Apply documented inlet and outlet geometry to estimate system-effect pressure and supported airflow changes.
Concepts behind this calculation
Estimate AMCA 201-23 installation-effect pressure bounds from specific inlet/outlet geometry, then optionally solve compatible fan and system curve consequences.
Installation system effect is the additional pressure allowance associated with a documented departure from the fan’s rating arrangement. Its coefficient multiplies velocity pressure at a source-defined reference area and actual density. A restricted inlet, short outlet or nearby damper therefore needs the appropriate geometry and velocity basis; a coefficient taken from another fan family or inlet plane is not a general fitting-loss value.
Ordinary resistance and the additional allowance must be accounted for once each. The damper method adds only the excess above an already-counted catalog drop. Coefficient ranges remain pressure ranges, and unsupported source cells are not assumed to have zero effect. Estimating an airflow consequence requires compatible fan and system curves; a pressure shortfall at one comparison flow does not itself determine the new flow.
How to use this calculator
Choose Calculation scope, complete Fan, geometry and rating-source reference, and establish Geometry and loss accounting.
Enter Comparison airflow, Actual air density, Ordinary system resistance and Available fan pressure on matching pressure definitions.
Select the applicable Inlet installation condition and Outlet installation condition, then enter their reference areas, source profiles, exact geometry ratios and any baseline damper drop.
For curve consequences, document Curve applicability and enter the Ordinary quadratic pressure, Ordinary flow-independent pressure term and Source fan curve. Review bounds and intersection support.
Calculation method and applicability
SE=C×ρ×(V/1097)²in. w.g., V=Qreference/Areference. Keep source coefficient bands. Obstruction velocity divides by free fraction. Damper extra=(source multiplier−1)×baseline catalog drop. Add inlet and outlet allowances once to ordinary resistance; optional fixed-geometry system curves add allowance×(Q/Qref)².
SE = system-effect pressure allowance (in. w.g.); C = applicable source coefficient; ρ = actual air density (lb/ft³); V = velocity (FPM) at the specified source reference area.
Input definitions and units
Calculation scope
Point mode estimates the pressure allowance. Curve mode also solves all supported baseline and allowance intersections; it requires fixed geometry and matching real fan/system data. Choices: Pressure allowance at one airflow; Allowance and curve consequences.
Fan, geometry and rating-source reference
Required: manufacturer/model/test arrangement, actual inlet/outlet geometry, referenced source figures/cells, inlet flow split, rating pressure/density and ordinary-loss accounting. Compare to source drawings. Source notes are not product or geometry authentication.
Geometry and loss accounting established?
Confirm selected effects are not already in the rating or ordinary pressure; at most one inlet and one outlet profile are combined, with interactions reviewed. Ordinary elbow loss remains separately included. Use manufacturer-specific data where available. Choices: Not established — withhold result; Documented matching geometry and accounting.
Comparison airflow (CFM)
Total fan airflow at the point. Inlet flow-share input handles the actual flow through a reference inlet; do not apply total flow to one of two equal inlets.
Actual air density (lbm/ft³)
Same air condition as the fan rating and pressure inputs. The printed I-P formula uses actual density with the1097constant.
Ordinary system resistance at this airflow (in. w.g.)
Includes normal duct/fitting losses and any baseline catalog damper drop, but excludes the selected additional installation allowance. Match fan static/total pressure definitions.
Available fan pressure at this airflow (in. w.g.)
Same pressure definition/density as ordinary resistance; reported signed margin uses the upper allowance. This point margin does not alone establish an operating point.
Inlet installation condition
Select one supported source geometry or explicitly no inlet allowance. Overlapping inlet obstructions cannot be added as independent rows. Choices: No inlet effect added; Centrifugal round inlet elbow — Figure 11.4; Axial inlet elbow — Figure 11.2; Wall or inlet cabinet — Table 11.10; Projected inlet obstruction — Figure 11.13.
Actual inlet reference area (ft²)
Full area of the source-defined collar/inlet plane. Obstruction mode computes adjusted free-area velocity separately. Cabinet/wall ratios must use the diameter defined by the source drawings. Applies to Inlet installation condition: Centrifugal round inlet elbow — Figure 11.4 or Axial inlet elbow — Figure 11.2 or Wall or inlet cabinet — Table 11.10 or Projected inlet obstruction — Figure 11.13.
