Taking a Close Look at Horizontal Wet Venting

Horizontal wet venting codes can vary from state to state, so carefully study which one fits what you are doing before the work starts.

Taking a Close Look at Horizontal Wet Venting

In my last column, I reviewed circuit venting and the variations of it to which different codes allow. This month we will “dive” into wet venting.

There are different wet venting systems that both model and state plumbing codes allow. Vertical and horizontal wet vents are the most common of all these systems. For this article, we will discuss the horizontal wet vent.

One of my favorite sayings that I often used while teaching plumbing code and which I’ve shared in the past with you is, “If I can identify it, I can size it!” So, let’s look at how the codes define a wet vent and then throw some sizing at it.

Code A — “Wet vent” means that portion of a vent pipe receives the discharge from other fixtures.

Code B — “Wet Vent” is a vent that also serves as a drain.

Code C — “Vent, Wet” is vent pipe that is sized and arranged in accordance with this code to receive the discharge of waste from a fixture.

As you can quickly recognize from the definitions, a wet vent is part of the plumbing system where a drain from one or more fixtures is sized in a way that allows for a passage of air through the top portion and, in turn, vents other fixture traps.

In Figure 1, you can see how the drain from the lavatory wet vents the shower and the common drain from the shower and lavatory, wet vent the water closet.

In concept, all codes are the same when it comes to this method of venting. However, from that point on, things start to “swish” around differently depending on which code you follow in your state.

Figure 2 illustrates the wet vented portion of the system. In some codes, the fixtures that discharge into the wet vent shall be a maximum of 1 to 2 DFUs (drainage fixture units) and the water closet shall be the most downstream fixtures vented by the wet vent.

When it comes to sizing the wet vented portion of the system as seen in Figure 2, most codes agree that the size should be sufficient to provide enough air to protect the trap seals involved. What that minimum size should be, however, tends to vary from code to code. From my research of these codes, I’ve found the following sizing requirements:

Code A indicates: (1) be not less than one pipe size larger than the required drain pipe size for its upstream fixtures, and (2) be not less than one pipe size larger than one-half the size of the largest connected drain piping being wet vented downstream.

Code B mandates: The wet vent shall be sized based on the fixture unit discharge into the wet vent. The wet vent shall be not less than 2 inches in diameter for 4 DFUs or less, and not less than 3 inches in diameter for 5 DFUs or more.

Code C requires: The wet vent shall be at least 2 inches in diameter. No more than 4 DFUs may discharge into a 2-inch diameter wet vent.

Another point that I find interesting, and varies by code, are the types of fixtures allowed on the horizontal wet vent, how they are intended to be used — be that public or private — and where the fixtures are physically connected to the wet vent.

For the most part, the horizontal wet vent is used in venting bathroom groups. But again, as I mentioned at the start, we must be able to identify it before we can size it. Check out these variations in the definition of a bathroom group:

Code A — Bathroom Group: A group of fixtures consisting of a water closet, lavatory, bathtub or shower, including or excluding a bidet, an emergency floor drain or both. Such fixtures are located on the same floor level.

Code B — Bathroom Group: Means a water closet, lavatory and a bathtub or shower located together on the same floor level.

Code C — Bathroom Group: Any combination of fixtures, not to exceed one water closet, two lavatories, either one bathtub or one combination bath/shower, and one shower, and may include a bidet and an emergency floor drain.

Next, we have to look at these fixtures and determine how they are intended to be used — public or private — to confirm that the horizontal wet vent system can be used. Code A and C allows the fixtures to be located in public and private use bathroom groups. Code B only allows the fixtures to be located in private use bathroom groups.

Again, these requirements relate to how, or who, will use these fixtures, not the type of building they are located in. For example, while Code B only allows the fixtures to only be horizontal wet vented for private use, the definition of private use in that code states in part that in the classification of plumbing fixtures, those fixtures in residences, apartments, living units of hotels and motels and other places where the fixtures are intended for the use by a family or an individual to the exclusion of all others.

Shaking your head yet? Personally, I have to chuckle when I read these plumbing codes. It does amaze me that we can have so many variations to the wording; but at the end of the day, these codes and the plumbers installing the plumbing make it work.

The last thing I’ll touch on is the where the plumbing fixtures are physically connected to the horizontal wet vent.  

Here in Figure 4, the lav connects to the tub/shower located downstream which then connects to the water closet. Don’t let the drawing fool you. Some might argue that the tub/shower is located upstream of the lav connection.

Remember where the wet vent starts. The moment the discharge from the lav enters its fixture drain the wet vent begins. See Figure 5.

In other words, follow the flow. Think of it like the current in a river or stream. The flow from the lav is the start point, so to speak, and is flowing towards the next drain connection which is the wye connecting the tub/shower. These two fixtures then continue to flow downstream towards the water closet to complete the horizontal wet vent as seen in Figure 6.

Figure 7 shows how the two lavs are upstream of the tub connection and the remaining fixtures connect downstream with the water closet being the last fixture vented by the horizontal wet vent.

One more thing to add; the fixtures to be vented by a wet vent have low discharge surges or surges that last for a short duration. It has been proven that they do not significantly impact the airflow in the horizontal drain. It is because of this that most codes require the water closet to connect downstream of all other fixture drain connections to the horizontal wet vent. With that said, I would state that not all codes or jurisdictions agree with this and that the water closet isn’t always required to be the last fixture.

By now, I’m sure I have a few of you shaking your heads and stamping your feet while mumbling under your breaths that I am out of my mind and have no clue what I’m talking about. Several of you are also pointing fingers at these systems and blurting out things like, “There is no way that would work in my state!” I get it and I can appreciate it. I’m actually laughing to myself as I type this because I did the same thing while doing the research for this article.  


Randy Lorge is a third-generation plumber and the director of workforce training and development for the International Association of Plumbing and Mechanical Officials (IAPMO). Lorge is also a member of the planning team for the International Water, Sanitation and Hygiene Foundation (IWSH). This 501(c) (3) foundation has completed water and sanitation projects for those less fortunate in India, South Africa, Indonesia and, more recently, the United States. He enjoys time with his family and spending as much time as possible in his deer stand. To contact Lorge, email editor@plumbermag.com.


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