Plumbing vents and drains help to remove sewage and grey water from a building. They also regulate air pressure in the waste-system pipes to facilitate flow. Every fixture in the house has its own drain line and the drain lines tie into a larger main line.
The most important component of a drain is the trap. It traps water inside, preventing sewer gases from coming back into the house.
A sewer pipe is usually at neutral air pressure compared to the surrounding environment. A column of waste water flowing through the pipe compresses air in the pipe, creating a positive pressure. This pressure must be released to prevent it from pushing back on the waste stream and downstream traps.
Excessive negative air pressure may siphon water from traps at plumping fixtures. An empty trap allows toxic sewer gases to enter a house. Excessive positive pressure may cause waste water to be pushed into the fixture, breaking the trap seal. This can pose serious health and hygiene problems.
Vents help to regulate the drain pressure to prevent siphoning of water at fixture and pushing back of water into the fixtures. Venting allows waste water to pass out of the drains easily.