Backdrafting, Wet Weather & Causes of Sewage or Septic Odors
BACKDRAFTING & SEWER/SEPTIC ODORS
- CONTENTS: What is backdrafting and how does backdrafting occur in buildings? Why is it dangerous? How backdrafting can cause sewer gases to enter a building. Leaks at bathroom fan cause sewer gas odors in building. Sewer gas backup to roof enters building through bath vent fan opening. How to diagnose sewer odors in wet or cold weather. Causes and cures for sewer gas odors related to wet or cold weather. How to find and cure bad smells in buildings
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.
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 where we discuss dangerous backdrafting conditions that can make heating appliances unsafe and can produce potentially fatal carbon monoxide gas hazards in buildings.
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 CODES SPECS.
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
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.
See CORROSION & MOISTURE SOURCES in PANELS for 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.
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