Which factor is considered when sizing vent piping?

• A. Size of Soil Stack
• B. Length of Vent
• C. DFUs Connected
• D. Material of Piping

Answer: (B)

The main idea is that vent sizing is driven by how far the air must travel to reach the drainage system. The length of the vent run directly affects how much air is needed and how easily air can reach the fixtures to prevent trap siphon. The longer the vent, the more potential there is for air to be delayed or impeded, so the vent diameter may need to be larger to maintain proper venting along the entire run.

Other factors aren’t the primary driver for vent sizing. The size of the soil stack relates to drainage capacity rather than how much air the vent must supply, and while the number of fixture units connected influences overall drainage/vent requirements, the specific consideration that dictates vent diameter for the run is the vent’s length. Material of piping doesn’t change the air-flow capacity in a way used by vent-sizing tables.