How a Plumbing System Works
Residential plumbing actually consists of two systems: the fresh or potable water and the drainage system, as well as the fixtures and appliances that the systems serve.
There are three possible sources of water: a municipal water works, a spring, and a well. Water that starts its journey to your house in a water works is purified there; it then travels through a series of pipes until it enters a single main service line (usually ¾-inch diameter) into your house. As it enters, it goes through a meter, which records its use.
From there it branches out into a pair of narrow pipes (usually ½-inch diameter) that travel side by side throughout the house and terminate at the appliances or fixtures that use the water. One of the lines is for cold water and one is for hot. The water becomes hot by first being routed through the hot water heater before it begins its journey through the house.
All fresh water travels under pressure, because it is pumped to your home. That's why it gushes out when you turn on a faucet. In a sense, the water is always ready to go—the faucet simply directs the pressure where you need it.
Different names have been given to water pipes. Those that travel vertically at least one floor are called risers. Lines that travel horizontally to fixtures and appliances are called branch lines.
At various points on the pipes there are valves, which are faucet-like devices for shutting down the water supply to the pipes. This way, when repairs are needed, you don't have to shut down all the water. Valves are on runs to control specific lengths and are usually (or should be) under sinks and toilets: hot and cold valves for the sink, plus one valve for the toilet tank.
Valves are also located at the bottoms of risers; these are used to drain the pipes. Additionally, homes usually have one or two main valves at the water meter. One is called the meter shutoff and is on the street side of the meter; the other is called the main shutoff and is on the house side. Turning either of these valves will stop all water flow to the house.
Every plumbing system should have air chambers (though not all systems do). These are vertical pieces of capped pipe that jut up from water supply pipes at the point where the pipe enters the fixture. In essence, an air chamber is a shock absorber. When the pipes are handling excess water, the excess enters the air chambers; otherwise, the pipes could start vibrating, a malady called waterhammer, and possibly loosen or leak from the shock.
There are three kinds of water-related fixtures in the house: sink, tub, and toilet. Outside faucets, also known as sillcocks, may also be considered fixtures, but they are called, like other faucets, fittings.
Traps are safety devices. They are shaped to trap water (hence their name), and thus they're constantly filled with water so that gases and vermin can't back up into the house through sink or tub drain holes. Sink traps are separate pieces of tubing; toilets are shaped to trap water. In addition there are drum traps for tubs and a trap and fresh air inlet where the soil pipe leaves the building.
The Drainage System
The second part of a plumbing system is the drainage system, technically known as a “DWV” system to describe what it does: It drains away wastes at the same time as it vents them. The DWV system is composed of waste pipes, a soil stack, traps, and vents.
How a DWV system works can be illustrated in the way a sink operates. Waste runs out of the sink to a series of pipes that lead to the waste pipes, usually pipes with a diameter of 1½ or 2 inches, which are sloped to empty into the soil stack. The stack is a large pipe, usually 3 or 4 inches in diameter, that runs from the lowest point in the house to the highest and emerges about 6 inches through the roof. (The stack is affectionately known in the trade as a “stinkpipe.”) In older homes cast iron was commonly used; in newer homes copper and plastic are used. From the stack, the waste goes to the building drain, a horizontal pipe that runs across the house and leads to the sewer line and to the disposal system—sewer, septic tank, or cesspool.
Venting is required to release gases. However, it has another important function: It keeps the air pressure in the system equalized. This prevents water from the drain or waste lines from backing up, by a siphoning action, into the fresh water system.
From Toilet to Stack
Pipes that go from the toilet to the stack are called soil pipes. There are no branch lines here. The soil pipe drains into the soil stack directly. Drain lines only carry sink waste; soil pipes only carry toilet waste. The entire system operates by gravity.
All water fixtures are connected to the soil stack. However, if the distance between fixture and stack is too far, there may be additional stacks. These also are vented. In essence, a stack has two purposes: It functions as a waste line and a vent line. It's a waste line below the fixture and a vent line above.
Cleanout plugs are located at the bottom of each soil stack and wherever the waste pipe changes direction, because that's where blockages often occur. These plugs can be removed and a cleanout tool (snake) inserted for clearing the blockage.