How a Plumbing System Works

Every building has two plumbing systems that must never mix. One brings clean water in under pressure. The other carries used water out by gravity, with air moving through it. Almost every plumbing rule, from pipe slope to the height of a vent, exists to keep those two systems working and apart. If you understand why, you can reason through problems you have never seen and remember rules instead of memorizing them.

What You Will Be Able to Do

• Trace water from the street to a faucet and from a drain to the sewer
• Explain what traps, vents, slope, and cleanouts each do
• Choose the right kind of fitting for a change of direction in a drain
• Name the common pipe materials and what each one is used for
• Read common symptoms, such as gurgling drains and banging pipes, and name the likely cause

The Supply Side

Water supply piping works under pressure, so it can run uphill, sideways, and through walls. Follow it from the street in:

1. Water service line: Carries water from the public main or a well to the building.
2. Meter and main shutoff: The meter measures use. The main shutoff stops water to the whole building. Know where it is before you touch anything.
3. Pressure reducing valve (PRV): Installed where street pressure runs high.
4. Cold water distribution: Mains and branches that feed each fixture.
5. Water heater: Takes cold water in and sends hot water out through its own set of lines.
6. Fixture stops: Small shutoff valves at each sink and toilet so one fixture can be serviced alone.

Pressure

Residential water pressure usually sits between 40 and 80 psi. Most codes cap static pressure (pressure with nothing running) at 80 psi and call for a PRV above that. High pressure wears out valves and hoses. Low pressure gives weak showers. To measure it, screw a gauge onto a hose bibb near the meter.

Height costs pressure. Water loses about 0.43 psi for every foot it climbs. If a gauge in the basement reads 60 psi, a shower head 24 feet higher starts with about 50 psi, because 24 x 0.43 is roughly 10. Friction in pipes, fittings, and the meter takes more.

Sizing by Demand

Fixtures rarely all run at once, so pipe is not sized by adding up every faucet. Instead, each fixture gets a number called a fixture unit, a standard measure of how much demand it is likely to add. Water supply fixture units (WSFU) size supply pipe. Designers also watch velocity, because fast water is noisy and wears out pipe. A common design ceiling is about 8 feet per second.

Expansion and Water Hammer

Water expands when it is heated. In an open system, that extra volume pushes back into the street main. A check valve, PRV, or backflow preventer on the service closes the system, so the pressure has nowhere to go and the relief valve on the water heater starts to drip. A thermal expansion tank gives the water somewhere to expand.

The relief valve on a water heater is the last line of defense against an overheated tank. Its discharge pipe must end within 6 inches of the floor or at another approved location, and it must never be capped or valved.

Water hammer is the bang you hear when a valve closes fast. Moving water stops suddenly and the pressure surge shakes the pipe. Washing machines, dishwashers, and other quick-closing valves are the usual causes. A water hammer arrestor absorbs the shock, and solid pipe supports keep the shock from rattling the line.

Protecting Clean Water

A cross-connection is any link between drinking water and something that could contaminate it, such as a hose in a bucket of chemicals. If supply pressure drops, that dirty water can be pulled backward. This is called backflow.

The simplest protection is an air gap, an open vertical space between the end of a supply pipe and the highest point the water below can reach. It must be at least twice the diameter of the supply opening and never less than 1 inch. Where an air gap will not work, a mechanical backflow preventer does the job. For high-hazard connections, such as an irrigation system injecting fertilizer, that means a reduced pressure zone assembly (RPZ).

The Drain Side

Drain, waste, and vent piping, called DWV, is not pressurized. Gravity moves the water, so everything depends on slope and air.

Traps

A trap is a curved section of pipe that stays full of water. That water seal stops sewer gas from rising into the room. Most codes require a seal of at least 2 inches and no more than 4 inches. Each fixture gets one trap. Two traps in a row trap air between them and block flow, so double trapping is not allowed.

Vents

Water going down a pipe pulls a vacuum behind it, like liquid in a straw with your finger over the top. Without air coming in, that suction would pull the water out of nearby traps. Vents let air into the system so traps keep their seal, and they let sewer gas escape above the roof.

Vents must stay dry and clear. A vent that runs sideways must first rise at least 6 inches above the flood-level rim of the highest fixture it serves. The flood-level rim is the top edge a fixture would overflow from. That way a clogged drain cannot fill the vent with waste.

