Plumbing trap schematic (C) Carson Dunlop Associates Plumbing Fixture & Trap Defects
A source of sewer gas smells and septic tank odors

  • TRAPS on PLUMBING FIXTURES - CONTENTS: How to inspect plumbing fixtures, loose toilets, & plumbing traps in order to track down building odors and sewer or septic gas smells, Definition of plumbing trap, definition of plumbing drain interceptor.
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Plumbing drain traps:

This plumbing traps (interceptors) article describes plumbing traps (interceptors) and how to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well.

We describe the common problems that occur at plumbing traps: odors, leaks, noises, and we discuss plumbing trap types, requirements, locations, connections, installation, repair and replacement. This article distinguishes between P-traps and older S-traps and explains trap siphonage and the dangers that can result from dry plumbing traps.

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Problems With Plumbing Fixtures or Fixture Traps (Interceptors) & Sewer Gas Leaks & Odors

Open indoor trap (C) Daniel FriedmanImproper plumbing drain traps can release sewer gas odors indoors.

[Click to enlarge any image]

Our photo (left) shows a plumbing trap that was abanaoned in a building basement. The chopped-off, open drain line combined with eventual dry-out of the water in the plumbing trap shown will provide a ready path for sewer gases to enter the structure - a sanitary hazard and even an explosion hazard.

This article series describes both plumbing drain traps and how properly-installed traps or interceptors should prevent or cure sewer gas odors from septic systems, building plumbing, & similar sources.

Checklists in this article series will help with sewer gas smell diagnosis or septic gas smell diagnosis. Other causes of sewage odors, septic odors, sewer gas, rotten egg, or other indoor gas odors are also described.

Article Contents

What's the difference between Plumbing Traps vs Plumbing Interceptors?

Note: recent changes in plumbing codes including ASME Standards, the Uniform Plumbing Code (UPC), Canadian Standards Association (CSA) and the International Plumbing Code (IPC) have dropped the term plumbing trap, and substitute a more inclusive word, "interceptor".

So what's being "intercepted"? In this usage, sewer gases are "intercepted" or prevented from entering the building from the drain system.

In some building codes and plumbing texts you will see increased use of the term interceptor where you may be more familiar with the term plumbing trap. In particular, grease traps are referred to now as hydromechanical grease interceptors or gravity grease interceptors.

Sewer Gas or Methane Gas Hazards & Plumbing Traps

Smelly and potentiallyi explosive sewer gases are lighter than air and will rise up through plumbing drain or sewer systems unless these gases are blocked from entering the building. The two principal features that prevent sewer gases from entering a building through its plumbing drain system are water-filled plumbing traps at sinks and tubs or other plumbing fixtures and the building plumbing drain vent system that vents sewer gases above the building roof.

What makes the smell in sewer gas? Sewer gases are more than an obnoxious odor. The page top schematic of a typical plumbing trap is courtesy of Carson Dunlop'.

Watch out: Because sewer gas contains methane gas (CH4) there is a risk of an explosion hazard or even fatal asphyxiation.

Sewer gases also probably contain hydrogen sulfide gas (H2S) In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).

Depending on the sewer gas source and other factors such as humidity and building and weather conditions, mold spores may also be present in sewer gases.

Technical note on sewer gas smells: because the "sulphur smell" that some people may associate with dangerous sewer gases can have other sources having nothing to do with building plumbing systems, readers should also see



ODORS, SEPTIC or SEWER and finally,

ODORS GASES SMELLS, DIAGNOSIS & CURE where we describe how to track odors to their source or cause.

Common Plumbing Trap or Interceptor Defects: home made or "expedient" plumbing traps

Here are some plumbing trap (interceptor) defects to check. Any of these can produce drainage problems and in turn may leak or cause sewer gas or septic odors to be produced in or even outside of a building.

Leaky, Missing, or Improperly Mounted/Located Plumbing Traps / Interceptors: auto radiator hoses & paint cans as plumbing traps?

If you are tracking down building sewer gases or smells don't forget to inspect the building plumbing traps for leaks, defects or improper or missing traps.

Photograph of a car radiator hose used as a plumbing drain trap Paint can used for home made plumbing trap (C) Steve Smallman

The photo at above left shows a plumbing "trap" that is guaranteed to produce odors: it's not a plumbing trap at all, but rather this plumbing drain uses a car radiator hose. This was a great idea for an emergency "Sunday night" plumbing repair, but not something to keep in a home.

Note that there is no bend in the trap to hold a water seal - sewer gases will pass readily out of this sink drain into the building. This home made drain pipe also leaks, as you can see by the black mold on paper located below the sink.

At above right we see a rusty paint can enjoying a second career as a plumbing trap. Perhaps this midnight-repair was installed as a stop-gap measure to address a leaky trap. Like topsy this repair just grew and grew with blobs of sealant, lagging, plaster or whatever the repair person found at hand.

