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Plumbing Engineer - Columns: February 2011: Designer's Guide

Venting: IPC vs. UPC

By Timothy Allinson, P.E.,
Murray Co., Long Beach, Calif.

As you may have noticed, I usually write about design in the context of the UPC, the Code that governs the work I do here in California. Well, ironically enough, I am doing a project for the Navy, which is governed by the IPC, even though it is California based, because the IPC governs all Navy projects nationwide. This project necessitated that I get up to speed with the latest IPC, and the biggest difference I discovered between the two Codes is that of venting.

For the sake of this article, it is worth noting that I am comparing the 2009 IPC with the UPC of that same year.

In the IPC, vent stacks are required for every waste stack of five branch intervals or more. In the UPC, this requirement doesn’t kick in until branch intervals reach 10 stories or more. Vent stacks must be connected to the base of the waste stack in both Codes. In the UPC, waste and vent stacks must be interconnected with a yoke vent every five floors, while the IPC requires relief vents every ten stories. The IPC also requires a relief vent for stack offset where there are five stories or more above the offset. This relief vent must be connected to both the upper and lower sections of the vent offset.

Horizontal vent terminations are allowed by the IPC, through exterior walls that are 10 feet from the lot line and at least 10 feet above grade and screened from birds, varmints, etc. This is a handy allowance that the UPC does not provide.

Oddly, the IPC requires that vents in freezing climates (0-degree design temp) be insulated above the roof. In contrast, the UPC requires that they be at least 2" in size or larger to satisfy design conditions. Each of these requirements is meant to prevent frost closure from freezing vapor emitted by the vents.
Trap arm distances — downstream side of trap to vent — also vary between Codes, the IPC being more lenient, double in fact. The UPC allows 4', 6' and 8' trap arms for 2", 3" and 4" trap arms, respectively. The IPC allows 8', 12' and 16' for these same trap arm diameters.

Common vents in the UPC are allowed for fixtures at the same elevation, such as back-to-back lavatories. In the IPC, common vents tread into the territory of wet vents as defined by the UPC. The IPC allows common vents to serve two fixtures of different types and elevation provided that they are on the same floor level, such as a lavatory backed up to a urinal. The IPC has a dedicated sizing table for common vents.

Wet venting also differs between the two Codes. The UPC allows vertical wet vents only for one and two DFU fixtures, with a maximum of four fixtures and six feet in length. In contrast, the IPC allows both horizontal and vertical wet venting for any combination of fixtures in two bathroom groups. The IPC has a dedicated wet-vent sizing table. Horizontal wet venting is quite similar in the two Codes, although the verbiage is different. The IPC has a dedicated sizing table that represents approximately what the UPC says in words.

Circuit venting in the IPC is roughly the same thing as combination waste and vent as it appears in the UPC. The primary difference between the two is that circuit venting in as-of-right in the IPC, whereas combination waste and vent is only allowed “where structural conditions preclude the installation of conventional systems” in the UPC. Translation: Combination waste and vent is up to the discretion of the UPC officials.

Combination waste and vent in the IPC refers to a collection of floor drains and/or sinks (not water closets, showers, etc.) piped together to a horizontal drain with one dry vent connected at any point within the system. The sizing of the horizontal combination waste and vent pipe has a dedicated table in the IPC.

Island fixture venting is quite similar between the two Codes, the difference being that the UPC requires the island vent to connect to the horizontal waste below the island, while the IPC requires the island vent to connect to the horizontal waste below a nearby wall. Required cleanouts also vary in location in this same respect. However, while the IPC does not specifically say so in the island fixture section, I would think this to be an ideal application for an air admittance valve.

The vent sizing tables differ dramatically between the two Codes, making a comparison somewhat difficult. To give a brief example, in the UPC, a 3" vent stack can carry 84 DFUs and be no more than 212 feet long. The IPC has a sliding scale of length versus fixture units, but to compare it with the UPC, a 3" vent 84 feet in length would be allowed almost 190 DFUs — more than double the UPC.

The IPC goes on to address the concept of waste stack venting (similar to what is known as the single stack system in Philadelphia) not addressed or allowed by the UPC. This design allows a single stack to serve as waste and vent for fixtures other than water closets and urinals. Again, there is a dedicated sizing chart for waste stack venting in the IPC.

Lastly, the IPC acknowledges computerized vent design, as long as it is determined by an approved computer program. This computerized design can result in dramatically reduced vent sizes as small as ½" and presumably made of copper for sizes of 1½" or less.

All told the IPC seems to be a very progressive Code with respect to vent sizing. However, on the project I mentioned at the beginning of this article, one of the first comments we received from the plan reviewer was, and I quote, “Circuit vents and wet vents will not be allowed. Even though the IPC allows it to be installed on Army and Air Force facilities, it will not be allowed on this Naval facility. This facility is to be flexible and expandable, which a circuit vent and wet vent system does not allow. Design a standard waste and vent system for entire facility.” So, regardless of which Code you use, the vent design can only be as progressive as the Authority Having Jurisdiction.

Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an mba from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters. He can be reached at

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