PTrap too far below sink .??

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by beachfront71, Nov 9, 2012.

  1. beachfront71

    beachfront71 New Member

    Messages:
    43
    Location:
    newport beach, ca
    Hello and thanks in advance,

    Just curious if a Ptrap can actually be installed too far below a sink and if so could that cause the force/speed of the water dropping down to leave the ptrap unfilled and letting sewer gas into the house?

    We are getting some terrible smells from the sinks from time to time and can not think of what else the problem could be. The smell is not constant which makes me think the above might be happening.

    We turned a piece of furniture into a sink and wanted to try and preserve the drawers below so we is put a 90 on the sink drain and ran that towards the back of the wall and then down to a trap, basically we went over and around the drawers so we could still use them..

    Thanks and good weekend to all.
    Last edited: Nov 9, 2012
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,642
    Location:
    Cave Creek, Arizona
    The length of the piping to the trap could become fouled and create the odor also.
  3. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    If the tailpiece is too long, it will siphon a trap. Is there a vent on this lav? Normally the vent takes off at the height of the p-trap traparm.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    Seem to remember 2' as a max fall on a sink before the trap, but don't quote me on that! As mentioned, the trap not only stops sewer gases but also decaying crud that may be accumulating on the interior of the pipe up to the trap. The longer the pipe to the trap, the more area that can accumulate hair, soap scum, etc.
  5. kreemoweet

    kreemoweet New Member

    Messages:
    371
    Location:
    Seattle. WA
    I doubt very much that your drain configuration is causing the problem. I assume (because you left out that info) that your trap is somewhere
    on the same floor, behind the drawers? I have done similar drawer-dodging setups before with no problems. Also, the drain routing is not all
    that different than one frequently finds in dual-sink configurations. I would look to a venting problem that is sometimes causing the trap
    to siphon. I have never encountered "terrible smells" from the usual gunk that collects on inner pipe walls, but I suppose there is wide variation
    in the sort of stuff people dump down their sink drains.
  6. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    Sometimes it's in the bathroom sink itself. If there is an overflow, it drains down to the pop-up drain. I've seen people force water down the overflow with some bleach to clean that.
  7. Hammerlane

    Hammerlane New Member

    Messages:
    252
    Location:
    Ohio
    Sounds good with the exception of washing machine standpipes which can usually be 30".

    Had these diagrams saved.

    Attached Files:

    Last edited: Nov 10, 2012
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    The outlet of a WM is pumped, and the increased velocity would tend to scour the pipe with each use, not the same case as with a sink, draining via gravity, especially if a large portion of it was horizontal.
  9. petrie

    petrie New Member

    Messages:
    56
    Location:
    Wisconsin
    Old post, but gave me a clue to my problem. In my basement I removed a shower stall and put a utility sink in using that drain. It's more than 2 vertical feet from the sink drain to the trap, plus there is no vent on this drain. The drain on this sink is a 2" pipe that runs 6' over and is connected to the closet bend.

    Every now and then I go down there and smell a stench. Would flushing that toilet be enough to siphon water out of the trap. Would dumping a gallon bucket of water into that sink be enough to siphon out trap? Also the same vent/drain stack services both upstairs and downstairs toilets. Could the slug of water from upstairs toilet be enough to siphon out the trap on the sink?
  10. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    A long trapway has enough height that it can siphon a trap. Using a shower drain for a lav doesn't work.
    Any plumbing between floors needs to be separated by venting.
    Was the home plumbing inspected at the time of installation?
  11. petrie

    petrie New Member

    Messages:
    56
    Location:
    Wisconsin

    Home was built in 1972. When I bought it 4 years ago the homeowner had a cheap shower stall set up. The closet elbow was actually mostly covered by the concrete floor. I thought it was a hole in concrete and was going to cement over it, and then realized it was a closet elbow that was attatched to the 4" cast iron soil stack. Picture is of it after I chipped away concrete

    [​IMG]


    Here's the finished product.

    [​IMG]

    I just came off my copper with pex, replaced shower with sink and put toilet in so I never had it inspected. I knew the sink should be vented, but the previous owners had been using the shower for years so I firgured I could get away with it. Didn't realize Trap couldn't be a mile from top of drain. dope. Drain from sink just runs parallel to wall over to the closet elbow.
    Last edited: Mar 24, 2014
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    You might get by by adding an AAV (sometimes called a Studor valve) on the sink. Not the best solution, but it might work. They aren't allowed everywhere - a 'real' vent is much preferred. I'm assuming that the drain line in by the toilet is coming from upstairs, and is not a vent (yet, maybe higher up - the pipe can't be both a vent and a drain, except in special situations, and then, only on one floor). If that line IS a vent, then you could tap into it for the sink.
  13. petrie

    petrie New Member

    Messages:
    56
    Location:
    Wisconsin
    The Pipe next to the toilet is the drain for the upstairs bathroom, and goes right up through the roof. I figured the toilet installation was ok because the drain pipe was a "wet" vent.

    Since the install there is never an all time stench coming from this room. Just once in a while you have a weird sewer gas type of smell. I usually just run some water in sink and next time I come back the smell is gone.
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    If your local codes allow it, the simplest fix would be an AAV. Properly installed, that should stop the siphoning of the sink trap.
  15. petrie

    petrie New Member

    Messages:
    56
    Location:
    Wisconsin
    I've seen those valves at the local diy center I'll have to took at them.
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    You need your trap arm from the sink to run to the (proper) T, with a stub going up for the AAV, and down to the drain. IOW, the same configuration as if it were going up to a standard vent.
  17. petrie

    petrie New Member

    Messages:
    56
    Location:
    Wisconsin
    Without major "surgery" I can't fix this installation. I thought about the AAV, but no matter where I but that it's going to be above the trap...which is in the floor. I disconnected undersink drain from the pipe comeing out of floor and the underslab trap stays full after dumping a bunch of water down it and flushing toilets. The water in both the downstairs toilet and underslab trap does move up and down when toilets are flushed, but it doesn't siphon water out of it instantly.

    I guess it's been this way for 2-3 years, so I'll just have to deal with it.
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