13.2 Waste Stack Vents, Circuit Vents, and Combination Waste and Vent
Key Takeaways
- IPC 913.2 requires a waste stack vent system to be vertical with no offsets between the lowest and highest fixture drain connections, and prohibits it from receiving the discharge of water closets or urinals.
- IPC 914.1 limits circuit venting to not more than eight fixtures connected to a horizontal branch drain, each connecting horizontally to the branch.
- IPC 914.2 requires the circuit vent connection to be located between the two most upstream fixture drains, and prohibits the circuit vent pipe from receiving soil or waste.
- IPC 914.4 requires a relief vent for circuit-vented horizontal branches receiving four or more water closets and connecting to a stack that receives discharge from upper horizontal branches, limited to 4 dfu.
- IPC 915.1 limits combination waste and vent systems to floor drains, sinks, lavatories and drinking fountains, and prohibits clinical sink discharge.
Waste stack vent — IPC Section 913
A whole stack acting as its own vent.
913.1: A waste stack shall be considered to be a vent for all of the fixtures discharging to the stack where installed in accordance with the requirements of this section.
913.2 Stack installation — four hard conditions
The waste stack shall be vertical, and both horizontal and vertical offsets shall be prohibited between the lowest fixture drain connection and the highest fixture drain connection. Fixture drains shall connect separately to the waste stack. The stack shall not receive the discharge of water closets or urinals.
- Vertical, with no offsets of any kind in the fixture-connection zone.
- Each fixture drain connects separately.
- No water closets.
- No urinals.
Conditions 3 and 4 are the reason this method exists at all: without the massive surge of a flushometer or a tank closet, the stack keeps a stable air core and can vent itself.
913.3 Stack vent
A stack vent shall be provided for the waste stack. The size of the stack vent shall be not less than the size of the waste stack. Offsets shall be permitted in the stack vent, shall be located not less than 6 inches above the flood level of the highest fixture, and shall be in accordance with Section 905.2. The stack vent may connect with other stack vents and vent stacks per Section 904.5.
Offsets are prohibited in the waste stack but permitted in the stack vent above the fixtures.
913.4 Waste stack size — Table 913.4
The waste stack shall be sized based on the total discharge to the stack and the discharge within a branch interval. The waste stack shall be the same size throughout its length.
| Stack size | Total discharge into one branch interval | Total discharge for stack |
|---|---|---|
| 1-1/2 in | 1 | 2 |
| 2 in | 2 | 4 |
| 2-1/2 in | No limit | 8 |
| 3 in | No limit | 24 |
| 4 in | No limit | 50 |
| 5 in | No limit | 75 |
| 6 in | No limit | 100 |
Two features to notice: the branch interval limit disappears at 2-1/2 inches and above, and the stack totals are far smaller than Table 710.1(2) allows for an ordinary vented stack. A 4-inch waste stack vent carries 50 dfu; a 4-inch conventionally vented stack over three branch intervals carries 500. You buy the simplicity of self-venting with a tenfold reduction in capacity.
Recall also IPC 904.2's exception: a drainage stack installed per Section 913 does not require a vent stack even at five or more branch intervals.
Circuit venting — IPC Section 914
One vent for a row of fixtures on a horizontal branch — the standard method for a bank of commercial water closets.
| Rule | Requirement |
|---|---|
| 914.1 | Not more than eight fixtures connected to a horizontal branch drain may be circuit vented. Each fixture drain connects horizontally to the branch. The horizontal branch drain is classified as a vent from the most downstream fixture drain connection to the most upstream fixture drain connection |
| 914.1.1 | Circuit-vented branches may be connected together; each group of not more than eight fixtures is a separate circuit vent |
| 914.2 | The circuit vent connection is located between the two most upstream fixture drains; the circuit vent pipe shall not receive the discharge of any soil or waste |
| 914.3 | The slope of the vent section of the horizontal branch shall be not greater than 1 unit vertical in 12 units horizontal (8.3 percent); the entire vent section is sized for the total drainage discharge to the branch |
| 914.3.1 | Interconnected circuit-vented branches are sized independently; the downstream branch is sized for the total discharge including upstream branches |
| 914.5 | Additional fixtures may discharge to the horizontal branch drain if they are on the same floor and are individually or common vented |
The relief vent — IPC 914.4
A relief vent shall be provided for circuit-vented horizontal branches receiving the discharge of four or more water closets and connecting to a drainage stack that receives the discharge of soil or waste from upper horizontal branches.
Both conditions must be met. Four closets on a branch at the top of a stack, with nothing above, needs no relief vent.
914.4.1 Connection: the relief vent connects to the horizontal branch drain between the stack and the most downstream fixture drain of the circuit vent, installed per Section 905.
