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.