Some basic principles for plumbing and drain systems are that water enters plumbing pipes under pressure. Drains depend on gravity to take wastes away from the building, but vents, traps and clean-outs are needed to make the drainage system safe and efficient.
In a building's plumbing system, water enters through a pipe that is of a size designated by the local plumbing codes. If the building gets its water from a municipal system, the pipe is connected to a meter that records how much water enters. There should be a shut-off valve near the meter, which is used in case of emergency. Without this shut-off valve, the building risks being flooded in case of a burst pipe. The fresh water pipe also branches off to attach to the water heater, and the hot water supply line emerges from the top of this heater.
The hot and cold water lines run parallel throughout the building to service sinks, tubs, showers, toilets and appliances that are dependent on water, such as washing machines. These lines are under pressure.
The drain-waste-vent system of a plumbing system not only carries away waste water to a sewer or septic tank, but vents gases to the outside. Traps stop sewer gas from entering the house by creating water seals that force the gas up the vents.