UPC 910: Combination Waste and Vent Systems

Section 910 is titled Combination Waste and Vent Systems:

910.0 Combination Waste and Vent Systems

The idea is simple. The horizontal drain pipe is built larger than a normal drain would need. That extra size leaves an open air space above the flowing water. Air moves freely through that space. So the drain vents itself as the waste runs past. There is no separate vent pipe going up into a wall.

The code limits which fixtures can use this method. A combination waste and vent system serves floor drains, sinks, lavatories, and drinking fountains. It does not serve toilets or urinals. Those fixtures push a strong surge of water and need their own vent. Each fixture ties into the horizontal drain through a short vertical tailpiece, not a long drop, so the air space above the flow stays clear.

This system solves the "no wall nearby" problem. Picture a floor drain in the center of a commercial kitchen, a market, or a mechanical room. There is no wall to run a vent up. An oversized combination waste and vent line lets that drain breathe on its own. The same goes for an island sink or a row of floor sinks set out in the open. The oversized drain reaches back to a vent connection near the main line, so the whole run stays vented.

A normal drain runs close to full when a fixture dumps. That leaves little room for air. In a combination system, the pipe is sized up so the water only fills part of it. The open space on top acts like a built-in vent. If the pipe were normal size, the flowing water would seal it off, trap air, and the fixture would gurgle or drain slowly. The extra size is what makes the trick work, so this is not a place to cut pipe diameter to save money.

If a floor drain or island sink gurgles, drains slowly, or smells, the venting may be built wrong. That is a design and pipe-size problem, not a clog you can plunge away. This method also has limits, like a maximum branch length before a real vent is required, so it is worth having a licensed plumber lay it out. For the basics, see what a plumbing vent is. For a related open-fixture fix, see UPC 909 island venting. For the trap and seal rules a floor drain itself must meet, see UPC floor drain requirements. To tell floor drains apart, see floor sink vs floor drain vs mop sink.

UPC Section 910 is part of the Uniform Plumbing Code. IAPMO publishes it and holds the copyright, so only the section heading and short notes are shown here. Phoenix enforces the 2024 UPC with local amendments. Read the section on the UPC viewer at UpCodes, review the official code at IAPMO, or confirm local amendments with the City of Phoenix Planning & Development Department at phoenix.gov/pdd.