Diagram of a typical residential plumbing system - Supply, drain, and venting systems.

The residential water supply system transports pressurized water from the water utility company (or a well on the property) into the house and all the way to the sink, toilet, bath tub, and other water using appliances such as the dishwasher and washing machine.

The residential water drain-waste system delivers used water and solid waste out of the house and into the sewer system or septic tank.

The residential water vent system transports noxious sewer gases away from the house and maintains appropriate pressure inside the drainpipes, also preventing the sewer gases from entering the house.

The sink trap is always filled with water and used to prevent noxious gases from entering house.

The vents in the water drain system are constructed to eliminate sewer gas and to prohibit pressure buildup in the pipes. The vents come off the drainpipes further down from the traps and exit through the roof.  This maintains proper air pressure in the pipes and prevents the transport of water from the traps.

Each plumbing fixture in the home has to be vented.  Most often, a house has a main vent stack (the upper most part of the soil stack) with 1.5 – 2 inch vent pipes connecting to it.  In most houses, especially single story residences, widely separated fixtures make it unrealistic to use a single main vent stack.  In this scenario, each fixture or group of fixtures has its own waste connection and its own alternate vent stack.