March     2010
Inspection News and Views from the American Society of Home Inspectors

Plumbing Vents — Why Basins Sometimes Talk Back


This article and the illustrations are excerpted from the ASHI@HOME Training System.

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 Vents

The 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 out of the way. 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. Traps stop sewer gases from
getting into the home. Vents allow air in to prevent a siphon.

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

AlanCarson1.jpgGurgling Sounds and Sewer Gases

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 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 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" per foot. The smallest drain line is 1-¼" diameter. There are five ¼" in a 1-¼" 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 feet or less, the low end of the drain is not flooded and air can get in.

With no 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).


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

We need one of the following: a vent, a larger diameter trap arm or an air admittance valve to break the siphon and prevent the trap from being emptied.

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-foot limit to 2-½ ft. It’s always good to know the rules in your area.

A word about ASHI@HOME: 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, call 800-268-7070.