For which plumbing component is a vent necessary?

A vent is necessary for the drainage system because it plays a crucial role in maintaining proper air pressure within the pipes, which ensures the efficient flow of wastewater. Without vents, negative pressure can develop in the drainage system, leading to slow drainage or even the potential for siphoning of water from traps, which would allow sewer gases to enter the living spaces. Vents allow air to enter the system, facilitating the smooth discharge of sewage and odorous gases while protecting the integrity of traps at plumbing fixtures.

While a water heater, plumbing fixtures, and showerheads have their own necessary components for proper function, they do not require vents in the same systemic sense as the drainage system does. The ventilation aspect is specifically related to how waste is moved through pipes and how air pressure is balanced, making it essential only for the drainage system.