AskCodeMan

IPC 2009 Table 912.3 gives sizing information for Combined Drain and Vent Systems.
The Code is not clear if this Drain must be of uniform size for the entire length of the Combined Drain OR if it can be "telescoped" in size based on d.f.u. load.

NOTE: A Circuit Vent (Section 911) must be uniform size for total length.

Thank you,
John.

John,

I suspect you are referring to a Sovent system, or a system similar to it.

If this is correct, then the size is constant all the way up. Here ( place cursor over link, select 'open in new window', http://www.sovent.com/downloads/SE%20SO ... 011007.pdf ) is a design manual for the Sovent system, note that, on page 1-3, the first item in the left column, A1, states: "The stack must be sized by fixture unit load using table B.". You will find Table B at the top of page 1-9.

Is this the type of system you are referring to?

This is the first I have seen of this "Sovent" system.
I would question if this is IPC 2009 Code Compliant????

The drainage system I am referring to is described in IPC Section 912.
The Combined Drain and Vent is a form of Horizontal Wet Venting used only for Floor Drains, Lavatories, Sinks (without food grinders), and Drinking Fountains.

When you said:

cxagent wrote:Combined Drain and Vent Systems.

I thought you were meaning the entire system, not just for one section of the system - my apologies, I should have asked for clarification before answering.

I've inspected a couple of building with those Sovent systems being installed at the time of construction. I am sure they met the requirements of the various affected code sections and chapter, but the simplest way to show that they are allowed by the IPC is by this section:
- 105.2 Alternative materials, methods and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material or method of construction shall be approved where the code official finds that the proposed alternative material, method or equipment complies with the intent of the provisions of this code and is at least the equivalent of that prescribed in this code.

Now, though, being as I am on the same page as you are - back to your question "The Code is not clear if this Drain must be of uniform size for the entire length of the Combined Drain OR if it can be "telescoped" in size based on d.f.u. load."

(bold and underlining are mine)
- 912.2.3 Vent size. The vent shall be sized for the total drainage fixture unit load in accordance with Section 916.2.

- 916.2 Vents other than stack vents or vent stacks. The diameter of individual vents, branch vents, circuit vents and relief vents shall be at least one-half the required diameter of the drain served. The required size of the drain shall be determined in accordance with Table 7 10.1(2). Vent pipes shall not be less than 1 1/4 inches (32 mm) in diameter. Vents exceeding 40 feet (12 192 mm) in developed length shall be increased by one nominal pipe size for the entire developed length of the vent pipe. Relief vents for soil and waste stacks in buildings having more than 10 branch intervals shall be sized in accordance with Section 914.2.

- TABLE 710.1(2) HORIZONTAL FIXTURE BRANCHES AND STACKS

Being as the combination drain and vent system is limited to as to what is allowed to be connected to it, and that "the only vertical pipe of a combination drain and vent system" is the connection between the fixture drain and the horizontal combination drain and vent pipe, and the height of such pipe is limited to 8 feet, the combination drain and vent system is intended for use in limited areas, not as the entire drain and vent system of the building, and with the size based on Table 710.1(2), the size for the combination drain and vent piping would be uniform for the entire combination drain and vent piping.

I agree, though, that the code could address that issue with a less circular route and with more clarity.