The distance from a trap to its vent is limited. Too much distance lets the drain run full and siphon the trap. Limits depend on your code. For a 1-1/2 inch drain, the International Plumbing Code (IPC) allows 6 feet and the Uniform Plumbing Code (UPC) allows 3-1/2 feet.

Slope

Horizontal drains must fall toward the sewer. Too little slope and solids settle. A common residential default is 1/4 inch per foot. The IPC requires that for pipe 2-1/2 inches and smaller, then allows 1/8 inch per foot for 3 to 6 inch pipe and 1/16 inch per foot for 8 inch and larger. Your adopted code and local amendments set the exact break point.

Worked example. A 2-inch drain runs 18 feet from a laundry to a stack. At 1/4 inch per foot, 18 x 1/4 gives 4-1/2 inches of total fall. If the start of the pipe sits 30 inches above the floor, the far end must sit at 25-1/2 inches or lower.

From Fixture to Sewer

Fixture drains join branches. Branches join stacks, which are the vertical drains. Stacks feed the building drain, the lowest horizontal pipe inside the building. Outside the foundation, it becomes the building sewer, which runs to the public sewer or septic tank.

Pipes grow as they collect more flow. Drainage fixture units (DFU) add up the load. A residential toilet is usually 3 DFU and needs at least a 3-inch drain. Lavatories commonly use 1-1/4 or 1-1/2 inch pipe, while showers and laundry standpipes use 2 inch.

Drain Fittings

Drain fittings are shaped so that water keeps moving in the right direction. Using the wrong one causes clogs and failed inspections.

A sanitary tee laid flat throws flow across the pipe and collects solids. That is why it is not allowed for a horizontal-to-horizontal connection.

Cleanouts

A cleanout is a removable plug that gives you access for a cable or jet. Cleanouts are required at the base of stacks, at changes of direction greater than 45 degrees, at set intervals along long runs, and at the point where the building drain becomes the sewer. Keep them reachable. A cleanout behind a finished wall is useless.

Materials at a Glance

Supports are general guides. Your adopted code sets the real number.

A few things to remember:

• Copper is marked by wall thickness. K is thickest and goes underground, L is medium and is widely used indoors, and M is thin and is allowed in many areas.
• PEX breaks down in sunlight, so it must not be left exposed outdoors.
• Standard PVC is for cold water and drains, not hot water. CPVC handles hot and cold supply up to about 180 F.
• Galvanized steel rusts from the inside and closes up over time. You will see it in old houses, but it is not used for new water lines. It also corrodes badly in soil, which is why it is not used for buried drains.

Testing the Work

Plumbing is tested before walls close.

• Water supply: Tested at working pressure or the pressure the inspector requires, and every joint is checked.
• DWV water test: Openings are plugged and the system is filled to a head of at least 10 feet of water for 15 minutes. The level must not drop.
• DWV air test: Some codes allow a 5 psi test for 15 minutes. Makers of plastic pipe warn against testing plastic with compressed air, because a failed fitting can explode. Use the method the manufacturer and your inspector allow.

Reading the Symptoms

Once you understand the system, symptoms start to explain themselves.

Common Mistakes

Check Your Understanding

1. A bathroom sink gurgles every time the tub drains. The trap has water in it right now. What is your first thought?

Air. The drain is likely pulling air through the sink trap because the vent is blocked or missing. Check the vent before you blame the drain.

2. A 3-story building's water service measures 70 psi at the ground floor. About how much pressure is lost before reaching a fixture 30 feet higher?

About 13 psi, because 30 x 0.43 is roughly 13. That fixture starts with about 57 psi before friction loss.

3. A new branch drain needs to join a horizontal building drain. Which fitting do you reach for?

A wye with a 1/8 bend, often called a combo. A sanitary tee laid on its back is not allowed there.

4. A homeowner added a check valve on the water service, and the water heater relief valve now drips. What do you suspect?

Thermal expansion. The system is now closed, so heated water has nowhere to go. An expansion tank is the usual fix.

Key Takeaways

• Supply is pressurized and clean. Drains run on gravity and air. The two must never connect.
• Traps block sewer gas, vents protect traps, and slope keeps solids moving.
• Residential pressure usually runs 40 to 80 psi, and water loses about 0.43 psi per foot of rise.
• Drain fittings are chosen by direction of flow, not by what fits.
• Fixture units size pipe by likely demand, not by adding up every fixture.
• Your adopted code and local amendments decide the final numbers.