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Heat tape used to clear a frost-blocked plumbing vent (C) Daniel FriedmanSewage or Septic Odors from Blocked Plumbing Vents

  • PLUMBING VENT BLOCKAGE ODORS - CONTENTS: causes of blocked plumbing vents & resulting smells & odors in and around buildings; why do some plumbing vents get blocked in freezing weather? Effects of snow cover on plumbing system vents & odors. Plumbing drain trap siphonage, odors, noises due to clogged plumbing vents.
  • POST a QUESTION or READ FAQs about diagnosing the source of septic or sewer odors in cold weather
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Clogged or blocked plumbing vents as a cause of sewer or septic system odors:

Diagnose odors traced to clogged, frozen or snow-covered plumbing vents. This article describes the causes of blocked plumbing vents & resulting smells & odors in and around buildings; why do some plumbing vents get blocked in freezing weather? Effects of snow cover on plumbing system vents & odors. Plumbing drain trap siphonage, odors, noises due to clogged plumbing vents.



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

Schematic of a plumbing stack vent (C) Carson Dunlop AssociatesWhat's different in cold weather that could cause indoor sewer gas odors only during freezing weather?

[Click to enlarge any image]

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. [Illustration provided courtesy Carson Dunlop Associates.]

A thoughtful reader, Jill Elwert, suggested insulating the vent piping to try to delay vent pipe blockage by freezing moisture. We are afraid that even if you insulate the vent line in a cold attic, it'll freeze above the roof surface outdoors. You could try it but if that proves the problem, ultimately you'll want to have a plumber or handyman install a 2" or larger diameter vent from the attic floor up through the roof.

See REMEDIES for SEWER ODORS, PLUMBING.

Diagnose frozen or snow-covered plumbing vent lines:

Plumbing vent beginning to clog with snow (C) Daniel Friedman Plumbing vent beginning to clog with snow (C) Daniel Friedman

See PLUMBING VENT DEFECTS & NOISESfor diagnostic details.

PlumbingTrap Siphonage, Dry Plumbing Traps, Sewer Gas Leaks & Freezing Weather-Related Plumbing Vent Problems

Reader Question: 16 Feb 2015 rose said: do I need to dig out the septic vents that are snow-covered?

I hear about the need to clear vents in the house in this snow, but can't find anything about when the septic vent pipes are buried in snow -- do those need to be dug out?

Reply: frost blocked or snow-covered vents can cause dangerous sewer gas backups

Rose,

My OPINION is that the answer is ... it depends.

On what your sewer piping connects to and where you live.

For example: North Dakota State University has advised that in deep-snow-winter weather, blocked sewer vents can cause a [dangerous in my opinion] sewer gas backup in homes that can "make people sick" (sewer gas is also explosive). - see "Clogged vents can lead to sewer gas backup, which can make you or your family ill.", Roxanne Johnson, Water Quality Associate, (2011), - ag.ndsu.edu

Johnson also notes that deep snow on a roof can block the roof vents of the plumbing system too; indeed I've published photos of that condition here at InspectApedia.

Recently, in New York, dealing with deep snow and ice dam leaks on a flat roof, we had what seems to me a clever solution: having to make an emergency "repair' by adding a heat tape to a leaky roof we just stuck an extra length of the heating tape cable into the rooftop plumbing vent stack. That small amount of heat won't damage the plastic piping of the vent, and the heat can avoid a frost-blocked plumbing vent that can occur in freezing weather whether there is deep snow cover or not.

Freezing weather can cause a complete blockage of a plumbing vent stack by frost that forms inside the portion of the vent that extends outdoors above the roof, or by deep snow cover that completely covers a short plumbing vent.

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 plumbing 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?

Schematic sketch of distance allowed between a plumging fixture and vent piping (C) Carson Dunlop Associates

[Click to enlarge any image]

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.

Sketch above courtesy Carson Dunlop Associates.

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 & NOISESfor diagnostic details of venting problems

and see PLUMBING FIXTURE TRAPS for fixture trap inspection and defect diagnosis.

Also see METHANE GAS SOURCES and

See PLUMBING DRAIN NOISES where we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises

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Continue reading at TRACK DOWN COLD WEATHER SEWER ODORS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see PLUMBING FIXTURE TRAPS for fixture trap inspection and defect diagnosis.

Or see PLUMBING VENT DEFECTS & NOISESfor diagnostic details of venting problems

Suggested citation for this web page

PLUMBING VENT BLOCKAGE ODORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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