Plumbing vent problems can lead to noisy plumbing drains or may cause odors, even unsafe sewer gas explosions (C) Daniel Friedman Plumbing Vent Noise Diagnosis
Building Noises traced to plumbing drain, waste, or vent piping problems: causes & cures

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This plumbing drain and vent noise article explains proper plumbing vent piping and how errors cause trap siphonage, odors, and plumbing noises such as gurgling or a glub-glub-glub noise at plumbing drains or fixtures. Building noises such as gurgling drains and some building odors (sewer gas, methane, toilet smells) are often traced to defects in the plumbing vent system. Here we explain the causes and cures of these problems.

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Plumbing Vent Noises – Why Basins Sometimes Talk Back

 Carson Dunlop AssociatesAlan Carson, Carson Dunlop & Associates

Definitions: Plumbing Vent Terminology

This article and the illustrations were prepared by Carson Dunlop Associates, with edits and additions by .

This material appears in the ASHI@HOME Training System and are used here with permission from the original authors.

The illustration at left, courtesy of Carson Dunlop Associates, shows the names of some of the pipes in a venting system.

Plumbing drain or sewer gas odors

Watch out: If you smell sewer gases in your building conditions could be dangerous (risking a methane gas explosion) or unsanitary. See REMEDIES for SEWER ODORS, PLUMBING and


Plumbing drain or fixture noises: Other advice about controlling plumbing noises in buildings is


You run water into the basin and everything seems fine, until the last bit of water leaves the fixture. The loud, slightly rude, gurgling noise draws puzzled looks from your client and the agent. What's going on? Does it matter?

The problem is in the plumbing venting system and the risk is not noise, but sewer gases entering the home because the trap has no water. Most home inspectors understand this problem to some extent, but it's hard to visualize what's happening. In this article we'll review the functions of vents, touch on the terminology, and then look at why things gurgle.

The Functions of Plumbing Vents: how do plumbing vents work & why are they needed

Photograph of a loose, improperly installed, slanted plumbing vent above the building roofThe venting system equalizes the air pressure throughout the waste piping system. Why does this matter? Let's look at four functions of vents.

1. The waste won't flow properly if it can't push the air in the pipe ahead of the waste out of the way. Plumbing vents allow air out of the waste pipes.

2. The waste won't flow well if it's held back by low air pressure or a vacuum in the pipe behind it. Vents allow air into the waste pipes.

3. We don't want the water to be siphoned out of the trap every time a fixture is used. It's the water sitting in a plumbing trap that stops sewer gases getting into the home. Vents allow air in to prevent a siphon.

4. Plumbing vents allow sewer odors to escape from the house, venting safely above the roof. Without venting, the sewer gases seep through the water in the trap and enter the house. Vents help sewer gases escape outdoors.

Gurgling Sounds and Sewer Gases at Sink Drains, Showers, Tubs

Plumbing drains work by gravity. Gravity pulls the water out of the fixture and down the drain pipe. Gravity wants to leave some water in the trap because it's a low spot. That's all good.

But when the sink basin is empty, how do we split that solid slug of water flowing through the pipe, so that some stays in the trap and the rest flows down the drain? The venting system allows air to get between the water going down the drain and the water staying in the trap. The illustration below shows this nicely.

Carson Dunlop Associates

The maximum distance from the sink basin trap to a vent is usually about 5 feet. What’s magic about 5 feet? Here’s a clue: The drain slope is about ¼ inch per foot.The smallest drain line is 1 ¼ inch diameter. There are five ¼ inches in a 1 ¼ inch pipe.As the last bit of water flows, we don’t want the lower end of the drain pipe to be flooded.

That creates a vacuum that results in siphoning. If the drain is 5 ft. or less, the low end of the drain is not flooded, and air can get in.

Our sketch (above left) illustrating the basics of plumbing traps is also provided courtesy of Carson Dunlop and appears in Drain Waste Vent Plumbing, a section of Carson Dunlop's Home Study Course for Home Inspectors.

Without an effective plumbing drain vent, a vacuum forms in the pipe between the water going down the drain and the water that needs to stay in the trap. A siphon is created and the atmospheric air pressure pushes the water out of the trap to satisfy the vacuum. This allows sewer odors to back up into the house. The gurgling sound is air forcing its way through the water in the trap. (See illustration below.)

