Is my fast-draining, gurgling sink the cause of sewer smells?

Question

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|______________________
\____>______________>____
                         |

Ecnerwal · Accepted Answer · 2022-08-08T12:21:38.333

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.)

score 14 · Answer 2 · answered Aug 08 '22 at 12:48

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.

Is my fast-draining, gurgling sink the cause of sewer smells?

2 Answers2