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Thread: PTrap too far below sink .??

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  1. #1
    DIY Member beachfront71's Avatar
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    Default PTrap too far below sink .??

    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 by beachfront71; 11-09-2012 at 11:35 AM.

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    Moderator & Master Plumber hj's Avatar
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    The length of the piping to the trap could become fouled and create the odor also.
    Licensed residential and commercial plumber

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    Forum Admin, Expert Plumber Terry's Avatar
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    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.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
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    DIY Senior Member kreemoweet's Avatar
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    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.

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    Forum Admin, Expert Plumber Terry's Avatar
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    I suppose there is wide variation in the sort of stuff people dump down their sink drains.
    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.

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    DIY Member petrie's Avatar
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    Quote Originally Posted by Terry View Post
    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.
    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?

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    Forum Admin, Expert Plumber Terry's Avatar
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    Quote Originally Posted by petrie View Post
    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?
    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?

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    DIY Member petrie's Avatar
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    Quote Originally Posted by Terry View Post
    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?

    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




    Here's the finished product.



    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 by petrie; 03-24-2014 at 03:22 PM.

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