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Sewer smell in basement (hopefully with pictures)

Evening. I'm another of those folks with intermittent sewer smells in the basement. I'll try to be brief ...

I moved into this house 6 months ago. During one of my multiple viewings, there was a sewer smell in the basement of the house. An inspector determined that the smell was coming from an open pipe, apparently disconnected from where the sink/dishwasher used to drain (it was moved when the kitchen was remodeled). He said once capped, the smell would go away. You can see the pipe/drain and where it was capped (white cap). The kitchen sink and dishwasher run from the pipes to the left - the only sources in the house that use this piping. I was fine with the explanation, it was capped, and no smell for several months, though I wasn't fully moved into the house.

Lately, the smell has returned, and seemingly comes from the area near where the main line goes thru the wall, and out to the septic - the same area as before. There's a picture of the main line attached. The smell occurs perhaps once a week, at most, and I have tried to pinpoint any actions on my part that may cause it. I have no ideas, though I thought it may coincide with doing dishes ... and I ran about 30 gallons of water thru the sink last night while sanitizing my water softener, and the smell was back this morning. That seems to be the only common denominator.

Upstairs ... the previous owner had installed a washer and dryer connection. The drain tubing for the washer is left exposed and open, and isn't capped; the washer and dryer are now down on the main level. The drain goes behind the wall (backing the hallway bathroom). While no smell comes from the opening, if I stick my nose in the tubing and inhale deeply, there is a faint sewer smell at times.

There are no floor drains in the basement, though there's a sump pit on the opposite end of the sewer line. The heat pump units (there are 2), both in the basement area and one unit close to the sewer line, apparently drain via a drain pump elevator, thru a copper line that elevates about 8', then continues outside of the house, though I see no evidence of any recent drainage where the copper line exits. I assume it would drain in the winter, but not sure.

I painted the basement floor with a white sealant about 8 weeks ago, and there's a very small rust colored splotch that obviously appeared after my paint job, about half the size of a dime, under the main line.

The line goes thru the wall to an older septic tank, that then drains (lifts?) to another, new septic tank that you see in the picture. The white pipe area is capped. There is no other clean out or "candy cane." I can hear constant dripping in this area of the septic. The septic inspector rated this septic set-up as one of the best he'd seen in the area, giving it an "A" grade. The dripping always was baffling to me but he wasn't concerned.

There's an older toilet on the main floor that I want replaced, and doesn't seem too powerful, if that makes sense - but it drains fine. The sink in that bathroom is fine. All other toilets and water/waste drain perfectly fine, too.

Any ideas? I haven't checked the roof vents ... if they are blocked, would that cause the smell in the basement, and not in other areas of the house? Would it be better to eliminate the capped line in the basement completely, and just have the line from the kitchen run directly into the main line? Could the trap on the capped piece be dry, and therefore the smell I smell is a result of the dry trap that I can no longer access?

Appreciate any ideas/help before I start spending some $$$. Thanks for your time and making it this far.


(sadly picture upload isn't working - msg me if you want to see pictures, I will link to another forum)

Re: Sewer smell in basement (hopefully with pictures)

Dried out traps are most likely. Your basement toilet is likely fine as long as there is water in the bowl. If you don't use the sink then the trap could have dried out. You can check by pulling the trap off or by just pouring water down it.

Your clothes washer should also be fine. Those traps are located in the wall and should always have water in it if you use the machine at all. Also a good sign that you don't smell anything there.

You mention a smell where the sewer pipe goes out the wall. Have you tried expanding foam between the pipe and the concrete? I would not be surprised if slight vapors/fluid is wicking backwards along the pipe, just enough to smell.

You might also try to flush your sump pump out by taking a fresh water hose and filling/rinsing it until the pump kicks on a few times and flushes all the stale water out of there. (Especially if anyone has ever peed in there).

Re: Sewer smell in basement (hopefully with pictures)

Thanks for the reply.

Well I think I ruled out the sink/dishwasher. I only use a portion of the house for now as I paint and clean elsewhere, so it is possible that the traps may be dry, though I try to run the sink water every 3 days. I use the washing machine maybe once a month (I use my current house's machine) and the toilet that is slow as rarely (maybe every 3 days). There is some green algae that will grow in the toilet if I don't flush it, if that matters. The sun shines directly on the toilet throughout the day, assuming that could cause that issue.

But when I flushed it most recently (again, trying to narrow down a source) that smell returned, though faintly. I assume the vent on the roof could be ... blocked? The wax ring could be bad?

I've thought about this possibility: "I would not be surprised if slight vapors/fluid is wicking backwards along the pipe, just enough to smell." It is hard to explain but I think this could very well be it. How do you mitigate the smell in this situation?

Thanks again,

Re: Sewer smell in basement (hopefully with pictures)

Not knowing what it looks like it is hard to guess, but the first thing I would try is expanding foam between the sewer pipe and the concrete wall.

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