Share of total airflow at this inlet (fraction)
Documented share 0 < share≤1. For two equal inlets use 0.5 with area of one inlet; do not multiply pressure by2 afterward. Applies to Inlet installation condition: Centrifugal round inlet elbow — Figure 11.4 or Axial inlet elbow — Figure 11.2 or Wall or inlet cabinet — Table 11.10 or Projected inlet obstruction — Figure 11.13.
Centrifugal inlet-elbow profile
Exact Figure 11.4 round two/three/four-piece source profiles. FC and BC families are separate; unknown shapes, guide vanes and interpolated geometries are unsupported. Choices: FC — two-piece round elbow; FC — four-piece round elbow; BC — two-piece round elbow; BC — three-piece round elbow; BC — four-piece round elbow. Applies to Inlet installation condition: Centrifugal round inlet elbow — Figure 11.4.
Axial inlet-elbow profile
Figure 11.2 test data cover stated commercial tubeaxial/vaneaxial hub-to-tip range25–61%. Missing cells and reported instability concerns are not waived by a pressure result. Choices: Tubeaxial — two-piece; Tubeaxial — four-piece; Vaneaxial — two-piece; Vaneaxial — four-piece; Propeller — two-piece. Applies to Inlet installation condition: Axial inlet elbow — Figure 11.2.
Straight inlet distance / source diameter (D)
Enter an exact source-table ratio. Centrifugal columns0, 1, 2, 5; axial columns0, 0.5, 1, 2, 3. A listed ratio may still have no data for the selected model; no nearest-row interpolation. Applies to Inlet installation condition: Centrifugal round inlet elbow — Figure 11.4 or Axial inlet elbow — Figure 11.2.
Wall or cabinet source profile
Match Figure 11.10 drawings, symmetry and the source-defined inlet diameter. Cabinet2 D/3 D height and lateral wall spacing are distinct dimensions. No automatic arbitrary AHU cabinet correction. Choices: Double-inlet centrifugal — plenum/wall; Single-inlet BC — cabinet 2D high; Single-inlet BC — cabinet 3D high; Single-inlet FC — cabinet 2D high; Single-inlet FC — cabinet 3D high; Axial flow near wall; Propeller near wall. Applies to Inlet installation condition: Wall or inlet cabinet — Table 11.10.
Lateral wall gap / source inlet diameter (L/D)
Exact Table 11.10 columns0.25, 0.3, 0.4, 0.5, 0.75, 1, 1.5, 2. This source contains unavailable cells; unlisted space is not treated as zero penalty. Applies to Inlet installation condition: Wall or inlet cabinet — Table 11.10.
Projected unobstructed inlet area (%)
Project the obstruction on the Figure 11.13 inlet plane; use one exact source row. Adjusted velocity is inlet flow divided by full area and this free fraction. Choices: 100; 95; 90; 85; 75; 50; 25. Applies to Inlet installation condition: Projected inlet obstruction — Figure 11.13.
Obstruction distance / inlet diameter (D)
Exact source columns0.75, 0.5, 0.33, 0.25 or0(at inlet plane). Collared and non-collared inlet planes follow different source drawing definitions. Choices: 0.75D; 0.5D; 0.33D; 0.25D; At inlet plane. Applies to Inlet installation condition: Projected inlet obstruction — Figure 11.13.
Outlet installation condition
Select one supported outlet source profile. A damper near the outlet is an alternative profile here, not a second overlapping outlet penalty. Choices: No outlet effect added; BC centrifugal short outlet duct — Figure10.3; Axial short outlet duct — Figure10.2; Axial outlet elbow — Figure10.4; Outlet damper multiplier — Figure10.7.
Actual fan outlet area (ft²)
Full fan outlet reference area, not blast area or enlarged downstream duct area. The area-equivalent diameter preserves area; it is not hydraulic diameter. Applies to Outlet installation condition: BC centrifugal short outlet duct — Figure10.3 or Axial short outlet duct — Figure10.2 or Axial outlet elbow — Figure10.4 or Outlet damper multiplier — Figure10.7.
Actual straight length before outlet condition (in.)
