2021 IPC Chapter 9
Lesson 11 covered where vents end and how far a trap can sit from its vent. This lesson covers the different ways to give a group of traps their air: individual, common, wet, waste stack, circuit, combination and island venting, plus air admittance valves. Then it covers how to size the vent pipe. Each method has tight limits, so an inspector asks two questions: is this fixture allowed here, and is the pipe big enough for its load? Houston enforces the Uniform Plumbing Code (Lesson 1), which vents differently in places, so these are 2021 IPC rules to check against the adopted code.
An individual vent serves one trap and connects to that fixture's drain (910.1). A common vent serves two traps on the same floor (911.1). Where the two fixture drains connect at the same level, the vent connects at or downstream of their junction (911.2). Where they connect at different levels, the vent is a vertical extension of the drain, the vertical drain acts as the vent for the lower fixture, and the upper fixture cannot be a water closet (911.3). Table 911.3 limits the upper fixture drain: 1 dfu on a 1 1/2 inch pipe, 4 dfu on 2 inch, and 6 dfu on 2 1/2 to 3 inch.
A wet vent is a drain that also carries air for its fixtures. In the 2021 IPC, any combination of fixtures within two bathroom groups on the same floor may be vented by a horizontal wet vent (912.1) or a vertical wet vent (912.1.1). Each wet-vented fixture connects independently, and fixtures outside the two groups discharge downstream of the wet vent. The dry vent that serves the wet vent is an individual or common vent for a bathroom group fixture (never an emergency floor drain). In a horizontal wet vent not more than one wet-vented fixture drain may enter upstream of the dry vent connection (912.2.1). In a vertical wet vent, the water closet drains join at the same elevation and the other fixtures join at or above them (912.1.1), and the dry vent serves the most upstream fixture drain (912.2.2). The dry vent is sized on the largest pipe in the wet vent system, and the wet vent is sized by the fixture units draining to it (912.3, Table 912.3):
| Wet vent pipe | 1 1/2 in | 2 in | 2 1/2 in | 3 in |
|---|---|---|---|---|
| Fixture load it may carry | 1 dfu | 4 dfu | 6 dfu | 12 dfu |
Wet venting turns up most in multifamily and dormitory bathrooms.
A vertical waste stack can vent all the fixtures that connect to it, if there are no offsets between its lowest and highest fixture connection, each fixture drain connects separately, and the stack receives no water closets or urinals (913.2). It needs a stack vent at least as large as the stack, and the stack is one size for its full length (913.3, 913.4). Table 913.4 caps the load: a 2 inch stack takes 2 dfu at one branch interval and 4 in total, a 3 inch stack has no interval limit and 24 dfu in total, and a 4 inch stack allows 50. It suits a run of lab or classroom sinks.
Circuit venting covers a horizontal branch of not more than eight fixtures, such as a restroom. Each fixture drain connects horizontally, and the branch itself acts as the vent from the most downstream fixture to the most upstream one (914.1). The vent connects between the two most upstream fixture drains, and it carries no waste (914.2). The vent section of the branch slopes no more than 1 unit in 12 (about 1 inch per foot) and is sized for the whole load (914.3). Where the branch receives four or more water closets and connects to a stack that receives discharge from upper branches, a relief vent is required, connected between the stack and the most downstream fixture (914.4). A relief vent that is also a fixture drain may carry no more than 4 dfu (914.4.2). Other same-floor fixtures may join if individually or common vented (914.5).
A combination waste and vent system serves only floor drains, sinks, lavatories and drinking fountains, never a clinical sink (915.1). The only vertical pipe is the drop from the fixture to the horizontal pipe, no more than 8 feet (915.2). The horizontal pipe slopes no more than 1/2 inch per foot and no less than Table 704.1 requires, and its length is unlimited (915.2.1, 915.2.2). It needs a dry vent, sized by 906.2 (915.2.3, 915.2.4). Sizes run from 2 inches (3 dfu on a branch or stack) to 4 inches (20 dfu) in Table 915.2.2. Island fixture vents serve only sinks and lavatories (916.1). The vent rises above the drain outlet before it offsets, and a vent below the fixture rim is built like drainage piping, with cleanouts, connecting full size to a vertical pipe or the top half of a horizontal one (916.2, 916.3).
An air admittance valve (AAV) is a one-way valve that lets air in and keeps sewer gas out. Stack-type valves meet ASSE 1050 and individual and branch types ASSE 1051 (918.1). They go in after the drain, waste and vent test (918.2). Individual and branch valves vent only same-floor fixtures on a horizontal branch, and a branch more than four intervals below the top of the stack also needs a relief vent (918.3.1). A stack-type valve may not terminate a stack with more than six branch intervals (918.3.2). An individual or branch valve sits at least 4 inches above the branch or fixture drain, a stack-type valve at least 6 inches above the flood level rim of the highest fixture, and any valve at least 6 inches above insulation (918.4). Valves need access and free air (918.5), and each system keeps at least one stack vent or vent stack open outdoors (918.7). AAVs are not allowed in plenums, on sumps or tanks unless engineered, or on outdoor terminals just to reduce clearances to air intakes (918.8). Local rules may limit them further, so check.
