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.
Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.
Alan Carson, Carson Dunlop & Associates
This article and the illustrations were prepared by Carson Dunlop Associates, with edits and additions by InspectAPedia.com .
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
also see ODORS GASES SMELLS, DIAGNOSIS & CURE.
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.
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.
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.
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.)
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.
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.
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.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Questions & answers or comments about diagnosing and fixing plumbing vent and drain noises
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References