Residential plumbing systems are easy to understand. In general, a home plumbing system consists of two networks of pipes. One is the supply system—the pipes that carry water into the house and distribute it to fixtures. The other is the drain, waste, and vent (DWV) system, which carries drain water, waste, and harmful gases out of the house.

Supply
In the supply system, an underground line from a water source connects to a meter that measures the amount of water entering the house. Next to the meter is a shutoff valve that, when closed, stops water from flowing into the house. The main supply line branches into two lines—one for cold water and one for hot. The cold-water supply line feeds the network of supply pipes throughout the house. The hot-water supply line goes to your water heater. From there, the hot-water pipes run parallel with the cold-water pipes to serve various fixtures and faucets.

Drains and Vents
Your home’s water-supply system is pressurized, but the drain-waste system depends on gravity. These pipes also are connected to vents, which allow sewer gases to escape harmlessly up a chimneylike vent stack. Because each new or moved bathroom fixture must connect with a stack, you must know the locations of your main vents, then determine a route for attaching new vent lines to them. Consult local building codes for the required size and general conformation of drainage, waste, and vent lines.