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How Your Home Plumbing System Works

In this article we are going to learn how your home plumbing system works. Before taking on any plumbing job, make sure you have a firm grasp of the three systems that work together to remove waste and provide your home with fresh water:

  • Supply system
  • Waste system
  • Vent system

The Supply System

The supply system brings pressurized water to your home’s plumbing fixtures (faucets, toilets, tubs, and so on). The water source may be a local water utility or private well. From the source, water flows into your house through:

  1. A main shut-off valve: Closing this valve shuts down the water to your entire home. You should know the where your main shut-off valve is and how to use it.
  2. A water meter (if provided by a water utility): This allows the utility to measure your water usage.
  3. Water heater: This provides your home with hot water.

Hot and cold water lines, called branch lines, carry water to various parts of your home.


A valve is a device that can open or shut off the flow of water. All the hot and cold water lines in your home end at a valve. You can operate some valves manually (such as those in sink faucets) and others automatically (such as those in toilets).

Branch and Fixture Shut-Off Valves

Every branch line in your home should have a branch shut-off valve, which lets you shut off the water to that region of your house without turning off the water to your entire house.

Newer fixtures may also have fixture shut-off valves, which let you turn off the water to just that fixture. You should know where all the shut-off valves in your home are.

The Waste System

The waste system uses gravity to transport solid and liquid waste from your home. Waste enters the system through your home’s fixtures, then moves through the trap and a T-shaped connecting pipe called a sanitary tee into either a branch waste line or the main waste line.

If the waste enters a branch waste line, it will eventually join the main waste line through another sanitary tee. The main waste line, also called the main or soil stack, continues past the main clean-out and empties into the municipal sewer system or a private septic tank.

Home Plumbing System


Between every fixture and the waste line in your home is a trap—a curved section of pipe that traps water within it. The trapped water forms an airtight seal that prevents sewer gas from entering the home. At the bottom of their curve, some traps have a clean-out plug that provides access to the trap, making it easier to clear out any clogs.

Home Plumbing System

There are three main types of traps:

  • P-traps: Designed for waste lines that come out of a wall, P-traps are shaped like the letter “P” lying face down. They’re located under sinks, tubs, and showers.
  • S-traps: Designed for waste lines that come out of the floor, S-traps can sometimes allow sewer gas into the home. This has led to the passing of laws that make it illegal to install S-traps in new homes.
  • Integral traps: Toilets have built-in “integral” traps, which work just like P-traps.

The Vent System

The vent system allows fresh air to flow into the waste system, which improves drainage and prevents siphoning.

A vent is connected to each fixture’s drain line past the trap. Depending on the layout of your home, vents may join together at a single main vent that opens into the outside air, or different vents may vent separately.

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