Basement septic smell

[GreenSheep]

I'm dog sitting for my brother for the week, and the guest room is in the finished basement. The basement also contains a full bathroom, living room, laundry nook, and utility closet. I mentioned catching a whiff of the septic system (it has a distinctive smell that you never forget -- slightly different from fresh poo!), and he said he's noticed it, too. He pours water down the drain under the water heater in the utility closet occasionally, but he's done that recently and it hasn't solved the problem. I don't see a drain under the washing machine (pouring water in that solved the same problem in a former house of mine), or any other drain that might be the culprit. The system was inspected when the house was purchased in late 2020, but I don't know if it was pumped then. I assume it would have been pumped if it needed to be, and there are only two people living in this 4-bedroom house, so I would guess that they haven't overfilled the septic system after less than 1.5 years. (Oh... but I doubt houses/septic systems built 40 years ago accounted for people working from home, which they do most of the time.)

Any ideas on non-invasive things I could try to get rid of the smell? It's not overpowering, and I can deal with it for the week, but it's just enough to be constantly noticeable and obviously gross. It would also be a nice thing to have solved for my brother when he gets home!

[reeshau]

You mention the water heater and washing machine; what about the furnace?  A forced air furnace A/C will also have a drain pipe and u-bend.  This is the one we always had trouble with in Michigan, as it dried out by the end of winter.

We had 2 units, and the one in the attic was always a bear to reach.  (and was always the dry culprit)  I had to rig up a contraption with come PVC pipe to reach it and reliably fill it.

[herbgeek]

Another possibility:  a dry drain.  You mention only 2 people, so perhaps the drains haven't gotten regular use.  Water in the trap keeps sewer gas from entering the house, and if this evaporates, well you get that smell.  We had this in a downstairs shower that we never use- I know exactly what you mean by "a distinct smell".  We found some organic liquid meant for cleaning drains.  We had to apply it a few times, but eventually it went away and now I make sure to run water to that shower every couple of weeks.

[Askel]

It's also possible that the venting on the drain system is blocked or inadequate. There should be some vent pipes outside venting this.   This can force air in the drains back up through the traps if bad enough.   

If it's a persistent problem, "cheater valves"- AAVs can be used to solve it sometimes.  https://www.oatey.com/faqs-blog-videos-case-studies/blog/3-things-you-should-know-about-air-admittance-valves

[GreenSheep]

Thanks for the ideas. They have a heat pump instead of a furnace, so I don't think that's the issue. Maybe a dry drain in the bathroom could be causing it, but they do use the bathroom somewhat. They wash their dog in there... not sure how often, but probably at least once a month, and they just did it before I arrived (and promptly took him out to play in the mud like any good dog sitter would...). I've been showering in there and using the sink for a few days now, so I'd like to think that would have taken care of it! I'll keep an eye (a nose?) on it and see if it starts to improve over the next few days after the drains get more use.

[reeshau]

Was any of this basement recently remodeled?  There could be a pinhole leak somewhere downstream of the u-bends.

[GreenSheep]

Was any of this basement recently remodeled?  There could be a pinhole leak somewhere downstream of the u-bends.

Not recently, as in the last couple of years, but before that, it's certainly possible. It's a 40 year old house that was a rental for most of its life, so I'm sure a lot was done to it. If it's like most houses in the neighborhood (I grew up here, a few doors down, and have been in most of the houses thanks to childhood friends, babysitting, and dog sitting), the now-finished basement was probably unfinished when the house was first built.

[GreenSheep]

Update for anyone who cares: Either I got used to the smell and didn't notice it as much, or it really did improve over the time I was there. I would guess maybe the latter, since I was using the sink, toilet, and shower much more often than they are typically used. So... for anyone who has a rarely-used basement guest area, I guess the lesson is to go run some water occasionally so you won't offend your guests' noses!