    Exactly! In this case, that hole is the left sink drain sucking in air needed for the right sink to drain.
  12. SHR

    SHR Member

    Messages:
    127
    Location:
    Minnesota
    I think this problem is being overthought. You have straightforward clogged drains. Clogged vents do not stop the flow of waste. Hire a qualified licensed plumber or drain cleaner to clear all of your clogged drains. Unfortunately you used chemical drain cleaners. Working on drains that have been filled with caustic chemicals requires precautions to be taken so be sure to inform whoever you hire.
  13. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    The analogy is flawed. In the case of drainage, fixtures themselves provide all the vent needed to evacuate the fixture. The purpose of the vent is to equalize the pressure within the plumbing system so that fixture traps do not siphon. An occurrence that is common with older, non-vented S traps. In any case, atmospheric pressure exerting its pressure on the liquid in the fixture is more than enough to insure that it drains. In the case of slow drains, it is always because of an obstruction of some sort in the drain line itself being either stuff in the drain or a rise or dip in the drain or a drain that is not properly pitched. All of those things cause the drain pipe to fill with water. Adding a vent only gives the air being pushed ahead of the water a place to go which gives the impression that the problem is the vent. It's not, it's the drain.

    http://youtu.be/RA2LHLT26aE
    Last edited: Mar 16, 2014
  14. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    Makes sense. But how would a clogged drain make the left (lower) drain gurgle when the right sink is draining? And if the left drain is closed, the right drain is significantly slower.
  15. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    A lot has to do with how the trap arms were plumbed.
  16. reybo

    reybo New Member

    Messages:
    14
    Location:
    Virginia
    "Rooter Guy" was here today.
    Sludge, not airway.
    Plumbers 1, Rey 0.
    Says "Drain Care" by Zep is crap. Recommends BIO One, a patented biodegradable bacteria culture. Anyone else like this to eat sludge?
  17. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,026
    Location:
    Cave Creek, Arizona
    quote; Place a drinking straw in a glass of water, cover the top hole with your thumb, then lift the straw. See where the water in the straw goes.

    You are not really trying to teach me about plumbing and venting, are you? If you noticed I said, "assuming the drain line is not also plugged", because if it is not the air will NOT be trapped and can exit down the drain line.
  18. SHR

    SHR Member

    Messages:
    127
    Location:
    Minnesota
    Do not buy the Rooter's crap. I strongly hate drain chemicals but I recommend biological bacteria/enzyme treatments. You can pick up the same type enzymes at any big box store for a fraction of the cost. Congratulations on solving your drainage problem.
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