Plumbing fixtures in most houses are located near the 3 or 4-inch drain-waste-vent (DWV) pipe. Also known as the main stack or soil stack; or auxiliary DWV stack. The DWV extends from the basement floor to the roof, serving as the drain and vent for multiple fixtures.

Typically, plumbing in residential structures involves not just a water supply system; it also involves fixtures and a drain system. It works something like the following:

• Fresh water enters the home through a main supply line. As it does it usually passes through a meter that registers the amount of water flowing into the house.
• After entering the home, the water line branches off; some of the water feeds into a hot water heater. Warmed water from the heater flows through a line that runs parallel to the cold water line, feeding fixtures and appliances within the home.
• Waste water from fixtures and appliances flow into a trap, then into a drainage system that travels downhill through a series of large-diameter drain pipes.
• The drain pipes are attached to a system of vent pipes that allow air into the system through a roof vent. This keeps the drain water flowing properly.

Eventually the drain water reaches a main waste and stack vent that directs the water into an underground sewer line. This carries the water away from the house and into a municipal sewer system or a septic tank.

Supply Pipes

It is the cold and hot water supply pipes that make up the home’s plumbing system. Before 1950 most supply pipes were made from galvanized iron. Homes built after 1950 generally used copper or plastic (CPVC) pipes. Many homeowners use a home renovation as an opportunity to replace old supply pipes with new.

Supply pipes are manufactured to withstand high water pressure. They are usually ½-inch to 1-inch in diameter and are joined with strong, watertight fittings. Valves are used to control the flow of water.

Water supply systems are generally located inside wall cavities, or strapped to the undersides of floor joists.

Drain Pipes

Drain pipes carry water out of the house via a DWV system; drain-waste-vent. This system uses gravity to draw water down vertical pipes and along a sloping path into a municipal sewer system or septic tank.

Although drain pipes found in older homes are generally made from copper or lead, pipes in newer homes are made from plastic or cast iron. Drain pipes are larger than supply pipes; usually 1¼-inches to 4-inches in diameter.

The curved section in the DWV is called the trap. The trap holds a supply of standing water. This prevents toxic sewer gases from entering the home. Every time the drain is used, old standing water is replaced with new.

Outdoor air is brought into the house by the vent system in the drain pipes. This lowers pressure in the pipes and helps keep the water flowing freely. Improperly constructed drain systems can create drain malfunctions, interfere with toilet flushing, and allow sewer gas to back up and enter the home.

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