A combination drain and vent system shall not serve fixtures other than floor drains, sinks, lavatories and drinking fountains. Combination drain and vent systems shall not receive the discharge from a food waste grinder or clinical sink.
The only vertical pipe of a combination drain and vent system shall be the connection between the fixture drain and the horizontal combination drain and vent pipe. The maximum vertical distance shall be 8 feet (2438 mm). A combination drain and vent system sized in accordance with Table 912.3 shall be unlimited in horizontal length.
The horizontal combination drain and vent pipe shall have a maximum slope of one-half unit vertical in 12 units horizontal (4-percent slope). The minimum slope shall be in accordance with Table 704.1.
The combination drain and vent system shall be provided with a dry vent connected at any point within the system or the system shall connect to a horizontal drain that is vented in accordance with one of the venting methods specified in this chapter. Combination drain and vent systems connecting to building drains receiving only the discharge from a stack or stacks shall be provided with a dry vent. The vent connection to the combination drain and vent pipe shall extend vertically a minimum of 6 inches (152 mm) above the flood level rim of the highest fixture being vented before offsetting horizontally.
The vent shall be sized for the total drainage fixture unit load in accordance with Section 916.2.
The fixture branch or fixture drain shall connect to the combination drain and vent within a distance specified in Table 906.1. The combination drain and vent pipe shall be considered the vent for the fixture.
The minimum size of a combination drain and vent pipe shall be in accordance with Table 912.3.
TABLE 912.3 SIZE OF COMBINATION DRAIN AND VENT PIPE
TABLE 912.3 SIZE OF COMBINATION DRAIN AND VENT PIPE
| DIAMETER PIPE (inches) | MAXIMUM NUMBER OF DRAINAGE FIXTURE UNITS (dfu) | |
| Connecting to a horizontal branch or stack | Connecting to a building drain or building subdrain | |
| 22 | 3 | 4 |
| 21/2 | 6 | 26 |
| 3 | 12 | 31 |
| 4 | 20 | 50 |
| 5 | 160 | 250 |
| 6 | 360 | 575 |
For SI: 1 inch = 25.4 mm.