Sewer smell in basement

I’m attempting to help a client of mine locate the source of a strong sewer smell in his basement. He has already paid a plumber to re-pitch his crawlspace plumbing, I scoped the vent for the house, all the traps have water in them, all the toilets are secured to the floor and he had flex-sealed his ejector pit lid to the floor because he thought that may have been the culprit

Looking for any ideas to try to locate this problem, the smell is exponentially worse when it gets cold out.


Why not perform a peppermint test on the sewer system to confirm it is sewer gas and not another source. A smoke test on the sewer system will reveal the source of the leak as well. I recommend you contact a plumber to perform the testing.

There is no way you would have been able to video the entire venting system in the home unless it’s a single bathroom home with kitchen off of the bathroom.

Hydrogen sulfide is heavier than air, so the smell may be coming from overhead. Heating gas has mercaptan “ rotten egg smell” added for leak detection, could it be a gas fired appliance in the basement?

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Check p traps in little used showers for water

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I didn’t scope the entire vent system, just the main stack, we did find a blockage which was cleared by a plumber once we found it, as for showers sinks and tubs all fixtures have had water run through them to ensure traps are not dry. A plumber has already done a smoke test with no conclusive results. I’m not familiar with a peppermint test, will have to look into that to see if its an option in this case.

You may want to consider it is not sewer gas.

Where are you located?

Which plumbing drains does the sewer ejector serve? did the client try not to use these drains for a few days to see if the smell goes away? What prompted to have the pit flex-sealed? it should be sealed with a gasket, not a flex-seal. The pit would be the first place to check for proper function. If you seal it properly, for example, and its vent is clogged or isn’t installed correctly… you will pull the ptraps dry that dump into the ejector pit. You need a plumber with a brain come out and figure it out. Get ready to spend some $$$

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Chicago suburbs, the ejector only serves the basement toilet and sink. He flex sealed the pit as a last ditch effort after he had paid a plumber 4k to re pitch the sewer lines in the crawl space which did not fix the issue. It was after this that he reached out to me to try to find the source.

There is potential that its not sewer gas, I have not smelled it as it doesn’t stink all the time, but he is very confident as it “smells like a bad fart”

Is there gas in the house?

How often is the basement toilet/sink used? can he not use them for a week after priming their ptraps without triggering the ejector pump?

Is the smell throughout the basement or only in parts? is it worse in some area of the basement?

There is basically no info provided other than the basement stinks and the plumber got paid $4k without solving the issue.

The fact that I have to ask all these things tells me you should get someone else involved that can check all these things.

Rock on brother!

The info provided was as follows

Basement smells like sewer
All fixtures were run to verify water in their traps
There was a clog in the vent line that has since been cleared
The ejector pit is sealed
A smoke test has been performed with no positive results by a plumber
The plumbing in the crawl has been re pitched in the crawl space
The smell is not there all the time

If thats considered no info then I don’t really know what to tell you…

As for your follow up questions I can ask him to run the fixtures until the pump comes on then attempt to not use them and he has told me the whole basement stinks, given that I have not smelled it I couldn’t tell you if its worse in one area or another. As far as I am aware there isn’t a noticeable gas odor in the home or around any of the gas appliances.

I’m just trying to gather information and pick peoples brains who may have more knowledge than me in this area. I apologize but I was under the impression that that was the primary purpose of a forum

what Simon said

It is it’s just this is a puzzle best solved with boots on the ground.


An industrial hygienist who used to post here a lot said that many times odors he investigated coming from the basement/crawlspace were actually from soil-borne bacteria. If this is the case and the odor is from bacteria-source gas rising out of the soil, cold air is denser and that may increase the gas concentration/odor.
He seemed to know what he was talking about. He had an impossible to pronounce Gaelic first name (kwee-veen) O’Connell.

Is the crawlspace vented or sealed? Along with Kenton’s line of thinking, maybe there is/are dead animals buried down there.

Jim McKee has not had any success with the “lick test”, I hear, however.

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Do you have a combustible gas detection meter? They are not that expensive, and a good tool to have in your bag.

Use that to try to find the source(s) of the odor. Dont rule out that you may have more than one source.

Correct me if wrong, but I am assuming the sewer pipe repairs involved cutting the basement concrete floor open to access and repair piping? If a exosed soil floor, that is important to know.

Odor could be due to Soil gas from decaying organics under the basement slab (I assume you have an enclosed basement). Check the slab to wall corner joints and any other cracks or joints in the slab with the gas leak meter for elevated readings. If sewage leaked during the recent sewer repair work, it may be just under the slab and the odor may be seeping through the open joints in the floor.

Make sure the sewer repair work area was thoroughly cleaned after repair was completed. Workers make a big mess, and may not have cleaned up all sewage spills well.

Is the basement ceiling finished? If not, use the meter to sniff all the drain piping. Your looking for cracked sewer pipes, whether they serve as drains or vents. Cracks or open joints may be at the top of the pie, so you won’t see water leaks , but gas can get out.

Did you plumber get permits and inspection by building department? Did they make him test the piping after repair? Check for test plugs left in roof top vent pipes… sometimes they forget to remove them.

Is there an exhaust fan in the bathroom? Should be, or a window that opens. Make sure the back draft flap is sealing properly. Check the fan for odors with the meter.

It also may simply be that your client has a highly sensitive sense of smell and is detecting an odor which has nothing to do with your sewer system. He might want to try having the entire basement professionally cleaned, deodorize, disinfected, and then Place some damp rid type devices down there control humidity levels. Elevated relative humidity has a musty odor to it that people commonly mistake for sewer gas smells. Place some relative humidity detectors in the basement to make sure the humidity is below 60%.

If the relative humidity is above 60% then mold can grow on surfaces, and mold in pain to have bad smells as well. Cleaning and disinfecting regularly and adding a dehumidifier generally controls those issues.

There could be several other possible explanations those are the most likely.

When investigators holders that are intermittent, I always encourage my clients to call me Community is bleed when they smell it oh, and if I can I will run over and check it out while his the smell is present. It’s very difficult to troubleshoot this type of problem when you can’t even smell it.

Good luck!

One of my friends works at a municipal water treatment plant. On occasion they have algae blooms in their supply reservoirs. They treat the water so there is no issue to the public, but when this happens there is a certain (low) percentage of the population that has a super sensitive sense of smell, they think there is something wrong with the water. The municipal has done extensive testing of the water from samples at these people’s houses to figure this out. The source of the odor may not be the water supply as in my example. But if you can’t smell anything when you are there, the client could just be sensitive to certain odors.

I’ve seen bathroom sinks collect scum in the overflow that can create a foul smell. It’s amazing how much crud can build up and how bad it can smell.

Dead mouse, rat, cat, etc in a wall, etc

Make sure you are not missing a trap in a floor drain that is covered by “something”. The only other thought I would have is if they did a remodel back in the day, eliminated a sink or fixture and didn’t seal/terminate it correctly. Might be a vent or drain that was cut off inside a wall.

If all else fails have them buy a bunch of those plug in air fresheners and plug them into every other outlet…seems to work for those people that need to cover up pet smells when they sell a house…I mean they work so well and fool everyone.