TIF 8800 combustible gas detector checking for sewer gases at a sink (C) Daniel Friedman Building Drains as the Source of Sewage or Septic Odors
How to trace odors to fixture drains or drain piping

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How to troubleshoot sewer or septic odors due to building drain problems: this article explains sewer gas odor troubleshooting for odor sources due to blocked or slow plumbing drains.

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Blocked or slow building drains as a sewage gas odor source

Photograph of sewer line leak in a basement/crawl area during drain clog diagnosis

Another indoor sewer gas odor explanation is that there may be a partial blockage and possibly even a leak (photo at left) in a waste line such in a crawl space, below a basement floor, or outdoors between house and septic tank or less likely between septic and fields.

The "glub glub" sound discussed above can also be caused by a partially-blocked drain which is simply slow.

When a building drain is slow due to partial blockage (or bad venting), sewer gases, smells, odors may travel backwards up into the building through the plumbing drain openings.

If the building drain waste vent system has inadequate venting a result may be the loss of water from the building fixture traps. In turn this loss of water from traps permits sewer gases to rise up drain piping and out into occupied space through the building sinks, showers, tubs, and in extreme cases, even toilets.

Don't forget that a loose toilet may itself be an odor source - with sewer gases leaking out around the toilet base. If sewer gas smells are peculiar to a single bathroom we might check for toilet leaks. Even if a toilet seems bolted securely to the floor, its wax ring seal may be damaged, permitting sewer gases and odors into the building. At Fixture versus system blockage (just below) we provide steps to diagnose plumbing fixtures as the prime odor sources.

Drain noises due to bad venting or due to a partial blockage will be invariant day or night, depending on drain usage, but in the more quiet still and cool night air, drain noises are more easily heard.

As we said earlier, partially blocked building drain, septic tank, or drainfield can generate odors indoors or outside.

Odors in this case will usually correspond to surges in system use, and odors should not correspond particularly to cold weather, except insofar as cold still air may permit gases to fall (from a plumbing vent stack, for example) , or different seasonal wind patterns may cause gases to move to areas where they're not noticed in warmer weather.

See Plumbing Vent Defects for procedures to diagnose problems with the building drain vent system.

See Using the TIF 8800 Gas Detector for tips on using a gas detection instrument to scan for sewer gas sources in a building.

Also see DRAIN PIPING & SEWER ODORS for details about odors coming from plumbing drains.

Also 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. Also see ODORS GASES SMELLS, DIAGNOSIS & CURE for procedures to diagnose and cure all kinds of odors in and around buildings.

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