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This article describes how to examine the building's plumbing vent system piping and connections in order to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or plumbing drain and fixture noises that may appear indoors or outside. Here we list common causes of plumbing problems related to improper plumbing vents.

We describe how to track down and correct these "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. What makes the smell in sewer gas? Sewer gases are more than an obnoxious odor. What causes that gurgling noise in a sink, tub or shower drain, and why do some fixtures "gurgle" when a toilet is flushed nearby? Here are the answers. Our page top sketch of a plumbing stack vent and other sketches included below are provided courtesy of Carson Dunlop.

At PLUMBING DRAIN VENTS we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises. We discuss how to diagnose and cure drain sounds (gurgling, bubbling, smells, poor drainage) at PLUMBING DRAIN NOISE DIAGNOSIS (gurgling drains means bad vent piping or clogged drains)

Other advice about controlling plumbing noises in buildings is at  SOUND CONTROL for PLUMBING. More details about plumbing drain line venting can be found at PLUMBING VENT DEFINITIONS & TYPES, also  PLUMBING VENT DEFECTS & NOISES and at CLOGGED DRAIN DIAGNOSIS & REPAIR. We discuss plumbing fixtures, such as loose toilets, as sources of sewer gas odors separately at PLUMBING FIXTURE TRAPS.

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Plumbing Vent Piping Defects May Cause Drain Noises or May Release Dangerous Sewer Gas Odors Indoors

Plumbing drain or sewer gas odors: If you smell sewer gases in your building conditions could be dangerous (risking a methane gas explosion) or unsanitary. Also see Remedies for Sewer Odors and see ODORS & SMELLS DIAGNOSIS & CURE for advice on finding the source of smells or odors in buildings.

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. Also see Wet Weather or Cold Weather Septic Odors or Sewage Odor Diagnosis & Repair Guide for additional odor tracing and cure advice for odors occurring during wet or cold weather.

Plumbing drain or fixture noises: Other advice about controlling plumbing noises in buildings is at  DRAIN NOISES and at SOUND CONTROL for PLUMBING.

• Missing, inadequate, or improperly installed drain-waste-vent (DWV) piping in the building: sometimes a building drain system has inadequate or missing plumbing vents. We list a variety of plumbing vent defects in this article. Our first photo (below left) below shows our client observing a very questionable plumbing vent on a new house. The vent is so far from vertical that we wondered if it had been connected to anything inside, and even if it was, we took this detail to suggest work by someone who lacked proper training - so we were alert for other plumbing defects in the building.

• Plumbing vents which have been accidentally closed off or blocked: some plumbing vents include a screen which can slide down, closing off the vent and causing sewer gas backups in the building. We've also found plumbing bents blocked by wasps nests, birds, and once even a frog. Our photos above show how we simply raised the screen back up to open an old sheet-metal vent.
• Leaking plumbing vent system piping: check that all of the pipe connections in the building's plumbing vent system have been properly made and sealed. Particularly with plastic piping it's possible for an installer to push vent pipe sections together while forgetting the glue.

• Our plumbing vent stack leak photographs above and just below show a hidden plumbing vent leak in a building wall cavity - exposed by drywall removal during an odor investigation, courtesy of reader Cheryl Lidawer, show what happens when plumbing vent pipe connections are not properly made. In this case the installer coated the pvc vent pipes with primer but not glue, then assembled them.
  
  After lots of detective work and difficulties tracking down sewer gas odors in this home, the owners found multiple sewer gas leaks including the one shown in the two photos above, inside of the building wall cavity. The defective vent piping and sewer gas leaks were discovered by pressure-testing the plumbing drain-waste-vent system.

Troubles and mistaken sewer gas odor-sources along the way to finding this plumbing vent sewer gas leak problem included

• a red-tagged gas-fired heating furnace (people thought the odor was due to the heating system - an apparent mistake since odors persisted when the heating system was not running
• suspecting odors traveling through heating air ducts - was sewer gas or LP gas leaking into the duct work?
• suspicions of a chimney problem - was there back-drafting into the gas fired heater?, and finally
• visual discovery of open plumbing vents in the building attic (photo above left - that gray pipe stub at center of the photo ends with a saw cut and you can see staining on nearby wall surfaces), and more
• confusion about odor and plumbing error responsibility among multiple contractors.
  
  Clues surrounding this problem included visual evidence of inexperienced or untrained workers such as the extensive use of mixed PVC/ABS plastic vent and drain piping in an older home. While modern glues can secure PVC to ABS plastic, in older homes, before such adhesives were available, a mix of black ABS and white PVC drain waste vent piping often meant leaks due to incompatible glues. Don't assume mixed plastic piping is properly secured.
  
  When you find evidence of un-trained workers having been part of the construction team, remember to be alert for other improper or even unsafe conditions such as this unsafe electrical wiring found in the same building attic as the chopped off plumbing vent pipe. .
  
