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Backdrafting, Wet Weather & Causes of Sewage or Septic Odors
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
This article discusses the role of back drafting in buildings, exhaust fans, or other building air movement in causing sewer gas or septic odors (and other building smells and odors).
This article is part of our series SEWER GAS ODORS that helps track down and cure the source of sewer gas or septic smells in buildings. Also see ODORS & SMELLS DIAGNOSIS & CURE for procedures to diagnose and cure all kinds of odors in and around buildings.
See CORROSION & MOISTURE SOURCES in PANELS and CORROSION in ELECTRICAL PANELS examples of how backdrafting or negative building air pressure can cause unexpected air and moisture movement, condensation, and moisture-related problems such as rust, corrosion, or odors, or mold.
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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
How Back-Drafting Can Cause Sewer Gas Odors in buildings
What is "Backdrafting" in buildings?
"Backdrafting" refers to indoor conditions that create sufficient negative air pressure inside a building such that gases may be drawn into the building from a plumbing drain system or such that heating appliances may lack adequate combustion air and may produce dangerous carbon monoxide.
Watch out: backdrafting in a building can be dangerous, causing intake of explosive methane gases from a sewer system, or potentially fatal carbon monoxide gas hazards from heating appliances. Separately at BACKDRAFTING HEATING EQUIPMENT we discuss dangerous backdrafting conditions that can make heating appliances unsafe and can produce potentially fatal carbon monoxide gas hazards in buildings.
In this article (below) we discuss the causes, effects, and cures for backdrafting in buildings that affects the building plumbing drain vent system. Unsafe heating appliance conditions caused by backdrafting are discussed at BACKDRAFTING HEATING EQUIPMENT.
What Causes Building Backdrafting Hazards?
 Any one or more of the factors listed below can create negative air pressure inside a building such that backdrafting occurs or may occur at building drains or plumbing vents:
- Bathroom or kitchen exhaust fans create negative air pressure at least in those rooms in order to exhaust bathroom or cooking odors. Also see BATHROOM VENTILATION.
- Doors open between a basement and upper building areas may provide a path for basement or crawl space air (and odors) to pass to upper building levels.
- Exhaust-only ventilation systems in buildings can create backdrafting conditions, especially in tight buildings. See VENTILATION, EXHAUST ONLY, and for a cure see VENTILATION, BALANCED.
- Fireplaces, when in use, particularly open fireplaces that do not include an air-tight glass fire screen or door, create significant air movement out of the building and up the chimney, potentially causing backdrafting in other building areas.
- Rain, wet weather, and local flooding can cause sewer gases to back up through building drains, moving (properly) up through building vent piping to above the building roof, or moving (improperly) into the building through dry traps and drains.
- Sewer gas reentry into buildings: can occur when a plumbing vent is improperly located too close to a window, door, soffit vent, or even a bath or kitchen vent duct as we illustrate in the field report given below. The reentry of sewer gases into a building is more likely when:
- Wind from a particular direction pushes sewer gases from a source towards building openings, windows, doors, vents
- Other site conditions (rain, sewer or storm drain flooding) create higher than normal levels of sewer gases venting through a building's plumbing vent system
- Tight building construction with minimal air leaks, a desirable practice for saving on building heating or cooling costs, reduces the ready supply of outdoor makeup air when any other building condition is causing air movement out of the structure. The lack of easy entry of makeup air can add to backdrafting dangers in a building.
- Warm air rising in a multi-story building by natural convection during cold weather creates negative air pressure on lower floors, potentially drawing sewer gases out of dry traps, drains, or faulty plumbing vent piping
- Whole house ventilation exhaust fans create very powerful negative air pressure in buildings sufficient to cause backdrafting affecting both plumbing system drain/waste/vent piping and heating appliances.
- Windows open on upper building floors and closed on lower building floors increase air movement upwards in buildings.
Field Report: Example of Rooftop Plumbing Vent Backdrafting Causing Sewer Gases to Enter a Building - and The Cure
 Sewer/Storm Drain Flooding: indoor sewer gas odors have been tracked to a combination of wet weather, flooding sewer and storm drains, and backdrafting in the building due to improper bathroom exhaust fan vent installation.
Our photo (left) shows a poorly-installed rooftop bath vent fan outlet, patched to try to stop leaks, and likely to be blocked by snowfall on the roof surface. But you can see the "flapper" that closes this vent opening when the fan is not running.
Reader Jose Iturraspe provided the following sewer gas odor diagnosis and cure field report:
A customer complained of sewer smell in the home that occurred only when it rained, and only in his master bath.
Inside, at the exhaust fan housing in the ceiling, we found that there was no flapper. [The flapper is designed to stop indoor air movement up out through the exhaust fan system when the fan is not turned on.]
Outside, on the roof, the exhaust fan gooseneck had no flapper either. [The rooftop flapper is designed to keep birds, insects, and in some climates wind-blown rain or snow out of the exhaust fan system when the fan is not running.]
The plumbing vent was located about 2’6” from the exhaust fan gooseneck.
We replaced both flappers in the bathroom exhaust fan system, and the sewer gas odor issue has gone away.
