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)