  
• Because a plumbing vent is passing only sewer gas, not liquid waste that causes a drain not to function, no one notices that it's leaking into the building until there is an odor complaint, the pipe falls apart (which may never happen), or there is a sewer gas explosion.
  
  An experienced plumber may spot a loose vent piping correction quickly, especially in an attic or basement. It helps if you've already traced the sewer gas smell to its strongest location.
  
  
• Pressure test the vent piping or drain piping systems: It is also possible to perform a pressure test of the vent piping system or the drain piping system - a procedure which will diagnose for sure whether or not the plumbing vent system is leaking sewer gas. A plumber installs seals at appropriate locations and pressurizes the piping to check for leaks. [Thanks to J.V. for this detail.]
  
  In the Lidawer sewer gas odor diagnosis case described just above, pressure testing the plumbing system disclosed un-glued drain-waste-vent piping joints.
• Missing plumbing vents: I've seen systems with no vents at all - which often leads to sewer gases reentering the building through sink and shower traps.
• 
  Plumbing vents which are too short in their above-roof projection can be blocked by snow and ice in freezing climates.
• Plumbing vents which are blocked by ice freezing inside the vent line: in freezing weather if a lot of water vapor is passing up the vent line, for example from long steamy showers or perhaps from a hot water fixture left running or leaking into the drain system, frost can form in the vent piping where it passes through a cold attic and outside. When the frost becomes heavy enough it can block the plumbing vent system completely.
  
  In freezing climates, check that a plumbing vent is not being blocked by frost or by snow-cover. Waste line sounds heard in piping or at plumbing fixtures, such as "drain gurgling" or "glub glub" sounds, or similar noises may indicate defective or clogged plumbing.
  
  We discuss how to diagnose and cure drain sounds at Plumbing Drain Noises, and we discuss diagnosing sewer gas odors in cold weather further at Diagnosing Sewer Odors.
• Remedies for sewer gas odors are discussed at Remedies for Sewer Odors. Also see Wet Weather or Cold Weather Septic Odors or Sewage Odor Diagnosis & Repair Guide

• Noisy drain piping or plumbing fixtures: Drain-waste line piping sounds heard in piping or noises heard at plumbing fixtures, such as "drain gurgling" or "glub glub" sounds, or similar noises may indicate defective or clogged plumbing or even if the drains are not clogged, odors and noises are often caused by blocked or improperly installed plumbing vent piping.
  
  At PLUMBING DRAIN VENTS we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises. We discuss how to diagnose and cure drain sounds at Plumbing Drain Noises. Other advice about controlling plumbing noises in buildings is at  SOUND CONTROL for PLUMBING.
• Plumbing vent distances to fixtures: Plumbing codes require that plumbing vent lines be installed sufficiently close to plumbing fixtures to vent those drains (typically 5' or less) and that the vent system conducts any sewer gases outside and above the building (not in the attic). See PLUMBING DRAIN VENTS for details.
• Plumbing trap arm too long or not properly sloped: can cause plumbing trap siphonage and lead to both poor drainage and sewer gas odors in buildings. See PLUMBING DRAIN VENTS for details.

House Traps, Vents, and Sewer or Septic Gas Smells: Some residential plumbers like to install a trap in the main house sewer line at the point where the line exits the house wall to head for the septic tank. Such traps may protect copper waste lines from corrosive sewer gases. They also may prevent gases from a septic tank from passing back up the waste line and up the house plumbing vent stack. See Diagnosing Sewer Odors. Our photo (left) shows a sewer line vent through the building exterior wall. Sewer gases exiting this vent empty next to anyone seated on the chair that has been placed on the patio that was added long after the house was built - a condition that was not anticipated by the installing plumber. Other examples of sewer line vents as a source of septic or sewer gases noticed outside are found at SEWER GAS ODORS in COLD WEATHER

Frequently Asked Questions (FAQs) about troubleshooting plumbing drain waste vent system noises & odors.

Question: what is "short stack" plumbing ventilation?

short stack ventilation? - John

Reply:

John if you are referring to plumbing vents that are too short, we discuss that in the article above. I'm unsure what else you're asking. If a plumbing vent stack is too short a risk is that in climates where there is snow cover, under heavy snow conditions the vent may become blocked.

Question: unused plumbing drain now seems to be clogged

after noticing a standing water in a newly installed vanity - the drain hasn't been used for long time - I tried to used water pressure to dislodge any obstacle there. Instead I got a leak around the vent in the ceiling of the adjacent closet. Any comments? Thanks. - Ad

Reply:

Reader comment: It sounds like your blockage is downstream and by using water pressure, you are pushing the water upstream toward the vent. There should not be any water in the vent pipe. - reader B

Question: faint sewer odors come and go in several areas of our house, worse towards septic tank

We notice faint odors that come and go in a couple of areas in our house. they happen to be closest to the end of the house where our septic tank is located. One distinct location is a bathroom where a toilet had previously been lifted by a plumber to remove a flushed toy and I am now suspicious that this wasn't sealed properly when replaced (how do I know if this is the case?). The other is an adjacent bathroom where a shower drain seems to smell despite regular addition of water - also notable is that this bathroom had a very pungent gas odor when the septic tank was pumped. Is it normal for odors to come back into the house during pumping? (or does this suggest a malfunction of the shower trap or possibly a ventilation leak?)

