Plumbing trap dry or defect causes sewer gas odors in the building (C) Daniel FriedmanTrack Down Hard-to-Find Sewer Gas Odors in Buildings
Odd or Unexpected Sources of Sewer, Septic or Sulphur Odors

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Tracking down septic or sewer gas smells:

How to find mysterious sewer or septic smells when the source is not obvious. This article suggests things to check when you have had trouble finding the source of a sewer gas, septic gas, or methane smell in or near a building.

Sometimes the odor source is elusive because it comes and goes, is weather dependent, fixture use dependent, or because the leak is in a building wall or ceiling cavity.

Start with the inexpensive and easy things like checking for dry or defective fixture traps (or fake traps as shown in our page top photo).

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Tips for Tracking Down Hard-to-Find Sewer Gas Odors in buildings

Septic odors from vent pipe (C) Daniel FriedmanThis page lists additional sources of sewer or septic smells in or around buildings.

Also be sure to review SEPTIC / SEWER ODOR SOURCE TABLE.

Other Sources of Sewer Gas or Methane Gas Odors in buildings

Others with whom we have worked on tracking down sewer gas odor problems have taken these steps to successfully track down and fix the source of cold weather sewage odors in or around buildings:

Question: sewer odor from upstairs laundry room; pouring water down the drain fixed it. Odors returned in cold weather.

Hi we have a newer home (only 3 years old) and have experienced a very strong sewer odor coming from our upstairs laundry room. Previously we have just poured a pitcher of water down the washing machine drain pipe and it has cleared up the smell.

Just two weeks ago we have rented the home and the renters are complaining of the foul smell but say it originates from the upstairs bathroom that shares a wall with the laundry room. We are approximately three hours away from the home but plan on checking all of the vents on the roof to see if any of the vents are clogged.

We will be moving out of the country in few weeks and would like to take care of this before we leave as we will have no control over the situation once we leave and can not afford what could possible be several costly handyman/plumbing fees trying to diagnose the problem.

I have read through all of the articles you have posted including the cold weather/rain one as the house is in Washington state and it's winter and snowing right now but any advise as to our specific situation would be greatly appreciated. Thank you for your help, Kristi - Kristan 1/16/11

oh and I forgot to mention below that we are on a gravity septic system.


If a sewer gas odor seems specific to just one area in the home I suspect a clogged drain, inadequate vent piping, or a loose toilet or leaky vent line or leaky drain line;

If the problem is system wide - that is all fixtures have drain troubles, then I suspect the main drain waste vent system or the septic system

Don't forget to check CHINESE DRYWALL HAZARDS & ODORS against the age of your house.

Question: we can't find the source of a sewer gas problem in the basement - worse in cold weather

We spoke last year about the sewer gas problem in my basement. We've lived in the house for 4 years, and this is the 4th year in a row that it has returned (typically returning when it gets cold, but we have had it occasionally in the summer).

We've had multiple plumbers, drain specialists, septic specialists, home inspectors, etc in to try to diagnose the problem and figure out where the smell is coming from, with no luck.

Would you be able to do a service call to my home? We are desperate to get rid of the smell - with a baby in the house now, we can't live here with the gas in the house. - J.E.


I'm sorry to read you're still being plagued with cold weather sewer or septic odors. I'm not available for field investigation work. Did you take a look at the cold weather sewer gas odor article above on this page.

Reader Follow-up on tracking down sewer gas odor

[I'm looking for an expert inspector - ] I just can't think of spending more time/money on people who really don't have a lot of experience in these issues - they just check the "typical" sources of sewer gas, which we've already ruled out time and time again.

I do have one question perhaps you can answer - we have a sprinkler system. My husband went to open the valve up outside to drain the water out prior to the sprinkler guys coming to winterize.

He said that, when the water came out, it had a sewer gas smell. Have you ever heard of an instance where the sprinkler system pipes somehow are emitting sewer gas?? I can't think of how it would be possible...not sure if there are P-Traps in there or not....but I thought I'd ask.

Also, we had a plumber try to find the source by "flooding the system" with water and looking to see if there were leaks (which we didn't find any). Do you think doing a smoke test would find a problem that the "flooding the system" method wouldn't?

We had another plumber out to the house this week, and he said that he didn't have any ideas on what else to do. It is so frustrating that no one can figure this out and that my family is living in a hazardous environment.


You might be able to get some onsite advice from one of the more senior and experienced inspectors at EXPERTS DIRECTORY. Be sure to discuss your particular concerns with the inspector so that s/he can advise you if their skills and your needs match.

But I would also be VERY sure that the odor you are suffering is from the sewer system and not something traced to unsafe heating equipment or chimney.

