Fans and energy

Fan and System Curve Calculator

Intersect entered fan data with a system resistance model to estimate the operating airflow and pressure.

Calculation inputs

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An example is loaded automatically. Replace it with your project values; the result updates as you edit.

Calculation Use one system model or compare two documented resistance conditions against the same fan curve.

Supplied fan curve (CFM, in. w.g.) One flow and matching fan pressure per line, sorted by strictly increasing CFM. Use actual manufacturer/test data at the selected speed/density and a consistent total/static basis. Example is synthetic, not a real fan.

System reference airflow (CFM) A known airflow used to define the system friction curve, not an assumed operating point.

Friction pressure at reference airflow (in. w.g.) Flow-dependent friction component only. Exclude the separate flow-independent term below.

Flow-independent pressure term (in. w.g.) Enter zero for a purely frictional closed air circuit. This is a system-curve constant, not fan static pressure or TESP.

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Result

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Results appear automatically when inputs are valid.

Formula and source basis

Concepts behind this calculation

Find and plot all intersections between your supplied fan pressure-flow curve and one or two quadratic system curves, without extrapolating beyond the fan data.

A fan operates where its supplied pressure-flow curve meets the modeled system resistance. The system model separates a friction component that grows with the square of flow from an explicit flow-independent pressure term. System reference airflow anchors that friction relationship; it is not an assumed operating point.

The fan curve is interpolated only between supplied data points. A nonmonotonic curve can intersect the system model more than once, and the arithmetic alone cannot choose a stable or safe intersection. Comparison mode changes system resistance while keeping the same fan curve, so fan speed, density, and configuration must remain compatible between scenarios.

How to use this calculator

Choose Calculation and enter Supplied fan curve on a consistent pressure basis.
Enter System reference airflow and, for a single case, Friction pressure at reference airflow and Flow-independent pressure term.
For comparison, document Scenario definitions and curve basis and enter each scenario’s friction and flow-independent pressures.
Review every intersection and the limits of the supplied curve.

Calculation method and applicability

P_system(Q) = P_constant + P_friction,ref × (Q/Q_ref)²; solve P_fan(Q) = P_system(Q) on each supplied linear fan segment for each condition

Q = airflow (CFM); P_constant = flow-independent pressure; P_friction,ref = friction pressure at Q_ref. Both pressures are in. w.g.; Q/Q_ref is dimensionless.

Input definitions and units

Calculation: Use one system model or compare two documented resistance conditions against the same fan curve. Choices: Single fan/system operating point; Compare two system conditions.
Supplied fan curve (CFM, in. w.g.): One flow and matching fan pressure per line, sorted by strictly increasing CFM. Use actual manufacturer/test data at the selected speed/density and a consistent total/static basis. Example is synthetic, not a real fan.
System reference airflow (CFM): A known airflow used to define the system friction curve, not an assumed operating point.
Friction pressure at reference airflow (in. w.g.): Flow-dependent friction component only. Exclude the separate flow-independent term below. Applies to Calculation: Single fan/system operating point.
Flow-independent pressure term (in. w.g.): Enter zero for a purely frictional closed air circuit. This is a system-curve constant, not fan static pressure or TESP. Applies to Calculation: Single fan/system operating point.
Scenario definitions and curve basis: Describe what differs between the two system cases. Use the same supplied fan curve only when speed, density and fan configuration are unchanged; describe system changes, not a second fan curve. Applies to Calculation: Compare two system conditions.
Scenario 1 friction pressure at reference airflow (in. w.g.): Flow-dependent component for system condition 1. Both scenarios use the shared reference airflow above. Applies to Calculation: Compare two system conditions.
Scenario 1 flow-independent pressure (in. w.g.): Explicit nonnegative constant term for condition 1; not fan static pressure or TESP. Applies to Calculation: Compare two system conditions.
Scenario 2 friction pressure at reference airflow (in. w.g.): Flow-dependent component for system condition 2. Enter measured or documented scenario inputs; no filter/damper multiplier is inferred. Applies to Calculation: Compare two system conditions.
Scenario 2 flow-independent pressure (in. w.g.): Explicit nonnegative constant term for condition 2; not fan static pressure or TESP. Applies to Calculation: Compare two system conditions.

How to interpret the result

Every reported intersection is a mathematical candidate within supplied support. Use the chart to understand resistance changes, then review operating range and stability with the actual equipment data.

Worked examples

Single fan/system operating point

Inputs: Synthetic fan points: (0 CFM, 2 in. w.g.), (1,000, 1), (2,000, 0). System friction is 1 in. w.g. at 1,000 CFM with zero constant term.

Calculation: At 1,000 CFM, the interpolated fan pressure and quadratic system pressure are both 1 in. w.g.

Result: One intersection: 1,000 CFM at 1 in. w.g. This is an illustrative mathematical curve, not manufacturer data.

Compare two system conditions

Inputs: Same synthetic fan curve. Compare system condition 1: 1 in. w.g. friction at 1,000 CFM, zero constant term; condition 2: 2 in. w.g. friction at the same reference flow, zero constant term. These are constructed conditions, not a real installation test.

Calculation: Condition 1 intersects at 1,000 CFM / 1 in. w.g. Condition 2 intersects where 2 - Q/1,000 = 2(Q/1,000)^2.

Result: Condition 2: approximately 780.776 CFM at 1.219 in. w.g., a difference of -219.224 CFM and +0.219 in. w.g. relative to condition 1.

Assumptions and limits

Fan data are interpolated linearly only between supplied points. No extrapolation, manufacturer data download or invented fan curve is performed.
System resistance is modeled as a quadratic friction term plus a nonnegative flow-independent term. Real equipment, controls and flow regimes may differ.
Multiple intersections are all shown. Dynamic stability, stall, surge, transient behavior, noise and safe fan operating range need separate review.
All fan/system pressures must use compatible total/static definitions and air-density conditions. The synthetic example is not a real product selection.

FAQs

Does no displayed intersection prove the fan cannot serve the system?

Not necessarily. The intersection may lie outside the supplied fan-curve range, where this tool does not extrapolate. Obtain adequate applicable curve data before drawing that conclusion.

Is the flow-independent pressure term the fan static pressure?

No. It is the part of the modeled system requirement that stays constant with flow. Fan static pressure is a fan-performance definition and must not be substituted as that constant.

Sources

ASHRAE 2024 Systems and Equipment, Chapter 21: FansPrimary fan/system operating-point and series/parallel context. Piecewise-linear interpolation is the disclosed numerical model, not a certified product selection.

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