What is the maximum horizontal run allowed for a 1-1/4" vent pipe?

The maximum horizontal run allowed for a 1-1/4" vent pipe is indeed 15 feet. This specification is based on plumbing codes designed to ensure proper venting and to prevent issues such as drainage problems due to inadequate venting pressure.

Vent pipes are crucial in maintaining the pressure balance in the drainage system and preventing sewer gases from entering the living spaces. The allowable horizontal run is limited to ensure that the venting system remains effective. If the run exceeds the specified length, it could lead to inadequate venting, which may cause drainage issues and potential backflow of gases, thereby impacting the hygiene and safety of the plumbing system.

Understanding these limitations helps plumbing contractors design systems that are not only compliant with building codes but also efficient in maintaining proper drainage and air balance in plumbing systems.