Section 917 lets a properly sized stack serve as its own vent (a single-stack system, for example a 3 inch stack with two water closets and 24 dfu at most in a stack under 75 feet). Section 919 covers engineered vent systems under Section 316.
A stack vent or vent stack is sized from the total dfu on the drainage stack and the developed length of the vent to open air, using Table 906.1. It is never smaller than half the drain it serves or 1 1/4 inches (906.1). Other vents (individual, branch, circuit and relief) are at least half the size of the drain they serve, sized by Table 710.1(2), and never under 1 1/4 inches. A vent longer than 40 feet grows by one nominal pipe size for its entire length (906.2). The developed length runs from the farthest connection to the drain to the vent stack, stack vent or outdoors (906.3). A common branch vent is sized for the drain branch that would carry the total dfu (906.4). Sump vents follow Table 906.5.1.
Here are two rows of Table 906.1, for a 4 inch stack (maximum developed length of vent, feet):
| 4 in stack load | 2 in vent | 2 1/2 in vent | 3 in vent | 4 in vent |
|---|---|---|---|---|
| 43 dfu | 35 | 85 | 250 | 980 |
| 140 dfu | 27 | 65 | 200 | 750 |
Worked example 1, vent stack. A four-story clinic has a 4 inch stack collecting 140 dfu. The vent stack runs 90 feet, developed length. From the 140 dfu row, a 2 1/2 inch vent tops out at 65 feet, which is not enough. A 3 inch vent allows 200 feet, so 3 inches passes. It also clears the floor of half the stack size (2 inches) and the 1 1/4 inch minimum.
Worked example 2, long individual vent. A 2 inch fixture drain needs a vent of at least half its size, 1 inch, but the code minimum is 1 1/4 inches. If that vent runs 45 feet, it exceeds 40, so it grows one nominal size to 1 1/2 inches for its full length.
Worked example 3, circuit vent. The figure shows a men's room branch: four public water closets (4 x 4 = 16), two urinals rated at 4 dfu each because they use more than 1 gallon per flush (8) and two lavatories (2), for 26 dfu on eight fixtures, the maximum for one circuit. A urinal rated at 1 gallon per flush or less counts 2 dfu (Table 709.1). A 3 inch branch holds 20 dfu, so the branch is 4 inches (limit 160). The circuit vent is half of 4 inches, or 2 inches, and becomes 2 1/2 inches if it exceeds 40 feet. Because the branch has four water closets and connects to a stack carrying upper floors, it also needs a relief vent (2 inches by the same rule).
In the field
A plan set shows nine fixtures on one circuit-vented branch in a school restroom, with the vent tied in between fixtures 5 and 6. Two problems: eight fixtures is the limit per circuit, and the vent belongs between the two most upstream fixture drains. Ask the designer to split the branch into two circuits (914.1.1 lets them join). At rough-in, also look for a relief vent when four or more water closets feed a stack that takes upper-floor branches.
1. Two fixture drains in a Group B clinic connect at different levels to a common vent. The upper fixture drain is 1 1/2 inches. Under Table 911.3, the greatest discharge it may carry is:
2. Two fixtures are common vented at different levels in a Group E school. Under Section 911.3, the upper fixture may not be a:
3. In a Group R-2 apartment building, how many bathroom groups may be vented by one horizontal wet vent under Section 912.1?
4. A horizontal wet vent in a Group R-2 apartment suite receives a discharge of 5 dfu. What is the smallest wet vent pipe Table 912.3 allows?
5. A designer proposes a waste stack vent for a school (Group E) stack that serves classroom sinks and one urinal. Under Section 913.2, this is:
6. A 3 inch waste stack vent in a laboratory building (Group B) is loaded with 30 dfu in total. Under Table 913.4, this stack:
7. Under Section 914.1, not more than how many fixtures connected to one horizontal branch drain may be circuit vented?
8. Where must the circuit vent connect to a circuit-vented horizontal branch under Section 914.2?
9. A circuit-vented branch in a Group B office receives four water closets and connects to a drainage stack that carries discharge from upper branches. Under Section 914.4, the branch:
10. Which fixture may be served by a combination waste and vent system under Section 915.1?
11. In a combination waste and vent system serving a Group B lab, what is the greatest vertical distance allowed in the connection between the fixture drain and the horizontal combination pipe (Section 915.2)?
12. An individual-type air admittance valve is being installed on a lavatory branch in a Group B restroom. Under Section 918.4, the valve must be located at least how far above the branch drain being vented?
13. An individual vent for a 2 inch fixture drain in a Group B building has a developed length of 45 feet. Applying Section 906.2, what is the smallest vent size?
14. A four-story clinic (Group B) has a 4 inch drainage stack carrying 140 dfu. The vent stack has a developed length of 90 feet. Using Table 906.1, what is the smallest vent stack size?