Fire Flow Analysis: Engineering Solutions for Municipal Water Distribution Systems

A fire flow analysis answers one question: can your water distribution system deliver the flow the fire code requires, for as long as it requires, while holding at least 20 psi of residual pressure? For a municipal system that takes three pieces of work: the required fire flow from the adopted fire code, hydrant flow tests showing what the system delivers today, and a calibrated hydraulic model that checks every hydrant at maximum day demand.

What does a fire flow analysis measure?

It compares the required fire flow with the available fire flow at each hydrant, both at 20 psi residual. Appendix B of the 2021 International Fire Code (linked as Seattle publishes it) defines fire flow as the flow available for fire fighting, measured at 20 psi residual pressure. A hydrant that delivers the gallons but pulls the main below 20 psi fails.

In Texas, a public water system must be designed to hold 35 psi everywhere in the distribution network at 1.5 gpm per connection, and, when it is intended to fight fires, 20 psi under combined fire and drinking water flow (30 TAC §290.44(d)).

Where does the required fire flow come from?

From the fire code your city or county adopted, as applied by its fire code official. Appendix B is the IFC's tool for this, but it is not mandatory unless the adopting ordinance references it. Where it applies:

• A one- or two-family dwelling up to 3,600 square feet with no sprinklers needs 1,000 gpm for one hour.
• Other buildings read Table B105.1(2) by construction type and floor area: 1,500 gpm for two hours at the low end, up to 8,000 gpm for four hours.
• Sprinklers under IFC Section 903.3.1.1 cut that to 25% of the table value, but not below 1,000 gpm (1,500 gpm under Section 903.3.1.2).
• The fire code official may reduce the flow for isolated buildings in rural areas or small communities where full fire flow is impractical, or raise it, up to double, where group fires are a risk.

NFPA 1142 does not set fire flow for a city grid: Appendix B points to it only where adequate and reliable water supply systems do not exist (Section B103.3).

Texas adds a floor for some utilities. Where 30 TAC §290.46(x) and (y) apply, in cities that meet the population tests in (x) or adopt (y) by ordinance, a covered utility must give residential hydrants at least 250 gpm for two hours at 20 psi, on top of maximum daily demand. Neither binds a city-owned utility.

Get the agreed number in writing before modeling starts. A short water system model and fire flow memo keeps everyone on the same target.

How does ISO's rating fit in?

Separately, for insurance. ISO's Public Protection Classification rates communities Class 1 to 10 to help set fire insurance premiums. In its Fire Suppression Rating Schedule, water supply is worth up to 40 of 105.5 points: supply beyond daily maximum consumption, hydrants within 1,000 feet of representative locations, and how often hydrants are inspected and flow tested.

How does a hydrant flow test work?

Use two hydrants, one to read pressure and one to flow water. Read static pressure at the test hydrant. Open the flow hydrant, take a pitot reading at its outlet for the flow, and read residual pressure at the test hydrant. Read static again after closing, as a check.

The University of Tennessee's MTAS guide to conducting a fire flow test notes that neither NFPA nor AWWA recommends a single-hydrant test, and quotes NFPA 291: flow enough to drop the residual hydrant's pressure at least 25 percent. Then project to 20 psi:

Q at 20 psi = Q test × ((static - 20) ÷ (static - residual))^0.54

Example: 70 psi static and 50 psi residual while flowing 1,000 gpm is about a 29% drop, and projects to about 1,640 gpm at 20 psi. Record tank levels and running pumps with each test, because both sit inside the number.

How do you build and calibrate the hydraulic model?

Build it at maximum day demand, match it to the hydrant tests, then check fire flow at every hydrant. Where the area is still growing, model buildout demand too. EPA's EPANET is public domain software for pressurized pipe networks; our EPANET water modeling post covers setup.

The EPANET 2.2 user manual gives Hazen-Williams C for new pipe (130 to 140 for cast iron, 140 to 150 for plastic) and warns that roughness can change considerably with age. Calibration adjusts C on older mains until the model reproduces the measured static and residual pressures.

For the fire flow run, the manual describes an emitter that computes the flow available at a junction at a minimum residual pressure. One trap: a demand-driven run delivers every demand, even at negative pressure. A hydrant showing full flow below zero psi has failed. Pressure-driven analysis is one way to avoid that false pass.

