Plumbing · Lesson 2 of 13Beginner11 min read

Water Supply: From the Street to the Faucet

Follow a gallon of water from the main under the street to your kitchen faucet. Along the way you’ll meet the valves that control it, the physics that pushes it, and the devices that keep the pressure safe and the water clean.

From the main to the house

A water main runs under the street. The U.S. Army’s plumbing manual describes mains as usually cast iron and 8 inches or more across. A plumber taps a small valve, the corporation stop, into the main, and the house’s service line runs from it toward the house. The Army manual gives three-quarters of an inch as the smallest practical size for a service line.

Near the property line is the curb stop, a valve set in a stop box so it can be reached from the surface. Next comes the meter, which measures how much water you use, either in a pit near the street or inside the house. Then, where the line comes through the wall, your home’s main shutoff valve. Close it and no new water can get in anywhere (Shut Off Your Home’s Water).

After the main shutoff, many homes have a pressure-reducing valve. Then the cold line branches out to every fixture, and one branch feeds the water heater. Hot water leaves the heater in pipes that run alongside the cold ones.

A typical layout; the order and location vary. The meter may sit in a pit by the street or inside the house, and not every home has a pressure-reducing valve (PRV). Sources: U.S. Army FM 3-34.471; HUD; City of Edmond, Oklahoma.

Where the pressure comes from

Pressure is force spread over an area. In a liquid at rest, the pressure grows with depth, because the water underneath has to hold up the weight of all the water above it. OpenStax’s physics textbook gives the rule as P = hρg: pressure equals depth times density times the pull of gravity. The width of the container doesn’t matter; only the height of the water does.

That’s why towns put water in tall towers and hilltop tanks, and why pumps push it up into them. A faucet 100 feet below the water’s surface in a tower feels the weight of a 100-foot column of water.

In U.S. units, the numbers work out neatly. NIST gives the pressure under one foot of water as 2,988.98 pascals and one psi as 6,894.757 pascals, so each foot of height adds about 0.433 psi. A faucet 115 feet below the water level gets about 50 psi. Climb 10 feet, about one story, and you lose a bit more than 4 psi. The Army manual puts pressure in a main usually between 45 and 60 psi.

A pressure gauge on a hose bibb reads gauge pressure: the pressure above the air pressure all around us. That’s the number plumbers and codes use.

P = h × ρ × g
1 foot of water: 2,988.98 Pa ÷ 6,894.757 Pa per psi ≈ 0.433 psi
pressure (psi) ≈ 0.433 × height of water above the tap (feet)

Drag the water level in the tower, change the ground level of the house, or add a pressure-reducing valve. Gauges show static pressure: the pressure with no water flowing. Real pressure while water flows is lower, because friction in the pipes uses some of it up.

Too much pressure: the pressure-reducing valve

Low ground means high pressure. The City of Edmond, Oklahoma, offers an elevation lookup for that reason: addresses below 1,150 feet automatically need a pressure-reducing valve. Edmond quotes the model code it enforces, the 2018 International Residential Code, section P2903.3.1: “The static water pressure shall be not greater than 80 psi (551 kPa).” Where the main’s pressure is higher, an approved pressure-reducing valve “shall be installed” where the water service enters. HUD’s standard for manufactured homes uses the same 80 psi line. That is the model code; your local code may differ.

A pressure-reducing valve, or PRV, uses a spring and a diaphragm to let water through only until the pressure on the house side reaches its setting. San Diego’s utility guidelines call for setting a new PRV between 45 and 75 psi. It lists signs that a PRV is failing: banging or a high-pitched whistle in the pipes, often when water is turned off; more flow and pressure at faucets than you’d expect; and frequent leaks in toilets and faucets.

EPA’s WaterSense home maintenance checklist says to check your service water pressure once a year and adjust or replace the pressure-regulating valve if needed.

You canYou can check your pressure

A simple pressure gauge threads onto an outdoor hose bibb or a laundry faucet. With no water running anywhere in the house, open the faucet and read it. That’s your static pressure. If it’s over 80 psi, talk to a licensed plumber about a PRV. Replacing a PRV means cutting into the main line, so check whether your town requires a permit or a licensed plumber.

Pressure isn’t the same as flow

Pressure is the push. Flow rate is how much water actually comes out each minute. OpenStax defines it as the volume that passes a point per unit of time, and shows that flow equals the pipe’s cross-section area times the average speed of the water (Q = A v).

Moving water rubs against the pipe walls, so some pressure is used up along the way. In smooth, slow flow, OpenStax’s Poiseuille’s law shows the resistance growing with the pipe’s length and falling with the fourth power of its radius. Halve a pipe’s radius and its resistance goes up 16 times. That’s why a long run of small pipe can feel weak at the far end, and why the main lines are bigger than the branches.