Watch out: as a basic safety caution, particularly where there are gas odor complaints, be sure you have working CO and smoke detectors

Follow-up from reader:

We are sure it is sewer gas - no question about that according to the experts who have been out here. We have a Carbon Monoxide detector and smoke detectors.

Reply: smoke testing vs. pressure testing to find plumbing vent leaks

You should be able to have a plumber pressure test the plumbing vent lines - a smoke test won't do much if the leak is hidden inside a wall

Follow-up from reader:

We already had that done a year ago.  It found nothing.  I thought that it would be easier to see smoke (even if you have to cut a couple of holes in the wall) vs. finding water leaking behind a wall.


If the pressure test didn't find a leak, and was properly done, that's important info. I'm doubtful that smoke will show up inside if the earlier test was proper though outside or at fixtures you might get a surprise. Sounds like a reasonable next thing to try.

Reader Question: on cool summer nights we smell septic gas, always at the same place, how do we fix this odor?

Septic odors from vent pipe (C) Daniel Friedman

Have septic system in mountain area of NC. Only on COOL summer nights do we smell septic gas (always in same place) at rear of house. Also noted that either dishwasher or bathtub or washing machine is emptying into system when smell is detected.

The roof vent pipe is located on backside of house roof where we detect the smell below. Have concluded that smell is associated with vent gas being displaced by emptying water from an appliance and since outside air is COOL the gas "falls" to ground level where it is detected.

Do you have any thoughts on what might be causing problem? If agree with my conclusion there are vent mounted activated carbon filters that mount on top of vent pipe that are claimed to solve the problem, any comments will be appreciated. - S.T.

[Our photo at left shows a common source of septic gas odors at an older home. This add-on plumbing vent may be a bit too close to that bedroom window, and also in cool weather its exhaust may be noticed by people on the ground below or on a nearby deck.]

Reply: Cause and Cure Suggestions for Septic Tank Smells in cool evening hours

Sewer line vent (C) Daniel FriedmanA competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem in the septic tank, drainfield, or plumbing venting system. That said, here are some things to consider:

A basic tenet of tracking down odors is just what you have done - relate the smell to other conditions like time of day, weather, etc. Without knowing details about your home I am just speculating:

If you have a slow plumbing drain, a sluggish drainfield that is failing, or a plumbing vent defect, pushing water into a drain at high rate can in turn result in an increase in the level of sewer gases (or septic tank gases) that are back-venting up the drain/waste/vent piping.

Those gases should vent through the roof and should not be noticeable to building occupants.

But in the cool of evening not only do temperatures fall, but typically wind velocity falls as well.

Under those conditions even a modest smell problem can become more noticeable because the gases are not being dissipated by air movement, and may even be falling towards ground level as escaping plumbing gases are cooled in the night air and, counter intuitively, might even ride descending air currents that during warmer daylight hours, would instead be rising air currents.

Your suggestion of adding a filter to try to reduce odors is a band-aid approach that has some risks: you might be better off to track down the odor source to see what it means. For example, improper plumbing venting can be unsanitary or even dangerous; and obtaining an early warning that the septic drainfield is failing may be useful in planning for repair or replacement of the system.

Tracking Down Sewer Odors Outside

Because you notice the odor at a particular location - at the rear of the house, and at a particular time - in evenings, that's a good time and place to start tracking the odor to its source.

If by "rear of the house" you mean outdoors, sniff towards the septic tank, drainfield, or any sewer line vents (see our photo at above left) or sewer line cleanouts that might be installed between house and septic tank.

Tracking Down Sewer Odors Inside

Open indoor trap (C) Daniel Friedman

If you meant that the odors are noted inside the house, see if you can track the smell to a particular room or plumbing fixture or drain. Sometimes we find that sewer odors are traced to a leaky plumbing drain, loose toilet, or even a poorly connected dry plumbing vent running through the ceiling.

In both old and new construction we also occasionally come across an orphaned drain or waste vent line that was just chopped off and forgotten (photo at left).

A plumber can pressure-test your drain-waste-vent piping if that step becomes needed in tracking down the odor.

Also remember to have an expert take a look into the septic tank to see if it's sewage level is normal, if the baffles are intact, etc.

See SEWER GAS ODORS for our checklist of sewer gas or septic smell odor track-down details.

Reader Question: septic issues in Connecticut during cold weather

Do you know of someone in the southern Connecticut area that has extensive knowledge about troubleshooting septic issues?

I have had 3 different septic companies and 3 plumbers to my house and no one seems to be able to troubleshoot the problem.

Cold weather sewer odor symptoms:

For example, when the shower is used for more than about 3 minutes.

When the toilet is flushed several times and the sink is used to wash hands within a short period of time.

