Fan pressure and modeled duct losses

Match the calculation boundary to the equipment rating before selecting a fan.

What the calculator adds

DuctStatic sums the modeled losses: straight-duct friction, fitting losses calculated as Co × Pv, and entered published pressure drops. The largest path sum is the critical path. If both inlet and outlet ductwork are modeled, the largest inlet path and largest outlet path are added. Parallel paths are not added together.

Adjusted loss = modeled critical loss × (1 + safety factor % / 100)

This is a loss budget at the assigned design airflows, not an automatic fan selection or a solution for how an unbalanced system will distribute airflow.

Define the system boundary

Identify the start and end of each path, including the room, intake, discharge, or equipment connection it represents. Include each required loss once: ductwork, terminals, dampers, filters, coils, silencers, and intake or discharge losses where applicable. The calculator only includes components you model or enter; a safety factor does not replace missing equipment data.

For a complete total-pressure balance, also account for the pressure conditions at the endpoints. A discharge jet, a pressurized room, or an unmodeled return path can change the required fan duty. Do not add a separate exit loss if the terminal's published rating already includes it.

Fan total pressure and fan static pressure

At any section, Pt = Ps + Pv. Fan total pressure is the total-pressure rise from fan inlet to fan outlet. Under the fan-rating convention:

Fan total pressure = Pt,out − Pt,in
Fan static pressure = Fan total pressure − Pv,fan outlet

The outlet velocity pressure uses the fan's rated outlet area and airflow at the relevant density. It is not necessarily the velocity pressure in the connected duct. Fan static pressure is also not simply the difference between the two static-pressure readings when inlet velocity is nonzero.

Only after completing a consistent total-pressure balance should you subtract fan outlet velocity pressure to compare with a fan-static curve. DuctStatic does not automatically convert its loss sum to that rating. Do not subtract an arbitrary duct velocity pressure from the result.

Worked conversion (independently checked)

Shown in the default English view; the header toggle switches the same values to SI (m, m/s, Pa).

Supply fan drawing from atmosphere, discharging into a 12-inch round duct at 1,200 CFM, standard air (0.075 lb/ft³), no inlet ductwork. DuctStatic reports an outlet-path modeled loss of 1.50 in. w.g..

  1. Because every modeled loss (duct friction, fittings, and the exit/discharge loss to atmosphere) is expressed in total-pressure terms and the inlet is atmospheric, the fan total pressure ≈ 1.50 in. w.g..
  2. Duct velocity: V = CFM / A = 1200 / (π·1²/4) = 1,528 fpm, so Pv = (V/4005)² = (1528/4005)² ≈ 0.146 in. w.g.
  3. Fan static pressure (fan-rating convention): FSP = FP_total − Pv,fan outlet = 1.50 − 0.146 ≈ 1.35 in. w.g.

Use the fan's rated outlet area, not the connected duct size, for step 3 when they differ. If the system's last modeled loss is an exit or discharge loss that already converts the outlet velocity pressure to atmosphere, do not subtract Pv a second time.

Equipment external static pressure

An equipment manufacturer's external static rating refers to specified connections outside the equipment casing, with a stated allowance for internal components. Check which filters, coils, accessories, and connection losses that rating includes. It is not interchangeable with either the raw duct loss sum or a bare-fan static-pressure rating.

Terminal and published ratings

Confirm whether each terminal or equipment drop is rated on a static or total pressure basis, at which inlet and outlet sections, and at what airflow and density. Published ΔP is added as entered; it does not scale automatically when airflow changes.

For a device without a fan, the total-pressure loss between two defined sections is ΔPt = Ps,in − Ps,out + Pv,in − Pv,out. A catalog static-pressure drop may therefore need a velocity-pressure correction before use in a total-pressure budget. Check the catalog definition first: inlet static pressure relative to a room is not the same rating as a static-pressure difference between two duct taps. Keep the basis consistent and avoid counting terminal velocity effects twice.

Before using the result

Pressure terminology references

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