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newSchoolWorks.com › VoTechWorks › Building & Construction › Plumbing & Pipefitting › Water Supply Systems: Design, Installation, and Testing › Lesson 5
BLD-PLUM06-04 · Lesson 5 · 10.5 clock hours · theory

Pressure, Flow, and System Hydraulics for Plumbers

Objective. Students will calculate static water pressure, pressure loss due to elevation change, and equivalent length of common fittings, and determine whether a pressure-reducing valve is required for a given building scenario.

Why Hydraulics Matter in Plumbing

Every fixture in a building depends on water arriving at the right pressure and flow rate. Too little pressure and a shower trickles; too much pressure and pipes hammer, joints leak, and water heaters fail prematurely. As a plumber, you must be able to predict what the pressure and flow will be at any point in a system — before you cut a single pipe. That skill begins with understanding static pressure, elevation change, friction loss, and pressure-reducing valves (PRVs).

Static Water Pressure and Feet of Head

Water pressure is commonly measured in pounds per square inch (psi). It can also be expressed as feet of head — the height of a water column that produces that pressure. The conversion between the two is fixed by the weight of water:

These two conversion factors are the backbone of elevation calculations. Memorize them.

Pressure changes with elevation. When water travels downward through a pipe, it gains pressure (energy is added by gravity). When water travels upward, it loses pressure (it must work against gravity). The rule is simple:

Example: if street pressure at the meter is 65 psi and a fixture is 20 feet above the meter, the static pressure available at the fixture is 65 − (20 × 0.433) = 65 − 8.66 ≈ 56.3 psi — before any friction losses.

Minimum Fixture Supply Pressures

Plumbing codes establish minimum flow pressures at fixtures. Common minimums include 8 psi for most fixtures and higher values for specialty fixtures such as flushometer-valve water closets. Always check the applicable code for the jurisdiction you are working in, because the authority having jurisdiction (AHJ) has final say.

Friction Loss and Equivalent Length

Water moving through pipe loses pressure to friction — the resistance between the moving fluid and the pipe wall. Friction loss increases with:

Fittings such as elbows, tees, and valves create turbulence that behaves like additional straight pipe. The equivalent length method converts each fitting into a length of straight pipe that causes the same friction loss. An elbow in a 3/4-inch line might have an equivalent length of 2 feet; a gate valve (fully open) is usually far less than an elbow; a globe valve is much more. Values are published in tables by organizations such as PHCC and by pipe and valve manufacturers.

Total equivalent length (TEL) = actual pipe length + sum of all fitting equivalent lengths.

Once you have TEL, you look up friction loss per 100 feet of pipe from a friction-loss table (at your design flow rate and pipe size), then calculate:

> Friction loss (psi) = (TEL ÷ 100) × loss per 100 ft

Subtract that friction loss from your available static pressure. If the result is below the code minimum at the end fixture, you must upsize the pipe, reduce the run, or redesign the branch.

Pressure-Reducing Valves (PRVs)

When street (inlet) pressure exceeds the safe or code-allowable working pressure for a building's piping and fixtures, a pressure-reducing valve is required. Many codes require a PRV when inlet pressure exceeds 80 psi; verify the threshold in your local code.

A PRV works by:

1. Receiving high-pressure water at the inlet.

2. Passing it through an internal spring-and-diaphragm mechanism.

3. Delivering a regulated, lower pressure on the outlet side.

The adjustment screw compresses or relaxes the spring, raising or lowering the outlet set-point. Most residential PRVs are factory-set around 50–60 psi, but the plumber sets the final pressure during commissioning. Typical residential outlet settings run 45–80 psi depending on the building's needs and the local code maximum.

Testing a PRV: attach a pressure gauge at the outlet (or at a nearby hose bib), read static pressure, compare to the set-point, and adjust if necessary. Always measure with no flow (static) and then with flow to verify the valve does not drop excessively under load.

Safety Notes

Putting It All Together

For any new branch or fixture, the design process follows this sequence:

1. Find available static pressure at the source (meter or main).

2. Adjust for elevation change (+ going down, − going up).

3. Calculate total equivalent length including all fittings.

4. Calculate friction loss and subtract it.

5. Compare the remaining pressure to the code minimum at the fixture.

6. If inlet pressure exceeds the code or safe maximum, specify a PRV.

This logical, step-by-step approach is exactly what the NCCER Plumbing curriculum and state licensing exams expect you to demonstrate.

Key terms8

Static pressureWater pressure in a pipe or system when no water is flowing; the baseline pressure before friction losses are subtracted.
Feet of headA unit expressing pressure as the height of a water column; 1 foot of head equals 0.433 psi, and 1 psi equals 2.31 feet of head.
Elevation pressure changeThe gain or loss of pressure caused by a change in height; pressure increases 0.433 psi per foot of downward travel and decreases 0.433 psi per foot of upward travel.
Friction lossThe reduction in water pressure caused by resistance as water moves through pipe and fittings; it increases with higher velocity, smaller diameter, and longer runs.
Equivalent lengthThe length of straight pipe that would produce the same friction loss as a given fitting or valve; used to simplify friction-loss calculations.
Total equivalent length (TEL)The sum of the actual pipe length plus the equivalent lengths of all fittings and valves in a pipe run; used to calculate total friction loss.
Pressure-reducing valve (PRV)A valve that automatically reduces and regulates high inlet water pressure to a lower, preset outlet pressure, protecting downstream piping and fixtures.
Minimum fixture supply pressureThe lowest pressure at which a fixture will operate correctly as defined by the applicable plumbing code; the design must deliver at least this pressure at the fixture outlet.