This ONLY happens when it is cold outside (below 40 degrees or so).

There is no smell when the temperature outside is above 40 degrees or so.

One of the septic companies removed the toilet and ran a camera and checked the structural integrity of the line.

He also dug to inspect the dry well and said that there were no issues that he could see.

He then had the dry well cleaned out. Since I didn't even know that this dry well existed, it had been 8+ years at least since the dry well had last been cleaned out. The clean out was performed in August 2013 and the gas odor returned in October 2013.

Since all the basement utilities are used daily, lack of water in the traps has been ruled out as an issue.

I have not been able to identify if there is a vent for the basement.

The rest of the house is on a separate septic tank and there are no issues with that. - C.E.T. 12/10/2013


At you might find an experienced home inspector who has septic system expertise too, or who may be able to make a competent suggestion - offer to pay them for a brief consult if necessary. I presume you've checked out the cold weather septic odor article beginning above


It's too bad but true that often to get competent help we have to learn something about the problem ourselves.

I'd be sure to rule out the obvious (loose toilet, slow drains, frost-clogged plumbing vent) before doing anything expensive.

If there are un-vented plumbing fixtures in the basement that's certainly likely to be a year-round problem not just a winter problem;

Ask yourself what's different in winter - not just that it's cold outside, but that the heating system is on, windows are shut, in sum: the air flow patterns in the building change, and there may even be backdrafting due to closed house plus operation of heating equipment (which would be unsafe).

And re "Since all the basement utilities are used daily, lack of water in the traps has been ruled out as an issue."

If there is a basement floor drain that connects to the sewer or other drain line, it can indeed run dry any time, or may not even be properly trapped. Try to narrow down the odor source by odor strength, time of day, equipment on or off, doors open or shut, etc. 

Reader follow-up:

Sewer gas odors at washing machine drain hookup (C) InspectApediaI was able to find the source of the smell by running the shower for a long period of time and then seeing where the odor was coming from first.

The odor is coming from the hole where the white tube that is running from the washing machine goes into. I imagine that the hole, which I am sure leads to the dry well, is not trapped or not trapped properly. I attached a picture.

[Click to enlarge any image]


Indeed it is not uncommon for an installer to fail to properly trap and vent a washing machine drain, and drywells, while not usually as smelly as septic tanks, can send odors back into a building.

Pending opening the wall to inspect the piping and install a proper trap on the washer drain you could stop odors from entering at the washer drain hose connection by makeshift-sealing the connection between the flexible washer drain and the pre-fab drain connection shown in your photo.

Reader followup:

Thanks, I was wondering if there was a way to seal it instead of opening the wall.  I will call a plumber and see what he can do to make the seal. 


First try a simple DIY project - you can use a bit of insulation and duct tape - just be sure you don't drop insulation or crud into the drain as you may block it if you do.

As a seal in this location is non-standard, your plumber won't be any smarter than you are. And I would NOT use caulk or something that would glue the washer drain hose to the connection or it'll be hell to remove it later.

Reader follow-up:

Ok, thanks for the tip.  I will give it a try.

and see PLUMBING FIXTURE TRAPS for fixture trap inspection and defect diagnosis.


See our broad-scope article on diagnosis and cure of sewer gas and septic odors:

SEWER GAS ODORS diagnosing, finding, and curing septic tank and sewer line smells.

Reader Comment: traced sewer gas odors to HVAC condensate drain connection to vent stack

22 May 2015 T. W. Straub said:

We just found the source of our septic smell in the East Wing of our home. It appears that the HVAC contractors who installed the system on this wing plumbed the drip line from the air handler into the vent stack without using a P trap. That might have eliminated the problem, but I doubt it would be code compliant.

The way we found the problem was by using a smoke bomb in a shop vac and blowing the smoke down the vent stack from the roof. While doing that, we looked for smoke in the walls where the stacks were and then looked in the attic.

When we opened the access to the attic, we could smell the smoke (Superior #2B - 8,000 cu. ft. smoke bomb) and could see the smoke billowing out of the seams of the air handler. I hope this helps someone else out there --- Tom

Thank you for this important comment, T.W. It's helpful to have a "real world" report confirming what experts have been telling us for a long time and that some installers or building owners simply don't accept.

The convenient shortcut of connecting the HVAC condensate drain to the plumbing stack is overwhelming for some people even though it's a bad idea. I'll add your comments to the article above and also at our discussion of condensate drain handling.


Continue reading at PLUMBING VENT BLOCKAGE ODORS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SEWAGE ODOR SOURCE LOCATION FAQs - questions & answers posted originally on this page.

Or see these

Sewer & Septic Gas Odors Articles

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