Circuit Venting Code Explained

Knowing the code for circuit vents can be tricky with the different variations, but needs to be known and followed.

Circuit Venting Code Explained

Figure 1

Sometimes things have different definitions. Take, for example, circuit venting.

Take your pick:

Circuit vent. The vent that connects to a horizontal drainage branch and vents two traps to a maximum of eight traps connected into a battery of fixtures.

Circuit vent means a method of venting two to eight traps or trapped fixtures without providing an individual vent for each trap or fixture.

Circuit vent. A vent that connects to a horizontal drainage branch and vents two traps to not more than eight traps or trapped fixtures connected into a battery.

Vent, circuit: The vent for a group of battery-vented fixtures that is connected to their horizontal drain branch between the first two upstream fixtures and extends to a vent stack or other vertical vent.

No matter which code you use to define it, circuit venting is a very useful system, which uses one vent to vent up to as many as eight plumbing fixtures. Some codes will allow the fixtures to be both wall outlet and floor outlet fixtures while others limit the system to floor outlet fixtures only. 

The circuit vent dates back to the early 1920s and as many things do, has evolved over the years and been adjusted and slightly tweaked to meet the times and more often than not, the interpretation of code officials. Regardless, it still stands as a very functional venting system almost 100 years later!

You should always refer to the specific code you are using in your jurisdiction, but the following is an overview of the circuit vent system and the proper installation and sizing of it.

The circuit vent can be used to vent at least two but not more than eight fixtures. Each trap arm shall connect horizontally to the branch being circuit vented. The horizontal branch to which the fixtures connect is considered the drain and the vent from the most downstream trap arm connection to the most upstream trap arm connection as illustrated below (see Figure 1).

The circuit vent is required to connect between the two most upstream fixtures. While there are variations of what is considered “between” the two most upstream fixtures based on the code being used, the illustration below (see Figure 2) shows a widely accepted practice.

Some codes allow multiple circuit-vented systems to be interconnected provided of course that the systems are sized appropriately. (See Figure 3)

A relief vent is typically required when the circuit-vented, horizontal branch drain receives the discharge of four or more water closets and connects to a drain stack which receives the discharge of soil or waste from upper horizontal branches. The relief vent is required to connect downstream of the most downstream fixture being circuit vented and the connection to the drain stack. (See Figure 4)

The vented section of the horizontal branch is required to be uniformly sloped. It is also required to be sized for the total discharge into the branch and is not permitted to diminish in size from the circuit vent connection to the most downstream fixture being circuit vented. (See Figure 5)

Most codes allow the discharge of additional fixtures “into” the circuit-vented, horizontal branch drain provided those fixtures are individually or common vented with a drainage fixture unit (dfu) of one or less. (See Figure 6)

As I mentioned earlier, depending on which code is being used, the circuit and the relief vent are allowed to be wet. Again, the fixtures discharging in these vents are restricted to one dfu or less fixtures as can be seen in the next figure. (See Figure 7)

The circuit vent can be installed in both public and nonpublic applications. Always refer to the specific code you are using and be sure to install clean-outs where required.

I think I have mentioned it before, but I believe it’s worth saying it again. Plumbing codes are developed to ensure that plumbing is installed in a manner that will protect the health and safety of the public. No matter where you live or the code you follow, that fundamental principle is the most important. 

As a plumber, it falls upon you and I to follow the code. If you don’t agree with the code, then you need to get involved with the code change process. You need to do the research and justify why the code should be changed. Either follow the code the way it is written or find a way to help make it better, take your pick.  


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 recent, the United States. He enjoys time with his family and spending as much time as possible in his deer stand. To contact Lorge, email


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