What is the purpose of a vent stack?

The purpose of a vent stack is primarily to allow air into the plumbing system, which helps to equalize pressure within the drainage system. This is essential for the proper functioning of plumbing fixtures. When water drains, it creates a vacuum that could hinder the flow if air is not allowed to enter the system. The vent stack prevents back-siphonage and helps maintain the correct flow of wastewater by ensuring that there is equal pressure in the pipes, thus preventing dangerous sewer gases from entering the home and allowing wastewater to flow freely.

Additionally, while some other choices may seem related to plumbing, they do not accurately describe the specific role of a vent stack. For instance, expelling sewer gases is a function of other venting methods but is not the main role of the vent stack itself. The functions of draining excess water and diverting water away from the foundation pertain more to drainage systems and gutters than to the specific purpose of a vent stack.