Our photo of the paint can plumbing trap was provided by Steve Smallman, a professional home inspector in Raleigh NC.

Plumbing S-Traps on Plumbing Fixtures & Drains

Sketch of illegal plumbing trap types (C) Carson Dunlop AssociatesUse of "S" Traps or other illegal and obsolete plumbing fixture traps where a "P" trap is required:

S-traps are often installed in older buildings where there is no venting provided for that plumbing fixture.

S-traps easily lose the water from the plumbing trap, especially if the S-trapped fixture is near a toilet or other large plumbing fixture.Cs sketch at left shows several types of illegal plumbing traps including the "S" trap.

When the larger fixture is draining, the sudden and large volume of water rushing down the drain creates a vacuum in the drain line that can siphon water out of the nearby plumbing traps.

When a plumbing trap has lost its water seal, sewer gases pass readily back into the building.

Look below the sinks for antiquated or un-vented drains - if you see an "S" trap rather than a modern "P" shaped plumbing drain trap, the fixture is almost certainly not properly vented.

Details about s-traps are now found at PLUMBING S-TRAP CODES & HAZARDS

Garbage Disposers (food grinders) and Plumbing Traps / Interceptors

Don't forget to check for smells at your garbage disposer drain too.

And in buildings where GREASE TRAPS or grease interceptor is installed, the garbage disposer (food grinder) drainage is required to bypass the grease trap. This requirement is notwithstanding that garbage disposers (and the pre-rinse function of some dishwashers) is a substantial source of fats, oils, grease (FOG) that clog plumbing drain systems.

If that is the odor source you may be able to remove the odor by cleaning liquids or even simple vinegar.

Causes & Cures for Dry Plumbing Traps / Interceptors & Dry Plumbing Trap Odors

Trap priming schematic for dry plumbing traps (C) Carson Dunlop AssociatesDefinition of Dry Plumbing Traps: a dry trap is a plumbing trap or interceptor that has lost its water seal, thus permitting sewer gases to escape back up building drain piping and out into the building through the fixture - an unsafe, unsanitary, and potentially dangerous condition.

When a plumbing trap remains un-used for a long time the water can simply evaporate from the trap, permitting sewer gases to back up into the building.

Watch out: Because sewer gas contains methane gas (CH4) there is a risk of an explosion hazard or even fatal asphyxiation.
and also

Some places where we often find dry plumbing traps include:

Remedies for Dry Plumbing Traps

There are various ways to deal with dry plumbing traps to stop smelly and dangerous sewer gas backups into the building. These include

Self-Sealing Traps for Floor Drains

Self-sealing plumbing traps (interceptors) containing check valves are available and are suited for floor drains. If water on the floor needs to escape it can flow out of the trapped floor drain, but the trap contains a mechanical seal which prevents sewer gas backups even if the floor drain trap is dry.

Clogged Plumbing Traps / Interceptors

Schematic of plumbing trap showing cleanout plug (C) Carson Dunlop Associates

Clogged or blocked plumbing traps can cause leakage, and organic debris in plumbing traps may itself be a source of odors which people mistake for sewer gas backups.

If your drains are slow at only certain fixtures one of the first things to check is for clogging of the individual fixture traps.

As Carson Dunlop's sketch shows, some fixtures such as laundry sinks are required to have traps which include a cleanout plug, making cleaning of the trap easier.

Otherwise you'll have to remove the entire plumbing trap to clean it.

(Remember to put a bucket under a trap before trying to remove it, and remember not to try working on plumbing traps on Sunday night when you can't dash out to the building supply store to replace parts you've broken.)

Loose Toilets Can Produce Sewer Gas Odors Indoors

Photograph of a loose toiletCheck your toilets for leaks at the toilet base. The wax ring used to seal the toilet base to the waste pipe at the floor may be deteriorated or leaky, especially if the toilet is loose or was previously loose.

A wobbly toilet compresses the wax ring seal, leading to leaks and sewer gas odors in the bathroom.

In the photograph shown here, stains around the toilet base suggested that this toilet had been leaking at its base - a condition both unsanitary and smelly.

How to check for a loose toilet: Straddle the toilet and gently pinch it between your knees. Then gently push on each side of the toilet to see if it moves.

If the toilet moves it may be leaking into the floor (and ceiling below) - an unsanitary condition. The toilet needs to be removed, any damaged floor repaired, and then the toilet is reinstalled [photo] using a new wax toilet sealing ring [image] before bolting it securely to the floor.


Continue reading at PLUMBING S-TRAP CODES & HAZARDS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see TRAPS, PLUMBING FIXTURE FAQs - questions, answers, comments posted originally at this article

Or see PLUMBING DRAIN NOISES where we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises



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