A U-shaped pipe fitting that retains water is known as what?

A U-shaped pipe fitting that retains water is known as a trap because it serves a specific purpose in plumbing systems. The design of a trap allows it to hold a small amount of water at all times, effectively creating a barrier that prevents sewer gases from escaping into the living space. This is critical for maintaining air quality and ensuring that unpleasant odors do not permeate through drains.

Traps are commonly used in various plumbing fixtures, such as sinks, toilets, and floor drains. Their U-shape allows for efficient collection and retention of water, while also enabling waste to flow through. The importance of traps in plumbing cannot be overstated; they help to ensure both functionality and hygiene in waste disposal systems.

In contrast, a vent stack serves to allow air into the plumbing system to equalize pressure and assist in the drainage process but does not retain water. A septic system manages sewage through natural processes and is not limited to individual pipe fittings. A soil pipe is part of the drainage system that conveys waste from toilets to the sewer or septic system, but it does not retain water like a trap does. Therefore, the characteristics and function of a trap clearly define it as the correct answer.