If your house is hooked up to a conventional municipal water system, fresh water travels through the water utility's pipes until it reaches the meter in front of your home. It passes through the meter, where usage is measured, travels through a large gate valve, and then runs through one or more water pipes to your house and property. If you've ever seen water gushing from a fire hydrant, you have some idea of the pressure moving the water along.
Inside your home, that cold-water pipe branches off to deliver cold water to all faucets and water-using fixtures and appliances, including your water heater (it may go to a water softener as well). A second run of pipes carries heated water from the water heater to all faucets, fixtures and appliances that use hot water. Often, hot and cold-water pipes parallel each other. See a drawing of the supply system HERE.
Parallel hot and cold water pipes slope slightly to the lowest points so the pipes can be drained through a valve or faucet.
Waste drainage systems take advantage of gravity to channel waste to the sewer line. The soil stack, a vertical run of pipe three to four inches in diameter, carries waste to a main drain usually underneath the house, which empties to a sewer or septic tank.
Vents prevent sewer gases from seeping into your house, while traps—water-filled bends in pipes—keep gases from escaping up the drain. Vents branch off below the soil stacks while gases vent through the roof. Plumbing generally is vented with a main vent stack, or there may be a vent stack for each fixture, if they are widely separated.
A pipe’s size and material can serve as a fairly good indicator of its function. White plastic, copper, and galvanized (silver-toned) pipes that are from 1/2 inch to 1 inch in diameter generally carry water, though some galvanized steel, black steel, and flexible copper pipes of the same sizes may carry gas.
Large-diameter (1 1/2 inch and larger) black plastic, cast-iron, and copper are often drain-waste-vent pipes.