914.4.2 Fixture drain or branch: the relief vent may be a fixture drain or fixture branch for fixtures within the same branch interval. The maximum discharge to a relief vent shall be 4 fixture units.
Worked circuit vent
A public restroom has six water closets on a horizontal branch that connects to a stack serving three floors above.
- Eight-fixture limit: six fixtures. Compliant (914.1).
- Circuit vent location: between the two most upstream water closet drains (914.2).
- Relief vent: six closets is four or more, and the stack receives discharge from upper branches. A relief vent is required (914.4), connecting between the stack and the most downstream closet drain, carrying not more than 4 dfu if it doubles as a fixture drain.
- Branch sizing: six public 1.6 gpf water closets at 4 dfu each = 24 dfu. Table 710.1(2) horizontal branch: 3-inch carries 20 (too small), 4-inch carries 160. Use 4 inches for the entire vent section (914.3).
- Slope: not greater than 1 in 12 through the vent section, and not less than Table 704.1's 1/8 inch per foot for 4-inch pipe.
- A lavatory row on the same branch: permitted under 914.5 if the lavatories are on the same floor and individually or common vented.
Combination waste and vent — IPC Section 915
A horizontal oversized drain that serves as its own vent, used where a vent cannot be run — the middle of a large floor, under a slab, in a lab bench island.
915.1 Type of fixtures
A combination waste and vent system shall not serve fixtures other than floor drains, sinks, lavatories and drinking fountains. Combination waste and vent systems shall not receive the discharge from a clinical sink.
Four fixture types, and no clinical sinks. No water closets, no urinals, no showers, no bathtubs.
915.2 Installation
The only vertical pipe of a combination waste and vent system shall be the connection between the fixture drain and the horizontal combination waste and vent pipe. The vertical distance shall not exceed 8 feet.
| Rule | Requirement |
|---|---|
| 915.2.1 Slope | Not greater than 1/2 unit vertical in 12 units horizontal (4 percent) and not less than Table 704.1 |
| 915.2.2 Size and length | Size per Table 915.2.2; horizontal length is unlimited |
| 915.2.3 Connection | A dry vent connected at any point within the system, or the system connects to a horizontal drain serving vented fixtures on the same floor. Systems connecting to building drains receiving only the discharge from one or more stacks shall be provided with a dry vent. The vent connection extends vertically to not less than 6 inches above the flood level rim of the highest fixture being vented before offsetting horizontally |
| 915.2.4 Vent size | The vent is sized for the total dfu load per Section 906.2 |
| 915.2.5 Fixture branch or drain | The fixture branch or drain connects to the combination waste and vent within the distance specified in Table 909.1. The combination waste and vent pipe is considered to be the vent for the fixture |
Table 915.2.2 Size of combination waste and vent pipe:
| Pipe diameter | Connecting to a horizontal branch or stack | Connecting to a building drain or building subdrain |
|---|---|---|
| 2 in | 3 | 4 |
| 2-1/2 in | 6 | 26 |
| 3 in | 12 | 31 |
| 4 in | 20 | 50 |
| 5 in | 160 | 250 |
| 6 in | 360 | 575 |
The second column is dramatically larger than the first, because a system discharging directly to a building drain or subdrain is not competing with stack surge.
Worked combination waste and vent
A warehouse needs four floor drains and two sinks in the middle of a slab, 90 feet from the nearest wall where a vent could rise.
- Fixture types: floor drains and sinks. Permitted (915.1).
- Load: 4 floor drains at 2 dfu + 2 sinks at 2 dfu = 12 dfu.
- Connection point: to the building drain. Table 915.2.2, building drain column: 2-1/2 inch carries 26 dfu. 2-1/2 inches suffices, though 3 inches or 4 inches is common for rodding.
- Slope: not greater than 4 percent, not less than Table 704.1 — for a 3-inch pipe, 1/8 inch per foot.
- Horizontal length: unlimited (915.2.2), which is the whole point.
- Vertical pipe: only between each fixture drain and the horizontal combination pipe, not exceeding 8 feet (915.2).
- Dry vent: required because the system connects to a building drain; it rises to 6 inches above the flood level rim of the highest fixture before offsetting, sized for 12 dfu per 906.2.
- Trap arms: each fixture connects within the Table 909.1 distance — 8 feet for a 2-inch trap (915.2.5).
Which fixtures may NOT discharge to a waste stack vent system under IPC 913.2?
What is the maximum number of fixtures that may be circuit vented on a horizontal branch drain?
Where is the circuit vent connection located under IPC 914.2?
Which fixtures may a combination waste and vent system serve under IPC 915.1?