I periodically smell sewer gas from the main shower drain--it isn't there all the time. It is used daily and I have monitored the water level in the trap and it doesn't drop more than 1/8" in 24 hours through evaporation, so there isn't a leak in the trap. The stack vent is clear (it has a screen over it). I have tried the bleach & hot water trick, the vinegar, salt and baking soda trick but the smell keeps coming back.
Any ideas where it could be coming from? The local plumbing supply shop suggested a enzyme/bacterial cleaner, but if the trap is closed by water, where could the smell be coming from?

Hmmmmm...

Besides the obvious issue with "stuff" clinging to the inside of the pipe... which the cleaners should have dissolved... I'm going to venture a guess that you may be having a venting or clogging issue with the drains.

Here, if the drain is partially clogged or starting to clog, or if say the vent to the shower or sink is clogged/clogging then that could present as an occasional sewer gas odor as the system basically BURPS to bring air in and that allows sewer gasses to escape... maybe?

To test this, fill up your sink(s) and then drain the sink and flush the toilet while having someone watch inside the drain with a flashlight... any burping/movement of the water? If so, then let me know and we can discuss the next steps to take. Repeat this test 3 or 4 times back to back to check well.

I'd remove the shower strainer to watch the water in the shower drain without obstructing the view.

Let me know what you find...

Mark

THanks Mark, I will give that a try when my wife gets home tonight and let you know what I find out.

Mark
Tried the test you suggested and did it 5 times in succession. Did not get any air bubbles coming up through the trap. The water level would rise maybe 1/4 inch in the drain when the sink drained and the toilet flushed, but no bubbles or sewer gas escaped.

Smells are almost always the most difficult thing for us to deal with... UGH!

Since you seem to have isolated this to the shower drain I'd clean the inner wall of the drain pipe and then wash the drain down with bleach again.

If that fails to finally resolve this you may need to start exploring for leaks behind the wall... could be mold you're smelling... maybe?

PS: Ceck the oveflow hole at the sink drain... these can also create some pretty bad smells under certain conditions and the odor/gas could float over to the shower... maybe/maybe not?

Sorry I couldn't help more...

Good luck!

Mark

Hello Busydog,

Is this a shower only or Shower/Tub?
If it is shower/tub then Mark has point about the overflow hole.
Also if it is shower/tub check your overflow does this have a plunger installed or just the face plate installed, please see the images below, Thanks.

John

Mark, John
Thanks for the ideas
It's a shower only, so no overflow to deal with. One of the most frustrating problems I have dealt with as I can't seem to isolate where it is coming from. It is a sewer gas smell and don't think it is mold and it is erratic.

Is it possible that the vent stack for the shower drain or the adjacent sink drain (they are about a foot apart coming out of the roof) might be partially blocked? Could I run a hose with one of those rubber expansion bulb fittings into the stack and blow out any potential/partial clog without damaging the plumbing system?

You could just use a regular hose... just make sure someone is inside watching... just in case.

Let me know how it goes...

Will have to wait until the weekend to try the hose trick, so will let you know what happens