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I recently started detecting a sewer smell coming from the laundry room in my basement. I think I only trigger it (and not every time) when I run my whole-house fan. I've been running this fan (with plenty of windows open) for at least a month without this problem, so it seems like something may have changed.

Here is a figure of the plumbing in that room. It contains a waste line for the laundry and a laundry sink attached to the cleanout off of the main stack. There is also a floor drain (with its own trap), but it's off in it's own corner and doesn't seem to have the sewer smell around it.

Diagram of laundry room plumbing

I've been doing some research, and seen one or two related questions here. The closest is Install air admittance valve to solve musty smell (sewer gas)?, but I'm not seeing quite the same symptoms. I don't notice any issues with the drain speed (though I'm not sure I could tell there are issues from the washing machine, the sink is fine), and I've only noticed the smell accumulating slowly when running the whole-house fan.

I originally thought that the lack of venting on this line could be the issue, with one of the traps being siphoned dry by the other, but I don't notice anything after running the laundry. I also tried adding water to the traps when I start smelling sewer gas, but it's not clear that it helps (the seeping gas might be coming in too slow to tell - the basement is not well ventilated, so it takes a while for the smell to clear).

One other concern I've come across is corrosion in the black iron drain/vent pipe (100 year old house) may have opened up a small hole that I can pull out sewer gas from with the negative pressure of the whole-house fan, but I'm not sure how I can test this.

Any thoughts on what I can try are appreciated.

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  • How often do you use the sink? If you don't use ot often, then ir may be that the water in the P-trap for the sink is drying out and allowing sewer gasses into the room.
    – statueuphemism
    Commented Oct 5, 2019 at 13:20
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    Actually, it is more likely the trap for the floor drain if the floor drain connects to the sewer. I missed it initially since it wasn't drawn in connection with the sewer lines.
    – statueuphemism
    Commented Oct 5, 2019 at 13:22
  • Possibly related: diy.stackexchange.com/questions/144592/…
    – statueuphemism
    Commented Oct 5, 2019 at 13:25
  • @statueuphemism For a while I've been dumping about 1/2 a gallon of water into it every few days (basement dehumidifier tank), so it at least shouldn't be drying out on its own.
    – blarg
    Commented Oct 5, 2019 at 13:37
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    I have used vegetable oil for years in floor drains it doesn’t dry out, the oil layer on top of the water keeps the water from drying out and the high humidity in the plumbing doesn’t dry the water in the trap, a high flow or dumping a large amount can wash the oil down the drain but it only takes about 1/2 cup even on a large drain.
    – Ed Beal
    Commented Oct 5, 2019 at 14:17

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Didn’t read entire article. Did you poor water down the floor drain. The water evaporates over time, I fill my monthly with a 1/2 gallon of water.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here.
    – Daniel Griscom
    Commented Oct 17, 2019 at 13:34
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tough situation. I had an old house and ran into a plumbing issue one with a bubbling toilet. After trying everything a normal person could I ended up getting my line scoped and found the actual drain pipe leading from the house to the sewer had been over run by tree roots. My blockage was on the outside of the home so it made it a bit easier to resolve, but what I remember about that situation that is relevant to your problem is that the plumber said these old pipes eventually break down and then the water/sewage will just pool in the area of the breakage, sometimes for years before people notice depending on how the soil is etc. It’ll cost you about 300$ to get a plumber in there with a proper scope but it might save you a lot of money in the long run and give you an idea of what’s going on.

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    Thanks for the suggestion. I just happened to have had my main line scoped (down the same cleanout in my diagram) around the same time I first encountered the sewer smell. Aside from some small roots, it was clear.
    – blarg
    Commented Oct 5, 2019 at 23:08
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    So that does confirm your line has been penetrated. It’s possible you’re getting some water seepage from your drains that is possibly pooling somewhere under your house. The fan probably draws the smell up from wherever that is happening. Can be through small cracks in the floor or walls. Have you tried sealing the room off (no open windows, no fans, no doors open) for a while then trying to pinpoint where the smell is coming from exactly. Once you’re able to do that you can explore further.
    – Steve
    Commented Oct 6, 2019 at 15:15
  • When you say around the same time the smell started, does that mean the smell started after you the the line televised? If so, is it possible the clean-out wasn't sealed properly?
    – PhilippNagel
    Commented Oct 9, 2019 at 18:40
  • @PhilippNagel The smell started before the cleanout was touched.
    – blarg
    Commented Oct 11, 2019 at 9:17
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If the basement is poorly ventilated and contains organic materials, it could be that you have some rot caused by moisture.

If there is such damage. Its not uncommon that smells start to appear once the basement gets properly ventilated.

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