Your Home’s Water Supply System Explained

Turn on any tap in your house and water arrives instantly, already under pressure. That readiness is not an accident. It is the defining trait of your home’s supply system: a sealed, pressurized network that holds water in tension everywhere at once, waiting for a valve to open. This is the “water in” half of residential plumbing, and it behaves nothing like the drains that carry water back out.

Understanding the supply side answers a question that confuses most homeowners. Why does a leak on a supply line spray or hiss even when no faucet is running, while a drain leak only drips when you actually use the fixture? The answer is pressure. Supply piping is live all the time. Drainpipes are empty until something flows through them by gravity, a contrast covered in our guide on the drain-waste-vent system (see post 003). This guide stays on the pressurized side: where the water comes from, how it splits into hot and cold, what keeps it under pressure, and the chain of shutoff valves that lets you control it.

From City Main or Well to Your Front Wall: The Service Line and Meter

The supply system begins outside your property with a single pipe. According to the EPA, a service line is the pipe that connects the public water main to the plumbing in a home or building. Everything your household uses passes through that one line before it branches anywhere inside.

If you are on municipal water, the line runs from the main under the street, across your yard, and into the house. Somewhere along that path sits the water meter, which records how many gallons you use. The meter is often near the street in a buried box or, in colder regions, inside the home near where the line enters. Ownership of the line typically splits at a point called the curb stop. The EPA notes that the system-owned portion usually runs from the water main to the curb stop, while the customer-owned portion runs from the curb stop toward the meter and into the home, though the exact dividing point varies by water system. Knowing which side you own matters if a buried supply line ever needs repair.

Homes on a private well do not have a city main or a meter. Instead, a pump draws water from the ground and a tank holds it under pressure for the house. The internal pressurized network past that point works the same way, but the source and pressure equipment differ. Well pressure tanks are their own topic, so see post 123 for how those work.

One older-home concern lives right here at the service line: the pipe material. The EPA estimates that several million lead service lines still serve properties across the United States, most installed before the 1940s. Lead in the service line is a health topic with its own dedicated coverage, so this guide simply flags that the service line material can matter and points you to our guides on lead pipes and finding a lead service line for the full picture.

The Main Shutoff and Why You Should Find It Today

Your main shutoff valve stops all water entering the house, and you should locate it before you ever need it. In a plumbing emergency, the minutes spent hunting for the valve are the minutes a burst pipe floods a room.

The main shutoff usually sits where the supply line enters the house, often in a basement, crawl space, garage, or utility closet on an exterior-facing wall. In warm climates without basements, it may be outside near the meter or against the foundation. You will see one of two valve types. A gate valve has a round wheel handle that you turn clockwise several full rotations to close. A ball valve has a straight lever handle that you rotate a quarter turn until it sits crosswise to the pipe, which means closed.

Find your valve now, while nothing is wrong. Confirm that the handle turns and that the family members who would respond to a leak know where it is. There is often a second main valve at the meter itself, controlled by the utility, but the one inside or at your home is the one you operate. If you ever face a burst line, closing this valve is what stops the flooding before help arrives, a sequence covered in our emergency shutoff guide (see post 131).

How One Supply Line Splits Into Hot and Cold

One pipe enters your house, but two travel to most fixtures. Just inside the home, the cold supply branches. One path stays cold and feeds cold taps, toilets, hose bibs, and the water heater’s inlet. The other path enters the water heater, comes out hot, and runs as a parallel hot line to fixtures that need it.

This is why a kitchen or bathroom has two supply tubes feeding a single faucet, and why a toilet, which uses only cold water, has just one. The cold network is essentially the whole house. The hot network is a loop that exists only because the water heater sits in the middle of it. Everything downstream of the heater is the hot side; everything else is cold.

The water heater’s internal workings, the burner or element, the tank, the thermostat, are a separate subject. This guide treats the heater as the junction where the hot loop begins. For how a tank heater actually produces hot water, see post 051. What matters for understanding supply is the branching itself: a single pressurized source dividing into two pressurized networks that arrive together at the fixtures you use.

What Keeps Water Pressurized Throughout the House

Supply water stays under constant pressure because the source pushes it, and that pressure is the force that delivers water the instant you open a valve. On city water, the pressure comes from the municipal system, often from elevated tanks or pumping stations that hold the entire distribution network under load. On well water, a pump and pressure tank do the same job for your house alone.

Pressure is measured in pounds per square inch, or psi. There is a real upper limit that matters for your plumbing. Under the International Plumbing Code, where static water pressure in a building exceeds 80 psi, an approved pressure-reducing valve must be installed to bring it down to 80 psi or less. The Uniform Plumbing Code carries the same 80 psi ceiling. That threshold exists because faucet cartridges, fill valves, supply tubes, and appliance valves are rated for ordinary household pressure and wear out faster or fail when pressure runs too high. Plumbing codes vary by jurisdiction, so verify the figure that applies where you live, but the 80 psi static maximum is consistent across both major model codes.