Worked examples4

1. The water meter is at street level and reads 70 psi static pressure. A bathroom fixture is located 28 feet above the meter. What is the static pressure available at the fixture?
  1. Identify the elevation change: the fixture is 28 ft above the meter, so pressure decreases.
  2. Calculate pressure lost to elevation: 28 ft × 0.433 psi/ft = 12.124 psi.
  3. Subtract from street pressure: 70 psi − 12.124 psi = 57.876 psi.
  4. Round to a practical value: approximately 57.9 psi static pressure at the fixture.
Answer: Approximately 57.9 psi static pressure is available at the fixture before any friction losses.
2. A hot-water branch has 18 feet of actual 3/4-inch pipe. Fittings on the branch include four 90° elbows (equivalent length 2.0 ft each), two tee branches (equivalent length 3.5 ft each), and one fully open gate valve (equivalent length 0.5 ft). What is the total equivalent length?
  1. List actual pipe length: 18.0 ft.
  2. Calculate elbow contribution: 4 elbows × 2.0 ft = 8.0 ft.
  3. Calculate tee contribution: 2 tees × 3.5 ft = 7.0 ft.
  4. Calculate gate valve contribution: 1 valve × 0.5 ft = 0.5 ft.
  5. Sum all values: 18.0 + 8.0 + 7.0 + 0.5 = 33.5 ft total equivalent length.
Answer: Total equivalent length = 33.5 feet.
3. Using the TEL from the previous example (33.5 ft of 3/4-inch pipe at the design flow rate), the friction-loss table shows a loss of 6.0 psi per 100 feet. Static pressure available at the start of the branch is 57.9 psi. The end fixture requires a minimum of 8 psi. Does pressure meet code? If not, what pressure shortfall exists?
  1. Calculate friction loss: (33.5 ÷ 100) × 6.0 psi = 0.335 × 6.0 = 2.01 psi.
  2. Subtract friction loss from available static pressure: 57.9 − 2.01 = 55.89 psi at the end fixture.
  3. Compare to code minimum: 55.89 psi > 8 psi minimum.
  4. Conclusion: pressure easily meets the code minimum; no upsizing is needed for this branch.
Answer: Pressure at the end fixture is approximately 55.9 psi, which exceeds the 8 psi code minimum. The branch design is adequate.
4. A building receives 110 psi of street pressure. Is a PRV required (assuming a common code threshold of 80 psi)? If so, to what pressure should the PRV outlet be set to deliver a reasonable residential supply pressure?
  1. Compare street pressure to the threshold: 110 psi > 80 psi, so a PRV is required.
  2. Select an appropriate outlet set-point within a common residential range; 60 psi is a typical, code-compliant choice that provides adequate pressure while protecting fixtures and piping.
  3. Verify the chosen set-point (60 psi) does not exceed the code maximum and is above the minimum needed by the highest fixture after elevation and friction losses are accounted for.
  4. The PRV should be adjusted to deliver approximately 60 psi on the outlet side.
Answer: Yes, a PRV is required because 110 psi exceeds the 80 psi threshold. The outlet should be set to approximately 60 psi (or as required by final system calculations and local code).

The math in this lesson2

Learners total fitting equivalent lengths with pipe length to find total equivalent length.
7.NS.3A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Pressure calculation worksheet: students solve 10 problems involving conversion between psi and feet of head, pressure change with elevation (gain going down, loss going up), and minimum fixture supply pressure from code; instructor reviews solutions on the board. · Equivalent-length exercise: given a partial isometric drawing of a hot water branch with four elbows, two tees, and one gate valve, students look up equivalent lengths from a standard table and compute total equivalent pipe length, then determine whether friction loss brings pressure below code minimum at the end fixture. · PRV hands-on demonstration: instructor shows a PRV removed from service, identifies the spring, diaphragm, and adjustment screw, and measures inlet vs. outlet pressure with gauges; students record readings and determine what the outlet setting should be for a building with 110 psi street pressure.
Materials: Pressure calculation worksheet, equivalent-length table (from PHCC or manufacturer reference), partial isometric drawing handout, PRV (cutaway or removed from service), two pressure gauges, whiteboard, calculators
Check: Pressure calculation worksheet: score of at least 80% (8 of 10 correct); equivalent-length exercise: correct total equivalent length within 5% of answer key; PRV determination: correct yes/no with supporting pressure calculation

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Plumbing — Water Distribution and System Hydraulics
1. How much pressure does a water column gain for every foot of downward elevation change?
NCCER Plumbing — Water Distribution and System Hydraulics
2. A meter shows 65 psi static pressure. A fixture is located 30 feet above the meter. What is the static pressure at the fixture before friction losses?
NCCER Plumbing — Water Distribution and System Hydraulics
3. What is the correct conversion for 1 psi expressed in feet of head?
NCCER Plumbing — Water Distribution and System Hydraulics
4. A 3/4-inch branch has an actual pipe length of 20 feet. Three 90° elbows have an equivalent length of 2.0 ft each. One tee (branch flow) has an equivalent length of 3.5 ft. What is the total equivalent length?
NCCER Plumbing — Piping Systems and Fittings
5. Which fitting typically has the HIGHEST equivalent length (greatest friction resistance) of those listed?
State Journeyman Plumber — Plumbing Codes and Regulations
6. When is a pressure-reducing valve (PRV) most commonly required by plumbing codes?
NCCER Plumbing — Water Distribution and System Hydraulics
7. Which internal component of a PRV is responsible for sensing downstream pressure and modulating the valve position?
NCCER Plumbing — Safety and Health
8. Before removing a PRV from a pressurized system for inspection, what is the correct safety sequence?
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