Which factors affect vent size?

• A. Size of Soil Stack
• B. DFUs Connected
• C. Length of Vent
• D. All of the Above

Answer: (D)

Vent size is driven by how much air the system needs to prevent trap siphonage as drainage occurs. That demand comes from three things: the number of fixtures connected (the fixture unit load), how long the vent run has to be (the distance air must travel to reach the other traps and the stack), and the size of the soil stack that the vent serves. More fixtures mean more air is needed, a longer vent run means more resistance to air flow and may require a larger diameter, and a larger soil stack can create bigger pressure changes that the vent must accommodate. Because each of these factors can change the amount of air needed to keep traps from siphoning, all of them influence the required vent size.