|Household Plumbing System At first glance, plumbing seems to be very complicated - but actually, once you understand the basics of plumbing systems, it's simple. Household plumbing consists of three systems operating together - supply, drain-waste, and vent. The SUPPLY system carries water under pressure from the well or underground water mains into the house and to all fixtures, like sinks, showers, and toilets; and appliances, like washing machines and dishwashers.
The DRAIN-WASTE system begins at the fixtures and appliances where the water supply system ends. Since drained water is not under pressure, gravity pulls the water through the pipes and out to the city sewer or private disposal system.
As water rushes down the pipes, a vacuum is created at the high end of the pipe - creating a siphon. However, most fixtures use traps to hold water and form a seal against sewer gas entering the house. Without a VENT system, the siphon action would remove all water, even the water in the traps, allowing sewer gas to pass through the pipes, into the house. VENT systems provide a way for sewer gas to escape while maintaining atmospheric pressure inside drainpipes, preventing the traps from being drained.