The waste flows from the fixture trap down the drain line and out to the sewer or private sewage-disposal system. The drainage-system flow is entirely by gravity, unlike the water distribution system where the flow is under pressure. Consequently, the drainpipes are larger in diameter than the water pipes, ranging from 11/2 inches to four inches, while the water pipes vary from ⅜-inch to one inch.Drainage System Inspection – House Traps and Fresh-Air Inlet Pipes
In some communities, it is mandatory that the house drain line leading to the sewer has a house trap. The trap is typically located inside the house, near the foundation wall, and is designed 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 be a fresh-air inlet pipe as well, connected to the main drain. This fresh-air inlet pipe is located on the house side of the drain, approximately one foot from the trap. In cold climates, it is located about five feet from the house trap in order to prevent the water seal from freezing during the winter.
When the outer end of the air inlet terminates on the outside of the foundation wall, it should be covered with a perforated metal plate that allows air to enter and prevents obstruction. When it is freestanding, it should be covered with a goose-neck or cowl. The fresh-air inlet is designed to maintain atmospheric pressure at the house trap and to ensure complete air movement within the drainage system.
However, if you have a private sewage system (septic tank), a house trap on the drain line is not necessary. The gases that are generated in septic tanks are generally discharged to the atmosphere through the house drainage vent system. Venting equalizes the air pressure in the drainage system by allowing air to flow into and out of the drainpipes. The free air movement maintains atmospheric pressure at the various fixture traps, which prevents waste water from siphoning the water seal out of the drain trap. Venting in the drainage system consists of vent pipes connected to the drain line near each fixture trap and to a pipe that terminates above the roof line. This pipe is called a vent stack.