Full effective length is equivalent diameter×[2.5+max(0, (V−2500)/1000)]. Point calculations require exact tabulated fractions: 0/12/25/50/100% for short ducts, 0/40/80/100% for axial elbows. At or above 100% is negligible for the selected source condition. Curve mode currently requires length 0 because a nonzero fraction varies with flow. Applies to Outlet installation condition: BC centrifugal short outlet duct — Figure10.3 or Axial short outlet duct — Figure10.2 or Axial outlet elbow — Figure10.4.
Blast area / full outlet area (%)
Exact source row. BC short-duct table uses this ratio; damper method multiplies the baseline catalog drop. This is not free inlet area. Choices: 40; 50; 60; 70; 80; 90; 100. Applies to Outlet installation condition: BC centrifugal short outlet duct — Figure10.3 or Outlet damper multiplier — Figure10.7.
Axial short-outlet profile
Figure 10.2 distinguishes tubeaxial and vaneaxial. Do not reuse a negligible tubeaxial row as a vaneaxial guarantee. Choices: Tubeaxial; Vaneaxial. Applies to Outlet installation condition: Axial short outlet duct — Figure10.2.
Axial outlet-elbow profile
Figure 10.4 uses effective-length fractions 0/40/80/100%, not the12/25/50%short-duct columns. Keep the source fan family and mitered-piece count. Choices: Tubeaxial — two-piece elbow; Tubeaxial — four-piece elbow; Vaneaxial — two-piece elbow; Vaneaxial — four-piece elbow. Applies to Outlet installation condition: Axial outlet elbow — Figure10.4.
Wide-open catalog damper drop at comparison airflow (in. w.g.)
Baseline drop already included in ordinary resistance. Add only(multiplier−1)×this drop; do not add the whole multiplied drop again. Match source rating and actual installation. Applies to Outlet installation condition: Outlet damper multiplier — Figure10.7.
Curve applicability documented?
Required: actual same-RPM/density/pressure-basis fan curve; ordinary system is fixed pressure plus quadratic resistance; selected geometry coefficients valid across the supplied flow range. A damper baseline drop must follow the stated quadratic model. No stability or stall approval is implied. Choices: Not established — withhold consequence; Documented same-condition fixed-geometry model. Applies to Calculation scope: Allowance and curve consequences.
Ordinary quadratic pressure at comparison airflow (in. w.g.)
Together with the fixed term this must equal ordinary resistance above. Installation allowance is added automatically and must not already be included. Applies to Calculation scope: Allowance and curve consequences.
Ordinary flow-independent pressure term (in. w.g.)
Explicit fixed term for the stated system model. This is not a synonym for the fan-static pressure definition. Applies to Calculation scope: Allowance and curve consequences.
Source fan curve: flow, pressure (CSV)
Two to500 actual same-condition points, increasing flow inCFM and nonnegative matching fan pressure inin. w.g. Piecewise-linear within supplied support only. Default points are constructed teaching data, not a manufacturer curve. Applies to Calculation scope: Allowance and curve consequences.
How to interpret the result
Use the upper allowance with the reported signed pressure-budget margin and retain the lower bound where present. Treat airflow reduction as available only when the curve result is uniquely supported; no allowance establishes stall-free or mechanically acceptable operation.
Worked examples
Pressure allowance at one airflow
Inputs: Constructed 5,000 CFM / 0.075 lbm/ft³case: BC four-piece inlet elbow with 2D straight distance, one 2 ft² inlet carrying all flow; BC outlet 2 ft², blast ratio 80%, no straight outlet duct. Ordinary pressure 1.2 in. w.g., available 2 in. w.g. Source geometry/rating match is assumed for this teaching case.
Calculation: Both reference velocities=2500 FPM. Figure 11.4 selectsR–U(C0.4–1.2), Figure 10.3 selectsT–U(C0.4–0.5). Printed I-Pvelocity pressure=0.389518443 in. w.g. Combined allowance=0.311614754–0.662181353 in. w.g.
Result: Required fan pressure=1.511615–1.862181 in. w.g.; conservative margin=0.137819 in. w.g. This is a source-table approximation, not a real fan installation test.
Allowance and curve consequences
Inputs: Same constructed zero-straight-length geometry; ordinary system=.2+1.0×(Q/5000)²in. w.g. Constructed fixed-speed fan points: (0 CFM, 3 in. w.g.)and(10000 CFM, 1 in. w.g.). No manufacturer performance is claimed.
