The size of the vent stack is based on what criteria?

The size of the vent stack is primarily determined by the drainage fixture unit load and the distance from the main stack. Drainage fixture units (DFUs) provide a standardized way to quantify the demand that various plumbing fixtures place on the drainage system. By assessing the total load from all connected fixtures, the vent stack can be sized appropriately to ensure that it can handle the potential air flow needed to maintain proper trap functioning and prevent siphoning.

Additionally, the distance from the main stack is critical because longer distances can impact the performance of the venting system, requiring adjustments in size to adequately balance the pressures and maintain proper venting characteristics. This ensures that the plumbing system operates efficiently, preventing issues like drain backpressure and poor drainage performance.

While other factors like the type of building, number of stories, and the material of the vent line can influence venting systems, they are not the primary criteria for sizing the vent stack compared to the drainage fixture unit load and distance from the main stack.