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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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.
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.
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.
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.
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.
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.
Use 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.
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.
Definition 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.
See ODORS, DRAIN & SEWER LINE SOURCES and also DRAIN PIPING & SEWER ODORS
see METHANE GAS SOURCES
Some places where we often find dry plumbing traps include:
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 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 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.)
Check 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 using a new wax toilet sealing ring before bolting it securely to the floor.
Continue reading at VENT PIPING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
At PLUMBING DRAIN NOISES we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises
Or see SEWER GAS ODORS
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Can a dried out floor drain trap cause a sewer line backup? - Daryl 12/27/12
Can a dry trap cause a sewer line backup?
No Daryl, a dried out floor drain trap doesn't cause a sewer line backup, at least not directly;
A dried-floor trap is simply an empty U-shaped pipe in the drain system. For a sewer line to back up we expect to find either that the drain system has become blocked downstream from the point at which the backup manifests itself, or the entire sewer system is flooded, such as during a storm, hurricane, or area flooding.
But in a more subtle way a dried floor drain trap *might* contribute to a downstream drain blockage if it served as an entry point for animals who nested in the dry drain, or if there is no screen over the floor drain and some fool sweeps dirt and debris into the drain system. Both of those are indirect contributions to a sewer line backup.
(Mar 10, 2013) Mike said:
If there is no visible trap beneath the bathroom sink, is it possible that the trap may be hidden behind the wall?
It's possible to hide a plumbing trap in the wall, but bad practice since that makes it inaccessible for inspection, cleaning, or replacement when it leaks. If your drain smells that may indicate that there's no trap. But to be fair, there are other non-visible plumbing traps commonly found in some buildings such as the trap below a shower or bath tub that is really hidden in the ceiling below.
If the drain line drops straight down and then makes a 90 degree turn into the wall with no trap it's an improper installation.
If there are other clues suggesting non-standard plumbing in your home there may be other problems that affect both function and safety - I'd ask an expert plumber to take a look.
(May 8, 2014) Anonymous said:
My older homes bathtub become clogged, so I removed the drain lever to see if it was malfunctioning and it was old however working properly however tub drain is now completely blocked. I also noticed that the toilet is now becoming clogged and wont drain properly. I tryed to look down tub piping to see if stopper was stuck however stopper came out when I removed lever. Any suggestions on what could be the problem and what if anything I can repair myself. Thank you
Anon, if you're sure you were able to fully remove the drain stopper assembly then there is probably a debris blockage od the drain or it's trap. Try a gentle use of a small diameter plumbing snake.
(Feb 7, 2015) Hulya said:
Help me please. I moved in to a new build flat in 2012. Within 2 months, we began getting sewage smells from the kitchen and bathroom. We also would get food smells from bathroom. Landlord told me to use a plunger. We also had very bad and strong sewage smells outside the building. The smell worsened in the bathroom, neighbours water gurgling noises from somewhere inside our bath, neighbours bleach smell coming through the bathroom increased to several days every week. Reported these to landlord who repeatedly ignored my concerns as the smell began making my son ill. Smells strengthened, lasted longer especially schools holidays being every day. 2 years 2 months later, July 2014,landlord acknowledged the smells and blamed water company. water company found several manholes owned by landlord blocked and unscrewed. The manholes were finally drained less than 2 months ago.
But the cause of blockage could not be found, landlord stated they would need to obtain the drawing of the building from council. Landlord promised to have it rectified after Christmas holidays. The smells have been every day for months now.I am on the first floor.No flat beneath me. Most neighbours have similar problems too but not as strong or long as mine. Ended up calling a plumber myself yesterday. He took off the bath panel and straight away said the fitting was wrong, the u bend thingy. Sorry, I am female so can't remember the terms he used.
Basically the while funny shaped thing which the water goes down, was for showers and not baths. He explained it all but it was too much to take in and understand. He said landlord would need to make a hole so a longer one could be fitted similar to the one in my kitchen. he also found a long wire with a small plactic box attached, which he claimed was electric. I took photos. he said he would write out a report so I could hand it to my landlord. please could you offer me any advice because we have to constantly leave home when the smells are very bad as it stinks out the whole flat, as it is quite small.
If you have notified a landlord of an unsanitary, and potentially unsafe (risking methane gas explosion) condition in your home both orally and in writing, and if nothing is happening, you may have little choice but to ask for help from your local health officials or building department. Do keep us informed of what happens.
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