Notice that pressure is only half the story. Pressure is the push; how much water actually comes out of a fixture per minute is a separate measure called flow rate. The two can disagree, which is why a faucet can feel forceful yet deliver a thin stream. That distinction deserves its own treatment, so see post 006 for pressure versus flow rate. If your concern is that the whole house feels weak, that is a diagnostic question covered in post 117, and the device that tames excessive pressure is detailed in post 119. This guide’s job is only to explain that supply water is pressurized by design and capped by code.

Supply Pipe Materials You’ll See Behind the Walls

Supply pipes have to hold pressure continuously, so they are built differently from drainpipes, and a few materials dominate modern homes. You will most often encounter copper, a rigid metal pipe joined by soldered fittings; PEX, a flexible plastic tubing in red, blue, or white that has become common in newer construction and repipes; and CPVC, a rigid cream-colored plastic rated for hot and cold water.

Older homes may still have galvanized steel, an early pipe that corrodes and narrows from the inside over decades, and the oldest service lines may be lead. Each material has different strengths, lifespans, and failure modes. Because pipe-material comparisons are a deep topic in their own right, this guide names what you will see without ranking them. For the full breakdown of PEX, copper, CPVC, and the rest, see post 100.

What unites every supply material is the requirement to be pressure-rated and watertight at every joint. A drainpipe only has to channel water flowing past on gravity. A supply pipe holds water under load even when no one is using it, which is exactly why a failed fitting on the supply side announces itself with a spray rather than a slow drip.

Fixture Shutoffs: The Local Valves Under Sinks and Toilets

Almost every fixture has its own small shutoff valve, so you can stop water to one sink or toilet without killing supply to the whole house. These local valves, sometimes called stop valves or angle stops, sit on the supply line right where it reaches the fixture.

Look under any sink and you will usually see one or two of them on the wall or floor, with the flexible supply tubes running up to the faucet. A toilet has a single shutoff on the cold line near the base. Most are small oval or football-shaped handles you turn clockwise to close, though quarter-turn lever versions exist too. These valves complete the chain of control that runs from the street to the fixture: the utility’s valve at the main, your whole-house main shutoff, and finally the individual fixture stop.

That layered chain is the practical payoff of understanding the supply system. When a faucet drips or a toilet runs, you do not need to shut off the whole house. You close the local stop, do the work, and leave the rest of the home with water. For replacing a worn or stuck fixture shutoff, which is a job with its own steps, see post 196. Locating and testing these valves before trouble arrives is one of the most useful habits a homeowner can build.

Frequently Asked Questions

What is a home’s water supply system?
It is the pressurized network of pipes that brings clean water into your house and distributes it to every fixture. Water enters through a single service line from the city main or a well, passes the meter and main shutoff, then splits into cold and hot networks that stay under constant pressure until you open a valve.

Where does my home’s water supply come from?
Either a municipal water main under the street or a private well on your property. City water passes through a meter; well water is drawn by a pump and held under pressure in a tank. From the point of entry inward, the pressurized piping works the same way.

What is a normal water pressure for a house?
Residential pressure is measured in psi. Plumbing codes set a hard ceiling: where static pressure exceeds 80 psi, a pressure-reducing valve is required to bring it down. Many homes run comfortably below that limit. Because guidance and local code vary, confirm the figure for your area and have pressure measured if you suspect it is too high or low.

How is hot water separated from cold in my plumbing?
The incoming cold line branches inside the house. One branch stays cold and feeds cold taps and toilets. The other branch runs through the water heater and emerges as a parallel hot line. Fixtures that need both, like sinks and showers, receive two supply tubes.

Why does a supply leak spray but a drain leak only drips?
Supply pipes are pressurized all the time, so any breach pushes water out continuously. Drainpipes are empty and rely on gravity, so they only leak while water is actually flowing through them.

This article is general information, not professional advice. For work on your specific plumbing system, especially anything involving your service line, water heater, or buried pipes, consult a licensed plumber.

Sources

• EPA, Lead Service Lines: https://www.epa.gov/ground-water-and-drinking-water/lead-service-lines
• EPA, Getting Started with Lead Service Line Identification and Replacement: https://www.epa.gov/ground-water-and-drinking-water/getting-started-lead-service-line-identification-and-replacement
• EPA WaterSense, Showerheads: https://www.epa.gov/watersense/showerheads
• U.S. Department of Energy, Best Management Practice #7: Faucets and Showerheads: https://www.energy.gov/cmei/femp/best-management-practice-7-faucets-and-showerheads
• International Code Council, 2018 International Plumbing Code, Chapter 6 (Section 604.8, maximum water pressure): https://codes.iccsafe.org/content/IPC2018/chapter-6-water-supply-and-distribution
• IAPMO, Uniform Plumbing Code Spotlight, Section 608.2 Excessive Water Pressure: https://forms.iapmo.org/email_marketing/codespotlight/2018/Jan4.htm