Methods to Venting Plumbing Fixtures and Traps in the 2009 International Plumbing Code - Part 4

By Lee Clifton, PMG, Director of Plumbing Programs, International Code Council

IPC venting methods save resources and lower costs while protecting the health of the public.

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Part 4 of a Four-Part Series

In Part 3 of this series, we continued to discuss various approaches to venting that are permitted in the 2009 IPC. In Part 4, our final article of this series, we are going to take a look at circuit venting and combination, drain and vent systems. You will find that these venting provisions also offer the installer and designer different paths to achieving an adequately vented system and could result in cost savings along with ease of installation in many types of construction.  

Circuit Venting
The International Plumbing Code® (IPC) allows use of a variety of methods to vent plumbing fixtures and traps. Circuit venting is one that has been laboratory and field tested, establishing a long history of satisfactory service. The State University of Iowa conducted extensive research into the performance of circuit-vented systems. The research concluded that the single vent for the eight fixtures provided the necessary protection of the trap seal. Circuit venting was included in Roy B. Hunter’s research at the National Bureau of Standards and reported in BMS 66. The plumbing community has long recognized this venting method and it is included in the American Society of Plumbing Engineers Data Book.

The principle of circuit venting is that the flow of drainage never exceeds a half-full-flow condition. The air for venting the fixtures circulates in the top half of the horizontal branch drain pipe. The flow velocity in the horizontal branch is slow and non-turbulent, thereby preventing pressure differentials from affecting the connecting fixtures. The circuit-vented fixtures must connect to the circuit-vented branch in the horizontal plane to limit the amount of turbulence created by fixture discharge.

Table 906.1
Maximum Distance of Fixture Trap from Vent

Size of Trap

(inch per foot)

Distance From Trap

1 1/4



1 1/2













The circuit venting method is similar to wet venting except that it allows you to combine a total of eight fixtures on a single floor that are not limited to the two bathroom groups. It might be easier to explain by examining how circuit venting differs from wet venting. The fixture drains shall connect horizontally to the horizontal branch being circuit vented. Again, the fixture drains are limited in length to those shown in Table 906.1 above.

Figure 1: Sizing a Circuit Vent System: To see a complete, full-size version of this illustration, click here.

Because circuit venting is only to be used on horizontal applications as opposed to wet venting, which can be both horizontal and vertical installations, the maximum slope for a circuit vent is one unit in 12 units horizontally, or an 8-percent slope. The entire length of the circuit vent portion of the horizontal branch shall be sized for the total drainage discharge to the branch. There is not a unique sizing table for circuit venting as there is for wet venting or common venting.

The circuit vent connection must be located between the two uppermost fixture drains and shall connect to the horizontal branch. It cannot serve as a drain for other fixtures; it is truly a dry vent. Where a circuit vent consists of four or more water closets and discharges into a drainage stack that also receives the discharge of upper horizontal branches, a relief vent shall be connected to the horizontal branch ahead of the connection to the drainage stack and after the most downstream fixture drain of the circuit vent (see Figure 2).

Figure 2: Circuit Vent with Relief Vent Connection. To see a complete, full-size version of this illustration, click here.

Additional fixture drains may be connected with the circuit vented branch, but they need to be vented by means other than the circuit vent, and the fixture-unit values would be added to the total fixture-unit discharge into the horizontal branch. Such fixtures also must be located on the same floor as the circuit vent to which they connect. Where the relief vent receives the discharge of other fixtures, the maximum discharge allowed is four drainage fixture units.

Combination Drain and Vent System
This system is based on the same premise as the circuit-vented system. Most plumbing codes place arbitrary restrictions on combination drain (waste) and vent systems because the system appears too good to be true. The performance of the combination drain and vent system was verified in tests conducted at Stevens Institute of Technology. If sized according to Table 912.3 in the IPC (see Figure 3 below), the study concluded that the distance from a trap does not have to be limited in length. This is a horizontal wet vent system limited to floor drains, standpipes, sinks and lavatories for venting, except that a vertical riser – not to exceed 8 feet in length – may be used to connect a fixture drain to the horizontal combination drain and vent system. Again, the idea here is that the top half of the horizontal drain acts as a vent. As long as both the horizontal drain and vent system and the maximum 8-foot riser to a fixture drain are sized in accordance with Table 912.3, the flow of free air will be sufficient for the propped design.

Table 912.3
Size of Combination Drain and Vent Pipe



Maximum Number of Drainage Fixture Units

Connection to a horizontal branch or stack

Connection to a building drain or building sub-drain



















Figure 3

Of course, to get that free flow of air, a vent to the atmosphere must be provided. The vent, which must be sized for the total drainage fixture load of the combination drain and vent system per Section 916.2 of the IPC, can be located anywhere on the system and must rise vertically at least 6 inches before offsetting. This type of system is unique in that a branch that is already vented can accept a fixture drain under this combination drain and vent method (see Figure 4). Remember, too, that this type of venting procedure is dependent on adequate sizing and maintaining a horizontal installation. For this reason, the horizontal portion must not exceed a slope of ½-unit vertical in 12 units horizontal, or a 4-percent slope.

Figure 4: To see a complete, full-size version of this illustration, click here.

Many experienced plumbers and Inspectors will agree that the venting requirements of a plumbing system are usually the most difficult to comprehend. The IPC contains more types of venting methods than other plumbing codes, but as you continue to examine these methods and illustrations I have provided in this series of articles, you will begin to comprehend them and will likely see how they provide a viable alternative to those with which you are probably most familiar.

The PMG Resource Center is dedicated to assisting plumbing, mechanical and fuel gas professionals with code and related product and service support, as well as using the collective expertise of the entire Code Council to help further sanitation and other PMG advancements across the country and around the globe. Contact the Resource Center today by calling 1-888-ICC SAFE, 4PMG or emailing us at

Click on links below to read the other articles in this series.

Methods to Venting Plumbing Fixtures and Traps in the 2009 IBC, Part 1, February issue

Methods to Venting Plumbing Fixtures and Traps in the 2009 IBC, Part 2, April issue

Methods to Venting Plumbing Fixtures and Traps in the 2009 IBC, Part 3, June issue