Intermittent septic smell in house

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We purchased a 35 YO house a year and a half ago. About 6-8 months ago there started to be a really bad septic odor that seemed to be coming from the upstairs guest bath. We have a septic system that was inspected thoroughly by a consultant when the house was purchased and tank was emptied at that time. The house is occupied by two people and other than doing a lot of laundry (with a speed queen HE stack unit purchased new when we bought the house) use is pretty normal.

Although initially the odor seemed to come from the guest bathroom, lately it has also seemed to come from a second upstairs bathroom and once was smelled in the basement. When the odor is present, it flows downstairs via a vaulted ceiling (or HVAC vents) and is in the entire downstairs.

Most recently, there was zero septic odor for a week and a half, then this morning it was back with a vengeance. The washing machine was run early (5:30 am), the temperature had dropped a little since yesterday. There was no rain (which often accompanies the odor). Then the house filled with a septic smell.

Initially we attributed the odor to atmospheric inversion that was causing slight odor and is not unheard of in houses with septic but now it permeates the whole house for hours and sometimes a day or two. It often happens where there is rain- nearly always, actually.

The plumber who was supposed to try to track it has failed to follow up. The septic consultant came back, gave us a list of things to consider (dry trap, cracked toilet seals, etc.). We religiously run water in any drains not used for a week, we resealed one toilet that rocked slightly. The septic consultant looked into the system and had me flush toilets, I assume to check for blockage.

I have recently spoken with someone who does smoke tests. He has only done 3 or 4 residential smoke tests in 15 years because unlike his commercial tests, in a finished house you need to cut into so many walls. Even the basement ceilings are dry wall so it will be extremely costly to have a smoke test.

I have read about peppermint tests but because roof stacks are on multiple levels, we are unable to do that ourselves. The septic consultants suspects the vent stacks but the person who does smoke tests says it’s only 1 in a million that it is a blocked roof stack.

Maybe forum members can suggest something simple we have overlooked. The washer installed for close to a year before the issue started – that would have been a simple answer if somehow a super HE washer was causing it but the timing seems wrong.

If members recommend smoke testing, are there any precautions to take or things to know to make it more likely to be successful while minimizing wall removal? Thanks in advance for any ideas.


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Peppermint test, like a smoke test, purpose is not to check for a blocked vent but to check for a leak.

If doing a peppermint test, it is important that whoever handles the peppermint oil does not come in for the sniffing.

I am not a pro. I think you could maybe pay a roofer to drop the peppermint oil, and solicit several sniffers. Include young noses. A roofer knows how to access roofs safely.

The smoke test does not necessarily lead to opening walls. If the smoke were to come from under a toilet or under a sink or from the basement, maybe no walls would need to be opened.

Can you correlate the smell to barometric pressure changes? is an example of a 3 day history for a place that may or may not be near you.
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Test, Don't Guess!
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We start by inspecting all of the fixture traps, including floor drains, and condensation drains. Floor drains ate commonly forgotten, and their traps do dry out, allowing the sewer gas to come back up the pipe.

Beyond that, we would be pressure testing. Often it is a fitting that didnt get cemented properly or sometimes a nail is found through a pipe. Drywall is taken down as needed, but that is the least costly of the trade work.
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