Which type of material is never acceptable for use in underground waste and vent systems?

Type K copper is never acceptable for use in underground waste and vent systems primarily due to concerns over deterioration and corrosion. In plumbing applications, particularly those involving waste and vent systems that may contain aggressive waste materials, the longevity and resistance to corrosion of the piping material are crucial.

While copper has favorable properties in certain applications, it is not suitable for underground installations where exposure to moisture, soil chemicals, and potential galvanic reactions could lead to pipe degradation over time. Additionally, plumbing codes often specify materials that must be used in these systems based on their durability, installation requirements, and performance reliability.

In contrast, materials like bell and spigot cast iron, Schedule 80 PVC, and ABS plastic have properties making them viable choices for underground installations. They are generally resistant to corrosion and can handle the conditions typical of underground waste and vent systems, thereby ensuring the integrity and performance of the plumbing system over an extended period.