HUD’s inspection guide gives a quick check: a kitchen faucet should fill a one-gallon jug in about 24 seconds. That’s 2.5 gallons per minute. The same guide calls 15 psi at each fixture, with water flowing, preferable, and 8 psi the least that will do.

flow rate: Q = A × v (area × average speed)
laminar resistance: R = 8ηl ÷ (πr⁴) (longer or thinner pipe → more resistance)

How much water fixtures use (EPA WaterSense and HUD)
Fixture	Typical or required maximum	WaterSense labeled
Bathroom sink faucet	2.2 gallons per minute (standard)	1.5 gallons per minute or less
Showerhead	2.5 gallons per minute (standard)	2.0 gallons per minute or less
Toilet	1.6 gallons per flush (federal standard)	1.28 gallons per flush or less

Hot and cold side by side

The hot water system runs alongside the cold, from the water heater to every fixture that needs hot water. Hot is on the left: HUD’s manufactured home standard, for example, requires faucets to deliver hot water from the left-hand side, the familiar arrangement in most homes.

Hot water cools in the pipes between uses, so a faucet far from the heater runs cold for a while first. The Army manual describes the fix used in bigger buildings: a two-pipe system that keeps hot water circulating. Some newer homes use manifold plumbing instead, with a separate line from a central manifold to each fixture. The Department of Energy notes that fewer joints mean fewer leaks, and one line can be shut off without turning off the rest (Lesson 5).

Water hammer

Moving water is heavy. Stop it all at once, as a quick-closing valve does, and it slams to a halt with a bang. The Army manual calls this water hammer and lists the effects: noise, rattling pipes and sometimes leaky joints. The classic fix is an air chamber: a capped pipe near the valve that holds a cushion of air. Air squeezes and water doesn’t, so the air works like a car’s shock absorber. Lesson 9 has a diagram.

Keeping dirty water out: backflow

A cross-connection is any point where something that isn’t drinking water could come into contact with the drinking water. Water in a pipe normally flows one way, but if the supply pressure drops, water can be pulled backward, and with it whatever the end of a hose or a faucet is sitting in.

Plumbing systems block that path in several ways. An air gap is the simplest: the spout ends well above the rim of the sink or tub, so nothing can be sucked back up. A vacuum breaker lets air into the line when the pressure drops, which breaks the siphon. HUD’s manufactured home standard, as one example, requires a listed nonremovable backflow device on outdoor hose bibbs, a fixed air gap inside dishwashers and clothes washers, and vacuum breakers mounted at least 6 inches above the fixture’s flood level on flush valves.

Two ways to stop backflow. HUD’s standard for manufactured homes, for example, requires a listed nonremovable backflow device on outside hose bibbs and a fixed air gap in dishwashers and clothes washers.

StopLeave the meter and curb valves alone

The curb stop and the valve at the meter belong to the water utility. Use your home’s own main shutoff, and call the utility or a licensed plumber for the others. If your main shutoff is stuck or won’t close all the way, have a licensed plumber repair or replace it (Being an Adult 101).

Check yourself

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

About how much pressure does each foot of water height add?
Show the answer
A bit under half a psi. About 0.433 psi per foot: NIST’s 2,988.98 pascals for a foot of water divided by 6,894.757 pascals per psi.
A tap is 140 feet below the water surface in a tower. About what is the static pressure?
Show the answer
About 61 psi. 140 × 0.433 ≈ 61 psi.
The model code quoted by the City of Edmond sets what maximum static pressure before a PRV is required?
Show the answer
80 psi. 80 psi. Above that, the code calls for a pressure-reducing valve where the service enters.
A pipe’s radius is cut in half. In smooth flow, its resistance to flow…
Show the answer
grows about 16 times. Poiseuille’s law: resistance goes with 1 ÷ r⁴, and 2⁴ = 16.
What does an air gap do?
Show the answer
Keeps a spout above the rim so dirty water can’t be pulled back in. With open air between the spout and the flood level, nothing can be siphoned back into the supply.

Go deeper

OpenStax College Physics 2e, 11.4 Variation of Pressure with Depth in a Fluid Pressure and depth, with worked examples.
OpenStax College Physics 2e, 12.4 Viscosity and Laminar Flow; Poiseuille's Law Why long, thin pipes lose pressure when water flows.
City of San Diego Public Utilities: Pressure Reducing Valve Residential Guidelines A utility’s plain guide to pressure-reducing valves.
Being an Adult 101: Shut Off Your Home's Water Find and test your main shutoff.
The Trades, Cars, Lesson 8: Brakes (Pascal's principle at work) Pressure at work in a car: Pascal’s principle in the brakes.

Sources

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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.