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sewer gas smell

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i have two bath rooms there is a small one right behind the wall of my washer ad dryer and hot water tank,after electric dryer has been running about 20 minutes i get a sewer gas smell everytime in the small bathroom,sometimes i wil get this smell when dryer is not running but not often.

I have had two plumbers out a total of 5 times,both toliet seals replaced and traps checked,also went into the wall between the utility room and small bath room and did not find anything wrong and no smell in the wall,had a camara ran thru the sewer pipe under the cement foundation also up both roof vents and up into both toleits and found no cracks anywhere,the only thing i changed in the last few month was i have a electric roof fan on the roof that pulls air out of the attic i noticed by the instructions the hole was tobe 17 inches and it was only 14 inches so i cut it out 3 more inches.this attic fan is pretty close to the large large sewer vent pipe that comes thru the roof for the larger bath room which does not have a sewer gas smell in it,

The smaller roof vent for the small bathroom thats getting the smell is a good distance from the attic roof fan

i,m out of options.

What do you think is causing this sewer gas smell?
asked in Plumbing by camaro (120 points)
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1 Answer

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Hi Camaro,
I’m not sure what you mean by: “The smaller roof vent for the small bathroom that’s getting the smell”. Is that a roof / attic vent that the small bathroom’s exhaust fan is connected to? Is there an exhaust fan in that bathroom at all? If so, does it discharge into the attic / close to the roof vent, or through the roof / how close is this discharge point from the vent stack pipe and the electric vent, does the bathroom exhaust have a functional damper?.
I don’t see how the electric dryer can be responsible for the sewer gas odor in that small bathroom. The only thing I can think of is that the dryer (that needs so called make-up air while running) or / and the electric attic fan depressurize your home causing imbalance in the sewer system.
Somehow, the sewer gasses discharging from the vent stack (assuming there’s no cracked sewer / vent pipes (maybe in the attic, hidden under insulation), or faulty drain trap) find their way back into the bathroom. 
Maybe through an open window (probably not), bathroom’s exhaust fan (possible), or some other way... I’m assuming that you cannot pinpoint the source /area of the bathroom that this odor is originating from.
Well, I would turn off the electric fan in your attic and repeat the dryer’s cycle, see if that changes anything. If you have that exhaust fan in your bathroom, seal it off with a piece of plastic and run the dryer. Maybe based on that information you’ll be able to narrow your investigation. 
Please let me know, click “comment” below if you have more questions.
answered by darekrudy (21,730 points)
what i mean is on the roof there is a electric roof fan that pulls heat out of the attic and about 3 feet straight accross up by the peak of the roof is a sewer vent that goes down thru the roof down in the wall in the lrge bath room which does not hav the smell.

then about 10 ft straight down from that sewer vent is the small sewer vent  that goesthru the roof down into the wall to the small bath room that i,m getting the smell from everytime the dryer is on and sometimes when the wind blows hard.

there is no kind of ceiling vent in the small bath room.

i,ve lived here for 35 years and the dryer has been in use for 3 years and i just started experincing the sewer gas smell about a month ago.

i checked the trap in the sink and its ok.

had the wax seal under the toilet replaced although the plumber used to regular seals on top of one another he said this was because of the tile although a salesman in lowes told me that was to much wax that a jumbo seal would have been enough.

another plumber cut to holes in the wall and found nothing and another one ran

a camara thru the sewer pipe and up the sewer vents and toilets and found nothing

any other suggestions?


Hi Camaro,
Well, the sewer gas must have a way of getting into your bathroom; one thing that could be helping it is air from a forced air conditioning system or any type of air circulating system (assuming that you have central air in your home). There is also a possibility of something happening under the floor, again, I don’t know if you have a crawlspace, slab, basement or another living area under the bathroom.
One of the scenarios I’ve seen a lot during my inspections was a dryer vent disconnected in a crawlspace and a leaking sewer pipe under the floor (or just moisture). Running dryer was causing air pressure to build up in a crawlspace and force contaminated / smelly air through the gaps and air ducts into the house. 
If you didn’t, check dryer’s vent pipe, where it discharges, check its entire run, maybe disconnected inside the wall, ceiling, or under the floor.