My best guess is that during a rain, the storm drain/sewer had its sewer gas displaced with the rain water runoff, pushing the gas out the plumbing vents.
At the same time we think that the winds combined with the missing flappers on the bath exhaust fan created a situation where the gas was being sucked in thru the exhaust vent goosenecks.
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Technical Reviewers & References
Use links just below 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.
Backdrafting & Sewer/Septic Odors - see detailed links at page top & left
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Thanks to reader Jose Iturraspe, Pulte Group Customer Service representative, Sarasota FL, Tel: 352-266-8589, email: Jose.Iturraspe@Pulte.com. Pulte Group is a nationwide builder of new homes in the U.S. with more than 55 years in the construction business.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Inspecting Septic Systems: Online Book, Inspection, Test, Diagnosis, Repair, & Maintenance: our Online Septic Book: Septic Testing, Loading & Dye Tests, Septic Tank Pumping, Clearances, details of onsite waste disposal system inspection, testing, repair procedures.
- Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389
(2006), Advanced Onsite Wastewater Systems Technologies.
Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going
for homeowners but is a text we recommend for professionals--DF.
- Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP;
Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
- Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP;
Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.
Design Manuals for Septic Systems
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US EPA Onsite Wastewater Treatment Systems Manual [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems,
Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers.
U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
- Eco John® Innovative Toilet Solutions, Global Inventive Industries, Fountain Valley CA, PDF, product brochure
- "International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
- "Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental
Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
- Manual of Septic Tank Practice, US Public Health Service's 1959.
Onsite Wastewater Disposal Books
- Onsite Wastewater Disposal, R. J. Perkins;
Quoting from Amazon: This practical book, co-published with the National Environmental Health Association,
describes the step-by-step procedures needed to avoid common pitfalls in septic system technology.
Valuable in matching the septic system to the site-specific conditions, this useful book will help you install a reliable system in
both suitable and difficult environments. Septic tank installers, planners, state and local regulators, civil and sanitary engineers,
consulting engineers, architects, homeowners, academics, and land developers will find this publication valuable.
- Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994 - one of the best septic system books around, suffering a bit from small fonts and a weak index. While it contains some material more technical than needed by homeowners, Burks/Minnis book on onsite wastewater treatment systems a very useful reference for both property owners and septic system designers.
- Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf
- Soil Percolation Tests soil perc testing guide and instructions
- Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
- Planting Over Septic System Component", Daniel Friedman (author/editor, InspectAPedia.com), The Innovator, Winter/Spring 2008, BCOSSA, British Columbia OnSite Sewage Association, 201-3542 Blansard St., Victoria BC V8X 1W3 Canada
- Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S. - easy to understand, well illustrated, one of the best practical references around on septic design basics including some advanced systems; a little short on safety and maintenance. Both new and used (low priced copies are available, and we think the authors are working on an updated edition--DF.
Quoting from one of several Amazon reviews: The basics of septic systems, from underground systems and failures to what the owner can do to promote and maintain a healthy system, is revealed in an excellent guide essential for any who reside on a septic system. Rural residents receive a primer on not only the basics; but how to conduct period inspections and what to do when things go wrong. History also figures into the fine coverage.
- Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design. Readers should also see Soil Percolation Tests and for testing an existing septic system, also see Dye Tests
- Grass is Always Greener Over the Septic Tank, Bombeck, Erma: $ 5.99; FAWCETT; MM;
This septic system classic whose title helps avoid intimidating readers new to septic systems, is available new or used at very low prices.
It's more entertainment than a serious "how to" book on septic systems design, maintenance, or repair. Not recommended -- DF.
- US EPA Onsite Wastewater Treatment Systems Manual Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm
- Water Wells and Septic Systems Handbook, R. Dodge Woodson. This book is in the upper price range, but is worth the cost for serious septic installers and designers.
Quoting Amazon: Each year, thousands upon thousands of Americans install water wells and septic systems on their properties. But with a maze of codes governing their use along with a host of design requirements that ensure their functionality where can someone turn for comprehensive, one-stop guidance? Enter the Water Wells and Septic Systems Handbook from McGraw-Hill.
Written in language any property owner can understand yet detailed enough for professionals and technical students this easy-to-use volume delivers the latest techniques and code requirements for designing, building, rehabilitating, and maintaining private water wells and septic systems. Bolstered by a wealth of informative charts, tables, and illustrations, this book delivers:
* Current construction, maintenance, and repair methods
* New International Private Sewage Disposal Code
* Up-to-date standards from the American Water Works Association
- Wells and Septic Systems, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992. We have found this text very useful for conventional well and septic systems design and maintenance --DF.
Quoting an Amazon description:Here's all the information you need to build a well or septic system yourself - and save a lot of time, money, and frustration. S. Blackwell Duncan has thoroughly revised and updated this second edition of Wells and Septic Systems to conform to current codes and requirements. He also has expanded this national bestseller to include new material on well and septic installation, water storage and distribution, water treatment, ecological considerations, and septic systems for problem building sites.
- The NSFC Products List has an excellent list of design manuals/modules available from their website or by telephone 800-624-8301
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