What is the maximum length of a horizontal run permitted for a 2-inch vent?

For a 2-inch vent, the maximum permissible length of a horizontal run is indeed 30 feet. This guideline is rooted in plumbing code standards, which dictate that the length of a vent must provide adequate air circulation to effectively balance pressure in the drainage system and ensure efficient flow of wastewater.

The limitation on vent length helps to prevent issues such as siphoning and backpressure, which can lead to drainage problems. Specifically, a longer horizontal run can increase the potential for obstructions and reduce the effectiveness of the venting system. By adhering to the 30-foot maximum, plumbers ensure that the vent maintains its intended function, facilitating the proper discharge of sewer gases and promoting overall efficiency in the plumbing system.

While other lengths suggested in the options (like 20, 25, or 35 feet) might seem reasonable, they do not conform to the established plumbing codes designed to ensure performance and safety in plumbing installations.