Overview of Typical Household Plumbing
The plumbing fixtures in your home are connected by two basic systems: water supply and drainage.
The water supply consists of an incoming cold water main, which splits off to the hot water heater or boiler and to each of the cold water taps. The hot water main begins at the water heater and branches off to each of the hot water taps. Supply lines can be steel, copper or plastic and are commonly range from 1 inch down to 1/2 inch. The water supply is under pressure, precisely what pressure varies by the water source (municipal supply, well, cistern) but 40-50 psi (pounds per square inch) is considered good. Too high of pressure, generally 80 psi or higher, can result in damage to fixtures, appliances and pipes. Too low of pressure results in weak flow at the tap and is simply inconvenient. Pressure boosters can be used to increase water pressure and regulators can be used to reduce pressure.
The drainage system is referred to as the DWV system for drain, waste and vent. DWV pipe can be iron, steel or plastic and typically ranges in size from 1 1/2 inches up to 4 inches or more. Most new installations use plastic pipe. The DWV system differs from the supply in that it is not under pressure. Waste water travels simply by gravity. The waste lines in your home are carefully arranged to allow for a downward slope all the way out to your sewer or septic system. In cases where a continuous slope is impossible or waste water must travel uphill, pumps are employed to move the waste water.
Each fixture in your home has a device called a trap. It is designed to trap a small amount of water in it after each use of the fixture. Your toilet has a trap that is visible because you can actually see the water in the bowl. A trap is simply a "U" shaped section in which water can pass through, but some always remains behind. By leaving enough water to completely fill the trap, sewer gas carrying unpleasant odors cannot enter your home. The water also deters rodents and insects from traveling through the pipe and into your home.
Another part of the DWV system is the vent system. If there were no vents on the waste system, a variety of problems would occur. The most basic problem is that as the water flows down, a suction would be created behind the water. That suction would then siphon the water out the traps in the fixtures and allow sewer gas to flow into your home.
Plumbing vents are typically installed near each fixture, although in some cases two or more fixtures may share a vent. The drain line from each fixture leads back to a branch line. Either at the connection to the branch or along the way to the branch, a vertical pipe allows air to enter the system and allows sewer gas to escape up and out of the system. Water does not enter the vents because gravity drainage keeps it flowing downward. An exception to this is when the drain becomes clogged and water backs up in the drain line and it can enter the vent lines to some degree.