22

My laundry has a washing machine, a large sink, a toilet and a small hand basin. The washing machine drains into the large sink.

We sometimes notice a sewer-like smell in the laundry. I'm trying to figure out what might be causing it.

I read this post, which seems to fit. The noise isn't troublesome, but the occasional sewer smell is.

The hand basin gets very little use. I wondered if its P-trap could be drying up and letting sewer gas into the laundry, so on a smelly day I ran water into the hand basin and ventillated the room thoroughly. The smell came back.

The large sink drains quickly, but gurgles once all the water has gone down. I wonder if this is the sound of the water being dragged out of its P-trap by the long sewer pipe that leads from our house down to the street sewer about 10m below. Water goes down through the plug hole, around the P-trap, then horizontally into a stud wall, then to who-knows-where.

|  sink  |
\____ ___/
    |v|
 __ | |___
/  _| |_  \
|v| | | |^|
| | | | | | P-trap       |
| | |v|_|^|              | wall
| | \__>_ /              |
|v|______________________
\____>______________>____
                         |
9
  • 2
    There's supposed to be a vent running from exit side of the trap through the roof.
    – Hot Licks
    Aug 8, 2022 at 2:29
  • 2
    @HotLicks There isn't one in this case. In fact I don't remember ever seeing a P-trap with a vent here in New Zealand or in the UK (I haven't looked at any plumbing in the UK since 2003). Aug 8, 2022 at 2:54
  • 2
    I guess venting every single P-trap is mostly an American tradition. I've never seen that done in the (western/central) EU, there's typically just one big vent for the main stack (toilets), which tends to be 100 mm or more. Individual branches to sinks are almost never vented.
    – TooTea
    Aug 8, 2022 at 11:23
  • 2
    That's an S-trap. If you're not ready for a plumbing project then you can use the sink as normal, wait for the gurgling to stop, and let some water flow slowly from the tap to fill the trap.
    – MonkeyZeus
    Aug 8, 2022 at 17:48
  • 3
    I am betting on the floor drain, which was not part of this scenario as laid out. A room with laundry machines should have a floor drain and they hopefully get very very little use. But they could let sewer smells up. Get down there and give it a sniff.
    – Willk
    Aug 9, 2022 at 19:32

2 Answers 2

21

It's folded back as drawn, but that's an S trap, not a P trap.

A P trap would enter the wall at about the same level (1/4" per foot lower - 2% slope) as the top of the trap. When you bend the outlet of a P trap down (before hitting a drain pipe with a vent), you make an S trap, and S traps siphon themselves dry with ease.

You might be able to lower the trap (by extending the sink drain connection downwards, and rearranging other pipes to fit) to the point where it would actually be a P trap. Like this.

|  sink  |
\____ ___/
    |v|
    | |
    | |
    | |  
    | |                  |
    |v|                  | wall
    | |                  |
    | |  ________________|
    | | /  _________>____
    | | |^|              |
    | | | | P-trap       |
    |v|_|^|              | wall
    \__>_ /              |

It is normally true that the vent associated with a P trap will be on the drain line in the wall, out of sight, unless it's a mechanical vent (air admittance valve, aka "studor" which is a brand name.)

5
  • 5
    Ah, interesting! Can I assume then that the drain smells are probably a result of the S-trap siphoning itself dry? Aug 8, 2022 at 4:07
  • 1
    @OutstandingBill sure looks like it.
    – Mołot
    Aug 8, 2022 at 10:48
  • Regarding S versus P: does it make a differencea? Water going up the other side of the U doesn't remember whether it came in from left or right.
    – Kaz
    Aug 8, 2022 at 23:16
  • 2
    @OutstandingBill If it gurgles at the tail end of the drain cycle, it must be sucking air through.
    – Kaz
    Aug 8, 2022 at 23:17
  • 2
    @Kaz Left/right makes no difference - up/down does. A P (however laid out) goes down once and up once, into a pipe that has air space on the top of it running to a drain pipe with a connection to a vent. An S (however laid out) goes down, and up once, and down again without venting, and fills with water to complete a siphon and suck the trap dry. There are ways to make something that appears to be an S but actually isn't (described in IPC code, even) by making the final downward part larger than the first down and up part, so that the second down part cannot be filled by the sink.
    – Ecnerwal
    Aug 8, 2022 at 23:31
14

To add to what Ecnerwal said, it might not be practical to replace the S-trap with a P-trap (i.e. the drain line is too low).

The solution there would be an air admittance valve(AAV)

|  sink  |
\____ ___/
    |v|
 __ | |___     _____
/  _| |_  \    \   / AAV
|v| | | |^|     | |
| | | | | |     | |      |
| | |v|_|^|     | |      | wall
| | \__>_ /     | |      |
|v|_____________| |_______
\____>______________>____
                         |

What this does is to break the siphon effect of your S-trap by letting air in, but not water out. This is a valid alternative in most locales.

10
  • I think it would need to connect to the top of the S to be effective, and then it needs to be far enough from the P exit not to be a crown vent.
    – Ecnerwal
    Aug 8, 2022 at 14:12
  • @Ecnerwal I've seen them hooked up both ways. The catch with the top of the S is it might be too close to the sink (without a photo I put it into the only logical place in the diagram).
    – Machavity
    Aug 8, 2022 at 14:16
  • 1
    @Ecnerwal I don't think connecting to the top of the S is necessary. The problem with siphoning occurs because the only place to pull from is the trap. If there's an AAV after the trap, it's easier to pull air from there than water from the trap. Aug 8, 2022 at 14:59
  • If the leftmost pipe below the sink (where water flows down) is indeed longer than the rightmost one (where water flows up), it'll always pull the water out of the rightmost one. You may get away with this if the pipes are wide enough; when the center pipe is drained of water, you'll suck air up the right pipe instead, and when the left pipe is empty, the water from the right one will flow back. But you'll still have the gurgling noise. Better connect the AAV to the top pipe if you have the space. Aug 9, 2022 at 5:38
  • 1
    @MichaelRichardson An AAV is, by definition, only one way. A vent stack (where the pipe exits the roof) is not the same thing.
    – Machavity
    Aug 9, 2022 at 16:42

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