Why do municipal systems fail fire flow?

Because the pipe, storage, pumps or valves serving a hydrant cannot deliver the required flow at 20 psi. The usual causes:

• Mains sized for domestic flow. Texas minimum line sizes "are for domestic flows only and do not consider fire flows" (§290.44(c)).
• Dead ends, where one path carries the whole fire flow. TCEQ requires designs with a minimum of dead ends (§290.44(d)(6)) and monthly flushing of those that exist (§290.46(l)).
• Old pipe that has roughened with age.
• Elevation. One psi equals 2.31 feet of water (Kansas State University's conversion sheet), so a hydrant 100 feet higher in the same zone starts about 43 psi down.
• Storage and pumps. 1,500 gpm for two hours is 180,000 gallons, which a low tank or a booster with its largest pump down may not cover.
• Valves. A pressure-reducing valve set too low, or a valve left closed after a repair, can starve a street.

What fixes a fire flow deficiency?

Loop it, upsize it, store water closer, or boost it. Pipe size is the big lever: in the Hazen-Williams formula EPANET uses, head loss varies with diameter to the power of minus 4.871. At the same flow, a 6-inch to 8-inch upsize cuts friction loss to about a quarter.

• Loop dead ends so hydrants draw from two directions.
• Upsize the controlling segment the model finds, not the whole street.
• Add storage inside the zone.
• Give a high area its own pressure zone and booster.
• Reset pressure-reducing valves and exercise mainline valves.

For utilities and municipalities, the useful output is a ranked list of fixes that drops into a capital plan or funding application.

Is your system ready for a fire flow analysis?

Yes to all six means you can start. Each no is the first task.

1. Has the fire code official confirmed the required flow and duration in writing?
2. Do you know which fire code edition and appendices your jurisdiction adopted?
3. Do you have recent hydrant flow tests, with tank levels and pumps recorded?
4. Is your pipe data complete: diameter, material, age and location?
5. Do you have maximum day demand from production or meter records?
6. Do you have pump curves, tank dimensions and valve settings?

Ask your engineer which hydrants control the result, which single fix helps the most of them, and what changes with the largest pump out of service.

Limits: this is general education on the 2021 IFC Appendix B, 30 TAC Chapter 290 and published testing guidance as of October 2026. Your adopted code, fire code official and engineer decide what applies, and nothing here predicts a review outcome.

Frequently asked questions

What is fire flow?

The flow a water supply can deliver for fire fighting while holding 20 psi of residual pressure, as IFC Appendix B defines it.

How long does fire flow have to last?

Under Appendix B, one hour for a small home without sprinklers, up to one hour for a sprinklered home, and two to four hours otherwise, rising with the flow. Where the Texas §290.46 floor applies, it is two hours.

I'm the public works director for a small Texas city, and our fire chief says hydrants on our old 4-inch lines can't deliver fire flow. Where should we start?

Run two-hydrant flow tests on those lines, project each to 20 psi, and compare them with the flow the chief requires under your adopted code. Then model the area at maximum day demand to find the segments that control, and fix those first.

I run a Colorado water district where homes at the top of our pressure zone sit about 120 feet above homes near the tank. Why do the high hydrants fail flow tests when the low ones pass?

Elevation takes about 52 psi off the high hydrants before any water flows (120 ÷ 2.31), and friction during the test comes off what is left. The options: a separate upper zone with a booster sized for fire flow plus maximum day demand, storage serving that area, or larger mains.

Related resources

• How to Conduct a Fire Flow Analysis for a Texas Development or Subdivision
• Water Storage and Pressure Zone Design for McKinney's Rolling Terrain: Engineering for Collin County Topography
• How to Read a Hydraulic Grade Line
• Capital Improvement Planning for Small Municipalities

Not sure your hydrants can deliver the fire flow your code requires?

We coordinate the hydrant tests, build and calibrate the model, and rank the fixes, so your capital plan targets the pipe that actually limits fire flow.

Modern Engineering Solutions is headquartered in McKinney, Texas, with 525+ projects completed. Call (214) 833-6748.

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