We bought and renovated our house two years ago. It was a near-total gut job. One thing we did was remove a gross jacuzzi tub from the master bath and replace it with a large walk-in shower.
We’ve lately noticed an intermittent sewer smell in the master bath. We use it daily, so dry traps seem unlikely. But while I was looking at the exposed drain pipes from below in the basement, I noticed that the vent pipe to which our contractor connected the shower drain is upstream from the trap, like so:
Could this be the cause of our smell? IANA plumbing engineer, but from what I can find online, the trap should lie between the shower drain and all other plumbing/vent connections.
That is very weird and could be the source. If that’s the only atmospheric vent, when you flush the toilet it could easily “burp” the shower or sink trap. Sounds like it should be relatively easy to fix at least.
Is there any other vent for the rest of the plumbing? IANAPlumber, but the reason for vents is so a slug of whatever down the drain does not suck the contents of the other traps with it. Your setup is probably fine as is, but if there’s no other vent for the toilet or sink, each time you flush it sucks the shower trap contents out. the vent as shown is not doing anything vent-ish.
It occurs to me it does allow a direct flow of cold air into the house, if you live anywhere northerly.
I don’t see how that would cause sewer smell. It might not be the best for insulation. A bathroom fan might draw air down the vent. Theoretically if it was near the main stack it could draw down stinky air.
ETA: As long as everything else is vented correctly.
A simple fix would be to bypass the trap and connect the vent on the sewer side of the trap - but ensure the slope/height location of the vent is such that any flow will not fill the vent pipe with water and prevent actual venting.
I think a smell would occur if flushing the toilet or such created a slug of water making a piston effect, and so sucking out along with it the contents of the shower trap (and any other traps). This is what a vent on the sewer side of the trap is supposed to prevent - it’s pressure relief.
Improper venting can occur if the vent pipe in some location fills with water due to improper drainage angles, allowing the vent to be blocked.
Basically you have no vent, and sewer gas backing into the room is exactly what this causes:
Yes it could as having no vent on the sewer side of the trap can pull the trap water out of the trap. the vent on the sewer side is what’s needed to prevent this. Basically a syphon can form and pull the trap water along with it, the vent breaks that suction.
The way it’s plumbed the vent is just acting like a additional drain, no different then having a double kitchen sink that all goes through one trap.
The other factor is the vent may be affected by wind which might help push water out of the trap. This could be amplified if the drain is not sloped correctly allowing standing water before the trap, or if the shower drain is somehow closes off by a drain cover or just something acting like that such as water, towel, or debris such as hair that is at a serious level wrapped around the drain screen. Additionally water backing up from the sewer side (say from a toilet, or sink) to the p-trap can also cause the syphon.
