While we normally think of this system simply as “the drain pipes,” to be technically correct, the system is known as the DWV system. The acronym stands for Drain-Waste-Vent, and it indicates that this pipe network is not only designed to drain wastewater and solid wastes to the municipal sewer or septic field but also serves as a vent system that allows fresh air into the drain system. To learn about the drain system, it can be helpful to trace a drain line from a single plumbing fixture, such as a bathtub or sink, all the way out to the municipal sewer main. While this is the most obvious component, it is relatively rare for drain problems to originate here. Except when gaskets or washers may cause a tub or sink bowl to leak, the most common problems—drain clogs—almost always occur downstream from the fixture drain openings. The one exception: when hair clogs a pop-up stopper in a bathroom sink or bathtub. From here, wastewater drops down below the fixture to the next component—the drain trap. The purpose of this drain trap is to hold standing water, which seals the drain system and prevents sewer gases from rising from the sewer system into your home. If you’ve ever returned home after a long vacation and noticed a faint sewer gas odor in the air, it’s probably because the standing water in the drain traps has evaporated, allowing the sewer smell into your home. Depending on the age of your plumbing installation, the standpipe system can be made of galvanized iron, brass, or PVC or ABS plastic. Soil stacks are larger pipes, typically 3 to 4 inches in diameter. If you have ever heard a drain gurgling as you empty water, you are hearing the sound of a slight air-pressure vacuum attempting to pull water out of the drain traps. If the vent system is working correctly, this suction never reaches a point where water is sucked out of the traps. The vent pipes serve two functions: This fitting is used when a major clog is blocking the main sewer line. With the cap removed, a motorized auger can be used to remove a clog in the main drain. Usually 4 inches in diameter, this pipe might be either ABS or PVC plastic, clay, or cast iron. The mainline is rarely visible since it usually runs under the home’s basement or foundation slab. You will probably see this drain line only during major repairs or additions to the system.
