What is the extension of a soil or waste stack above the highest horizontal drain connected to it called?

The extension of a soil or waste stack above the highest horizontal drain connected to it is called a stack vent. This component is crucial in the plumbing system as it allows for the release of gases that accumulate in the drainage system, maintains atmospheric pressure in the pipes, and facilitates the proper flow of wastewater.

A stack vent serves an important role by allowing air to enter the waste system, which prevents vacuum conditions that could hinder drainage. Furthermore, stack vents help to prevent the backflow of sewer gases into the building, thus contributing to overall health and safety.

In comparison, air gaps, drain vents, and Venturi effects have different applications within plumbing and fluid dynamics. Air gaps prevent backflow by creating a physical separation, whereas drain vents are typically smaller piping that serve different purposes in venting. The Venturi effect pertains to fluid dynamics and pressure changes in flowing fluids, which is not directly related to the function of a stack vent within a plumbing system.