8.4 Island Fixture Venting, Combination Waste and Vent Systems, & Engineered Vent Systems (UPC 909, 910, 912)

Island Fixture Venting (UPC 909.1)

A sink in a kitchen island has no wall to carry a vent up through. Section 909.1 allows a returned (loop) vent:

1. Trap above the floor: The trap is roughed in above the floor.
2. Rise: The vent rises from the fixture drain as high as possible, but not lower than the drainboard height.
3. Return bend: Under the drainboard, the vent turns down with a one-piece return fitting or an assembly of a 45°, a 90°, and a 45° elbow, in that order.
4. Return connection: The vent comes back down and connects to the horizontal sink drain immediately downstream of the vertical fixture drain, through a wye-branch fitting.
5. Foot vent: A foot vent is taken off the vertical fixture vent through a wye branch immediately below the floor. It extends to the nearest partition and then through the roof, or it connects to other vents at a point at least 6 inches above the flood-level rim of the fixtures served.
6. Drainage-grade vent: Below the floor, the vent uses drainage fittings and slopes at least 1/4 inch per foot back to the drain, because it can receive water if the drain backs up.
7. Cleanout: An accessible cleanout goes in the vertical portion of the foot vent.
8. Dedicated drain: The island sink drain upstream of the returned vent serves no other fixtures.

Pipe sizes follow the normal rules for drains and vents. For Part 2 drawings, recognize the loop shape and check each element: the wye connections, the foot vent, the cleanout, and the slope.

Why the wye and the foot vent? If the sink drain stops, water can rise into the returned vent. The drainage-pattern fittings and slope let it drain back out. The foot vent keeps a true air connection to the vent system even if the loop fills.

Combination Waste and Vent Systems (UPC 910)

The UPC defines a combination waste and vent system as a specially designed system of waste piping that horizontally wet vents one or more sinks or floor drains through a common waste and vent pipe. The pipe is sized large enough that air moves freely above the flow line. It is used for large floor areas, such as markets and warehouses, where floor drains and sinks are far from walls.

Appendix B (adopted in Washington) gives explanatory notes on designing these systems.

Example: A floor drain normally needs a 2-inch trap and drain. On a combination waste and vent system, two pipe sizes larger is 3 inches, counting 2 to 2-1/2 as one size and 2-1/2 to 3 as the second. If the combination drain is 3 inches, its vent must be at least one-half of the 3-inch area: 7.07 ÷ 2 = 3.53 square inches. A 2-inch vent is only 3.14, so use 2-1/2 inches (4.91).

Engineered Vent Systems (UPC 912)

• 912.1: A vent system may be designed and sized by accepted engineering practice. It must be designed by a registered design professional and approved under 301.5, alternative engineered design.
• 912.2: It must still protect trap seals to the 1-inch water column limit of 901.3.

Single-Stack Systems

Section 310.4 prohibits single-stack drainage and venting systems with unvented branch lines. Washington's amendment to 310.4 keeps the exceptions of 908 through 911 and Appendix C. A combination waste and vent system is one of those exceptions. An unvented branch on a single stack is not.

Quick Comparison of Special Venting Methods