A plumber has suggested that the vent stack for this bathroom has been placed improperly on our flat roof, too close to a parapet (actually only 2 to 3 inches away)& that we are probably having downdrafts into the ventilation pipe (although I am not sure how this could explain the substantial flow of odor into that bathroom during pumping). The last area is in our garage where there is a utility sink & this also had a horrendous odor during the pump out. Oddly, these 3 problematic areas are in very close proximity to each other at one end of our ranch style house and I am not sure if there is a common problem between them or multiple separate problems. I have had it looked at by septic companies and plumbers and have not really gotten a solution. How do I find someone that can solve this mystery? - Arizona

Reply: use a smell patch test or a person with good sense of smell to track down strongest odor source, check toilet wax ring

Arizona

I'd try a simple sniff test to see if you can track down the strongest odor source. It's not a big job to pull a toilet and replace a doubtful wax ring if that seems to be the trouble area.

Sometimes drains smell even if not blocked, and even if the vent system is working - due to accumulation of hair, sludge, and bacteria in the drain trap. You might try cleaning that smelly shower drain.

It's worth considering with care your observation about the near-location of all of the smell-suspect fixtures. See if you can determine that all of them share the same drain or vent pipe.

Finally, your plumber's suspicion is worth considering, but you might be able to sort that theory out by taking note of the history of the smell problem. If the problems are long-standing and relate to weather/wind, a vent problem could be more suspect.

Question: long horizontal plumbing vent runs can leak into ceilings below

One potential issue that was not mentioned in the article:
I have a horizontal vent run of ABS in my attic, approximately 20 feet long. I'm assuming multiple vent stacks are being joined. I recently had a water leak from the horizontal pipe section onto the 2nd floor ceiling. Upon inspection I found that the horizontal run had a split along the side of the pipe. It appears that rain water had been collecting in the horizontal section and must have frozen, splitting the pipe.

I cut-out and replaced the split section, but it appears that I need to modify the horizontal run so it has some slope on it to allow the rain water to drain out. Just an FYI for folks out there. - Mike T

Reply:

Thanks Mike. - DF

Question: sulphur smell in closet adjacent to a toilet/bathroom

We have an issue with a sulfur smell in a closet on the second floor of our home. The closet shares a wall with the toilet in the bathroom. The bathroom and toilet do not smell, but there is a strong sulfur smell in the closet that comes and goes - it seems to be stronger on sunny days. There is also a window in the closet and a soffit vent right outside of the window. We had the plumber come out. He said he checked the vents in the attic and they were connected properly? Any ideas? Thanks!

Reply:

Sulfur smells that seem to relate to a septic system or plumbing drains are often traced to a loose toilet, missing or leaky toilet wax ring, dry plumbing traps at unused fixtures, inadequate plumbing vent piping by location or routing, or on occasion, a loose leaky connection in the plumbing vent piping as it runs concealed through building walls, floors, or ceilings. If checking the easy accessible problems like toilet wax rings and dry plumbing traps doesn't solve the problem, and if you believe the plumber's view that all of the drains have adequately located and sized plumbing vent piping, then next might be a pressure test of the vent system to see if you can prove that it is leaking.

Question:

I have 2 toilets that when flushed drain into the bathtub. I have a septic tank that has been checked and is working ok, Per the repair guy. We have had 3 separate plumbers come out and no one has been able to fix the problem. Toilets overfill with water by themselves, and when running anything like a dishwasher or Washing Machine everything overflows into the tub.... any suggestions?? I'm dying here.. - Shaun

Reply:

Shaun I read more than one problem here:

1. your toilets over-fill: if you mean that the toilet tank becomes too full, that's not a drain waste vent problem, it is a toilet fill or flush valve control problem; if you mean that the volume of water in the toilet tank is normal (fills to the "fill line" marked in the tank) but the toilet bowl becomes too full when the toilet is flushed then

1.a. the toilet drain is clogged somewhere in its routing

1. b. the toilet drain is not properly vented and the result is slow draining

2. the toilet flush water volume and delivery rate is improper - check the flush valve

The observation that the toilets overfill and that "everything overflows (presumably backs up into) the bathtub when a dishwasher or clothes washer is running is a very strong indicator that you have a blocked or partially blocked drain. Sometimes the blockage can be pretty far downstream from the toilet(s) or even from the building itself. A clue that suggests that condition is that when no water has been run for 4 hours or more, you get a few good toilet flushes before things begin to back up (you're filling up the drain piping with wastewater).

If the toilets back up immediately, any time, regardless of how long since any water was run into the drain system, then the blockage is probably closer to the fixtures.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Cheryl Lidawer generously provided both case history and photographs of efforts to track down the source of sewer gas in a building. March 2010.

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