Plumbing · Lesson 1 of 13Beginner6 min read

The Big Picture

A house has two plumbing systems that must never mix. One brings clean water in under pressure. The other carries used water and waste away by gravity, with vents that let air in. This lesson gives you the map; every later lesson zooms in on one part of it.

Two systems: pressure in, gravity out

The supply system is a closed web of small pipes that stays full of water all the time. The water in it is under pressure, pushed by a pump or by water stored high up in a tank or tower. That push is what lifts water to a second-floor shower and makes it spray out of a faucet the moment you open it.

The drain-waste-vent system, or DWV, works the other way. Its pipes are bigger, they run only partly full, and nothing pushes the water along. Every drain slopes downhill and lets gravity do the work. Vent pipes connect the drains to the open air above the roof so air can move in and out as water rushes past.

The two systems must stay apart. HUD’s federal standard for manufactured homes puts it plainly: no part of the water system may be connected to drainage or vent piping. Plumbing codes for other homes have the same goal, because a connection could let sewage get pulled back into the water you drink (Lesson 2 explains how that can happen).

The two systems never connect. Supply pipes stay full and pressurized; drains run partly full, so they need slope and a vent to the outside air.

A map of the house

Here is a whole house cut open. Switch between the water supply and the drains and vents. Point at a system (or its name below) to see what it does, and click it to open its lesson.

Point at a part of the plumbing.

The house is drawn with a basement so every part shows. In a house on a slab or a crawlspace, the same parts are there; the pipes just run in different places.

The systems at a glance
Part	What it does	Signs of trouble
Water service and meter	Brings water from the street main through the meter and the main shutoff valve.	No water anywhere, low pressure everywhere, a wet spot in the yard.
Cold water lines	Carry water under pressure to every fixture and appliance.	Leaks, banging pipes, frozen pipes in cold weather.
Water heater and hot lines	Heat water and carry it to the taps that need it.	No hot water, running out fast, rumbling, a leaking tank.
Fixtures and appliances	Sinks, toilets, tubs, showers and the washers that use the water.	Drips, running toilets, weak spray.
Drains and traps	Carry used water downhill; each trap holds a plug of water that blocks sewer gas.	Slow drains, clogs, sewer smells.
Vents	Let air into the drains so water flows smoothly and traps keep their water.	Gurgling, slow drains, sewer smells.
Building drain and sewer	Collect everything and carry it to the public sewer or a septic tank.	Backups in the lowest drains, several fixtures slow at once.
Gas line	Feeds fuel to a gas water heater, furnace, range or dryer.	The smell of gas: leave the house and call from outside.

Where the water comes from, and where it goes

Most homes get water from a public system. The U.S. Geological Survey says about 87 percent of Americans get their water from a public supply. The utility treats the water, tests it, and pushes it through mains under the street. Most of the rest rely on a private well with a pump. With a well, the owner is in charge: the CDC notes that government officials do not regulate, treat or monitor water from private wells, so testing it is up to you (Lesson 7).

Used water leaves the same two ways. In towns it flows into a public sewer that carries it to a treatment plant. In the country it often goes to a septic system: a buried tank where solids settle out, and a drainfield where the soil treats the liquid (Lesson 3).

City or country: four combinations
	Public system	On your own property
Water in	A utility’s mains, a service line and a meter	A private well and pump
Water out	A building sewer to the public sewer main	A septic tank and drainfield
Who maintains it	The utility up to its part of the service line; you, inside the house	You, with licensed or certified pros for the well and septic system
Testing	The utility, which reports the results	You: CDC says test a well at least once a year

Who owns which part

The pipe that carries water from the public main to a building is called the service line. Near the street there’s usually a valve called the curb stop, and HUD’s inspection guide notes it is usually the water department’s responsibility. Being an Adult 101 gives the rule of thumb: leave the valve at the meter or property line to your water utility or a licensed plumber, and learn to use the main shutoff inside your home (Shut Off Your Home’s Water).

Inside the house, the plumbing belongs to the owner, and so does the job of keeping it working.

NoteThe local code and the authority having jurisdiction decide

This course explains how plumbing works in general, with U.S. sources. Your state or local plumbing code, enforced by the building department or other authority having jurisdiction, decides what is allowed where you live. Many jobs need a permit and an inspection, and some work must be done by a licensed plumber. Where this course quotes a code, it’s an example; your local code may differ.

Check yourself

Tap an answer to check it, or open “Show the answer.”

What moves water through the supply pipes?
Show the answer
Pressure from a pump or from water stored up high. Supply pipes are full and pressurized, pushed by a pump or by the height of water in a tank or tower.
What moves water through the drains?
Show the answer
Gravity, with pipes sloped downhill. Drains aren’t pressurized. They slope downhill, and vents let air move so the water flows smoothly.
About what share of Americans get water from a public supply, according to USGS?
Show the answer
About 87 percent. About 87 percent. Most of the rest use private wells.
Why must the supply and the drains never connect?
Show the answer
Dirty water could get pulled back into the drinking water. A connection could let waste flow backward into the water supply. Codes keep the two systems apart.

Go deeper

Being an Adult 101: Shut Off Your Home's Water The first plumbing skill: find and use your main shutoff.
USGS Water Science School: Domestic Water Use Where home water comes from, from USGS.
U.S. EPA SepticSmart: How Septic Systems Work EPA’s picture of how a septic system works.
Lesson 2: Water Supply: From the Street to the Faucet Next: follow the water from the street to your faucet.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how plumbing works in general and follows U.S. sources; your local plumbing code, the authority having jurisdiction and manufacturers’ instructions are the final word.