Calculation: Solve the piecewise-linear fan against the baseline and both bounded quadratic system curves: baseline=6232.124598286 CFM, lower allowance=5643.942352395 CFM, upper allowance=5157.445724587 CFM.
Result: Within the stated constructed model, airflow reduction=9.437909–17.244181%. Other shapes, unstable intersections or unsupported fan range cannot be assigned this loss percentage.
Assumptions and limits
The pinned method is AMCA 201-23. Its installation factors are approximate guidance, not exact performance guarantees, current code compliance or a replacement for manufacturer-specific measured data.
Implemented lookup profiles are centrifugal round inlet elbows, axial inlet elbows, source wall/cabinet configurations, inlet obstructions, BC short outlet ducts, axial short ducts/axial outlet elbows and outlet damper multipliers. Centrifugal outlet-elbow tables and generic user-entered coefficients are not implemented here.
No nearest source row or geometric interpolation is used. Explicitly negligible cells yield zero only for that source condition; missing cells stop results. Letter ranges remain bounds rather than a midpoint.
Outlet full effective length uses the printed continuous I-P rule: minimum 2.5 area-equivalent diameters below2500 FPM, plus one per additional 1,000 FPM. Actual length fractions must match source columns; axial outlet elbows have different fractions from short ducts.
Source fan families, hub-to-tip range, elbow piece count, source-defined diameter, cabinet height/wall relation, symmetry and reference inlet plane must match. Some close inlet elbows can create instability; a pressure allowance does not approve such installation.
The inlet flow share refers to the actual area entered. For two equal inlets use half total flow with one inlet area; do not double the resulting pressure. Obstructed inlet velocity uses full-area velocity divided by free fraction.
One selected inlet and one selected outlet allowance may be added when independently applicable. Multiple overlapping outlet restrictions, unverified interactions, pre-rotation, asymmetry and accessories already present in the rating must not be blindly added.
A damper catalog pressure must already be in ordinary resistance; only the extra factor is added. The page does not model part-closed control dampers or infer catalog pressure from angle.
Curve mode currently supports zero straight outlet length or no outlet table effect; nonzero length makes effective fraction flow-dependent. Coefficients must be valid over supplied flow support, and damper/ordinary dynamic terms must follow the declared quadratic model.
Only unique supported intersections permit an airflow reduction percentage. Multiple, coincident or out-of-range curves retain pressure estimates and explicit unsupported status. No fan-speed, energy, noise, stability or structural certification is derived.
All complete worked installations and fan curves are constructed examples. Sources establish equations and discrete geometry coefficients; actual fan/rating conditions and installed performance require independent verification.
Unlabeled dashes in the inlet axial-elbow and wall/cabinet tables are treated as unsupported here; their meaning is not generalized from the explicit negligible legend in the obstruction figure. This conservative access to the table is distinct from cells explicitly defined as having no added effect.
This page uses the printed I-P1097 formula. The printed SI1.414 formula and rounded Annex A unit constants are not algebraically identical after conversion; a peer using SI can differ slightly in Pa. Source profile and native units must be recorded; no exact cross-unit equivalence is claimed.
FAQs
How do I represent a fan with two equal inlets?
Use the area of one reference inlet with half the total flow in Share of total airflow at this inlet. The resulting allowance is a pressure, so do not double it merely because there are two inlets.
Why can a point allowance work while the curve calculation is unsupported?
The curve method needs a valid coefficient over its flow range and a unique supported intersection. A nonzero straight outlet length changes its effective-length fraction with flow, so the implemented constant-coefficient curve method excludes that condition.
Sources
AMCA 201-23 Fans and SystemsPublic PDF accessed without login. Section 9/Table 9.1,I-P1097formula,Figures 10.1–10.4/10.7,11.2/11.4/11.13andTable 11.10. Point and curve model assumptions are explicit; full publication is not redistributed.
AMCA 2023 system-effect webinarPrimary industry context on installation versus laboratory performance; numerical coefficients come from the pinned201-23 publication.
Greenheck Understanding Fan System EffectsHistorical manufacturer guidance explains adding system effect to ordinary resistance and evaluating fan/system curves; no generic CFM-loss multiplier is copied.
