The drainage system is more complex than the water distribution system, since it consists of three parts-traps, drainpipes, and vents. The drainage system begins just below the fixture with a water-filled trap. The trap is generally U-shaped and should have water at the bottom portion. The water in the trap forms a seal to prevent sewer gases usually in the drain line from entering the room.
The wastes flow from the fixture trap down the drain line and out to the sewer or private sewage-disposal system. Unlike the water distribution system, where the flow is under pressure, the drainage-system flow is entirely by gravity. Consequently, the drainpipes are larger in diameter than the water pipes, varying from 11⁄2 inches to 4 inches, compared to 3⁄8 inch to 1 inch for the water pipes. In some communities, the house drain line leading to the sewer must have a house trap. The trap is usually located inside the house near the foundation wall. Its purpose is to provide a seal and prevent the gases that occur in the sanitary sewer from circulating back through the plumbing system. When there is a house trap, there should also be a fresh-air inlet pipe connected to the main drain. This air inlet pipe is To sewer located on the house side of the drain approximately 1 foot from the trap. (See FIG. 13-2.) In cold climates, it is located about 5 feet from the house trap to prevent the water seal from freezing during the winter. When the outer end of the fresh-air inlet terminates on the outside of the foundation wall, it should be covered with a perforated metal plate that admits the air and prevents obstruction. When it is freestanding, it should be covered with a cowl or gooseneck. (See FIG. 13-3.) The function of the fresh-air inlet is to maintain atmospheric pressure at the house trap and to ensure complete air movement within the drainage system. With a private sewage system (septic tank), a house trap on the drain line is not needed. The gases that are generated in septic tanks are usually discharged to the atmosphere through the house drainage-vent system.
Venting is needed in the drainage system, since it provides a means to discharge to the atmosphere gases that develop in the system. It equalizes the air pressure in the drainage system by allowing air to flow into and out of the drainpipes. This free air movement maintains atmospheric pressure at the various fixture traps, which prevents the waste water from siphoning the water seal out of the drain trap. (See discussion on fixture traps in chapter 10, page 119.) Venting in the drainage system is achieved by vent pipes connected to the drain line near each fixture trap and to a pipe that terminates above the roof line. This pipe, called the vent stack, is visible from the outside. Vent pipes must be unobstructed. They carry no water or wastes.