Proper drain pipe slope (C) Carson Dunlop AssociatesIf the drain pipe (trap arm) is longer than 5 feet distance from the vertical plumbing vent stack, or the plumbing drain pipe slope is more than ¼ inch per foot, a 1 ¼ inch diameter pipe will flood.

That’s the same as having no vent. The illustration below shows that air from the stack can’t get into the trap arm, and a siphon results.

As Carson Dunlop Associates sketch (left) shows, the slope for drain piping should be between 1/8 and 1/4 inch per foot for 3 inch or larger diameter pipe, but it should be a minimum of 1/4 inch for smaller pipe such as the sink basin drain in our example above.

The sketch shows what happens if a drain pipe is sloped too little (it doesn't drain and drain pipe clogs develop) or too much (water drains too fast and leaves solids behind, and pipe logs develop).

In larger diameter waste pipes, such as a blackwater line from a toilet or a 6" sewer pipe, a slope that is too low or a slope that is too steep can lead to clogging as solids are left behind in either case. If we try to apply the 5 foot rule for distance between the sink basin trap and the soil waste stack/vent pipe (at the right in our illustration above), and if we have a slope that's greater than 1/4 inch per foot, we will create a siphon.

That's why the Uniform Plumbing Code (UPC) limits the 5 foot distance in our general (not code specific) illustration above to 2 1/2 feet.

A plumbing trap siphon is created and the atmospheric air pressure pushes the water out of the trap to satisfy the vacuum. This allows sewer odors to back up into the house.

The gurgling sound heard at a plumbing drain is air forcing its way through the water in the trap. (See illustration below.0

If the drain pipe (trap arm) is longer than 5 feet, or the slope is more than ¼ inch, a 1 ¼ inch diameter pipe will flood. That’s the same as having no vent.The illustration at left shows that air from the stack can’t get into the trap arm, and a siphon results.

To Avoid Plumbing Trap Siphonage We Need One of the Following

Plumbing vents in Buenos Aires (C) Daniel Friedman

  • A plumbing vent. Our photo (left) shows a gaggle of rickety plumbing vents atop a Buenos Aires apartment building.
  • A larger diameter trap arm or
  • An air admittance valve (siphon breaker)* to break the siphon and prevent the trap from being emptied. Siphon breakers are not code approved in all jurisdictions, or may be approved on an "exception" basis.

We can say it another way. The fall of the trap arm should be less than one pipe diameter over the distance between the trap and the vent. Remember, you’ll rarely see this in the field, but it’s important to understand the principle. When you hear the gurgling, the problem is a health issue and the answer is venting. You may not hear the gurgling if you only run a little water.

Some authorities (e.g. The 2006 Uniform Plumbing Code) add a safety factor and reduce the 5 ft. limit to 2 ½ ft. It’s always good to know the rules in your area.

* STUDOR is a good example of an approved air admittance valve. These devices do not include a spring, only a diaphragm. This makes them more mechanically reliable. OATEY also makes an air admittance valves, possibly also spring-loaded auto vent types. We believe ProVent and the V-200 are spring-loaded siphon breaker valves and are not approved in some code jurisdictions.

The Sealing Disc of the V-200 automatic plumbing vent anti siphon valve [air admittance valve] must be located at least 6" above the top of the trap arm. NOTE: An Angle Mounting of no more than 15 degrees from the Vertical is acceptable. Location of the V-200 is no way related to the flood level of the fixture that it's venting.

The V-200 functions equally well above or below the flood level of any fixture in the home. - V-200 ABS Auto Trap Vent product description, V-200 anti siphon valve installation instructions from manufacturer Oakville Stamping & Bending.

See UPC Section 909.0 for plumbing code discussion of the use of air admittance valves in residential buildings.

A word about ASHI@HOME for home inspectors: You can take any one of the 10 courses and receive ASHI CE Credits. The Plumbing course for example, gives you 36 CE Credits. For more information contact Carson Dunlop - more information is below at Technical Reviewers & References. (Scroll down).

The above article is reprinted, with minor edits and amendments, with permission from Carson Dunlop Associates. Also see this example Drain Waste Vent Plumbing excerpt from the ASHI@HOME education program, courtesy of Carson Dunlop appeared in the ASHI Reporter.



Continue reading at PLUMBING VENT REPAIR or select a topic from the More Reading links shown below.


Plumbing noises from supply piping, water pumps, or other sources are discussed separately at PLUMBING NOISE CHECKLIST

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PLUMBING VENT NOISE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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