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How to Diagnose and Eliminate Sewer Gas Odors InspectAPedia®  -     How to diagnose traps and drain siphonage as a source of sewer smells and odors Causes and cures for sewer gas odors related to wet or cold weather How to find and cure bad smells in buildings

This article discusses the diagnosis and correction of sewer gas or septic odors (and other building smells and odors with focus on diagnosing odor sources and causes in cold weather. Some of the diagnostic steps pertain to all seasons. Also see ODORS & SMELLS DIAGNOSIS & CURE for procedures to diagnose and cure all kinds of odors in and around buildings.

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. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted.

Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical reviewers are welcome and are listed at "References." This is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links 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.

Diagnosing and Eliminating Sewer Gas Odors

Smelling sewage odors inside only, or mostly inside

Here are some places to look and perhaps to fix if you're smelling sewage or septic like odors inside of a building:

• Blocked or Inadequate Plumbing Vents can cause odor backdrafting into a building. See Plumbing Vent Defects for diagnostic details.
• Dry Plumbing Traps can serve as a Sewer Gas Odor Source indoors - check all of your fixtures, especially ones which have not been used in some time or ones which are lacking proper venting. A plumbing trap serving an unused sink or shower or tub can dry out and permit sewer gases to enter the building up through the fixture. See TRAPS on DRAINS for diagnostic details.
• All drains and traps may smell and produce a little methane:: But beware, a little gas odor is detectable at many fixture drains and traps since there is likely to be decaying organic matter there. Check for dry plumbing traps.
• Fixtures which are not properly vented may lose their water trap: If the fixture is not properly vented, even if it's in-use, the water that is supposed to be in the fixture trap may be siphoned away, permitting sewer gases to enter the building. See Plumbing Vent Defects for diagnostic details.
• Plumbing vents as an odor source: a leaky plumbing vent could be an odor source indoors. The odor may not be coming from plumbing drains: If none of the building drains are clogged or slow and if the septic system is otherwise working one's first guess would be a problem with the plumbing the vent system. This condition can occur year round. See Plumbing Vent Defects for diagnostic details.

You can usually spot inadequate or blocked plumbing venting by noticing, for example, that tub, shower, or sink drain makes a gurgling or "glub glub" sound when a nearby toilet is flushed or when a full fixture is being drained. Inadequate or blocked plumbing venting can also mean that a plumbing fixture does not drain well (even though the drain line is not blocked). Inadequate venting produces a glub glub sound as water passes down a drain and cannot draw air in after itself through a plumbing vent stack.

Instead the drain has to draw air in to itself through various plumbing traps which, as they normally contain water, produces the glub glub sound as air passes through the trap. This defect can siphon water out of traps and let sewer gases back into a home - smelly and potentially dangerous (methane gas).

Also see Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds

Cold Weather and Plumbing Vent Blockage Problems as a Source of Building Smells, Septic or Sewage Odors

What's different in cold weather? An under-sized vent (less than 2" diameter or too short above the roof line) can become blocked by frost in the vent line above the roof, blocked by snow cover, or have its gases blown down to a lower level by cold or varying air movement. If it's a plumbing vent diameter problem in a freezing climate, you'll see the vent basically blocked by frost above the roof line.

Look also for short plumbing vent stacks above the roof line - they can be covered and blocked by snow.

If in cold weather a plumbing vent pipe is frosting up, sewer gases may vent backwards out through building drains. In that case, even re-priming building drain traps won't prevent sewer gas entry as flushing a toilet or running a nearby drain can siphon water out of a nearby trap.

See Plumbing Vent Defects for diagnostic details.

Plumbing Vent Problems and Trap Siphonage, Dry Plumbing Traps and Sewer Gases

Freezing weather can cause a complete blockage of a plumbing vent stack by frost. For example steam from long showers moves up the vent pipe where it freezes in the pipe section extending above the building roof.

The absence of venting (missing or blocked) causes trap siphonage and loss of water in building traps. (Water flowing down a drain line without a nearby supply of makeup air to follow the water creates a vacuum that pulls water out of nearby plumbing traps. Flushing a toilet can siphon out a nearby sink or tub trap.)

What happens when the water seal is lost from a plumbing trap?

When the water seal is lost from a plumbing trap sewer gases can back up out of that fixture and not just smell bad. Sewer gases contain methane which is an explosive gas - possibly quite dangerous. In addition to occasional methane gas explosions inside buildings, I've had a report of an outdoor septic tank explosion too when an owner built a brush fire atop the tank.

In sum, dry plumbing traps are caused by evaporation at an unused fixture, leaks at the trap, or siphonage due to improper plumbing vent line installation. Dry traps can leak smelly or dangerous sewer gases into a building. Check for dry plumbing traps, particularly at un-used basement or lower floor fixtures and at floor drains which might be connected to the sewer line. "Dry trap" means that there is no water in the trap bend or weir.

See Plumbing Vent Defects for diagnostic details of venting problems and see Plumbing Fixtures or Traps for fixture trap inspection and defect diagnosis.

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ANIMAL ODORS IN BUILDINGS
CARBON MONOXIDE & GAS HEAT ODORS
CARPETS & PADDING ODORS IN BUILDINGS
GAS MEASUREMENT TOOLS
MOLD ODORS, Musty Smells in Buildings
OIL HEAT ODORS
OIL TANK LEAK ODORS
PAINTS & COATINGS ODORS IN BUILDINGS
SEPTIC SYSTEMS HOME
SEPTIC INFO ARTICLES
SEWER GAS ODORS
  SEWER GAS ODORS in COLD WEATHER
  Short Answer in Cold Weather
  Diagnosing Sewer Odors
  Cold Weather Plumbing Vent Blockage
  Trap Siphonage and Sewer Gases
  Building drain odor source
  Fixture versus system blockage
  Odors in Wet Weather
  Indoor Septic Odors Outdoor Causes
  Outdoor Odor Sources
  Sewer Odor Tracking
  Remedies for Sewer Odors
  Odors and Drain Lines
  Failed Drainfields and Odors
HOME BUYERS GUIDE to SEPTIC SYSTEMS
MVOCs & MOLDY MUSTY ODORS
ODORS & SMELLS DIAGNOSIS & CURE
SEPTIC SYSTEMS ONLINE BOOK
SEPTIC TANK PUMPING SCHEDULE
SEPTIC TANK PUMPING PROCEDURE
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SEPTIC AUTHORITIES
SEPTIC SYSTEM BOOKS REFS CODES
SEPTIC SYSTEM DESIGN MANUAL - Online
SEPTIC SYSTEM DESIGN BASICS
SEPTIC DESIGN ALTERNATIVES
SEWER LINE REPLACEMENT
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• Herb Reed County Extension Director, Agricultural and Natural Resources Educator, Calvert County Maryland - private email to DF 9/5/2006 adding comments about odors and partial blockages.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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More Information on Building Diagnostic Inspections and Repairs

• Sewer Gas Odors diagnosing, finding, and curing septic tank and sewer line smells.
• Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds


• The Septic System Information Website Home Page: public information website about septic system design, installation, maintenance, and repair
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• What is a Septic System An Engineer's View & Septic System FAQ's
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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