Compare louver catalog data with airflow, opening, pressure, and weather limits to identify eligible candidates.
Concepts behind this calculation
Compare documented louver sizes and test curves for free-area velocity, pressure drop and exact static or wind-driven-rain conditions.
A louver must fit the opening while admitting the required outdoor air within the pressure and weather limits. Gross face area describes its envelope; free area describes the open air passage; core area supplies the velocity basis for the wind-rain comparison. Dividing the same airflow by these different areas produces different velocities, so a favorable face velocity cannot stand in for a tested free-area or core-velocity condition.
The two weather methods ask different questions. The static-air screen compares free-area velocity with a documented water limit reduced by the project-selected fraction. The wind-driven screen requires matching wind and rainfall conditions and a supported core-velocity interval. Pressure support is checked independently on each candidate’s supplied curve. Missing weather or pressure evidence leaves a candidate unsupported even when its dimensions fit.
How to use this calculator
Choose Performance method and document Model, size and test-source reference for the exact assemblies.
Enter Outdoor intake airflow, Maximum available louver width and height, and Maximum allowed pressure drop. Select the Pressure curve coordinate and Selection objective.
For static air, enter the Project-selected still-air cap fraction and still-air candidate and pressure rows. For wind-rain, enter Test wind velocity, Test rainfall rate, Required water-rejection effectiveness and matching candidate and pressure rows.
Compare support flags and margins before accepting the ranked candidate; retain unsupported rows as requests for additional product data.
Calculation method and applicability
Free-area velocity = Q / Afree. Core velocity = Q / Acore for wind-driven data. Pressure is exact-point or piecewise-linear interpolation only within the supplied candidate curve. Static screen: free velocity < waterLimit × selected project fraction. Wind screen: exact wind/rain condition, core velocity interval and rejection ≥ required.
Input definitions and units
Performance method
Static-air and wind-driven-rain tests answer different questions. Do not treat a static water-penetration point as a wind-driven rating. Choices: Static-air pressure and water screen; Wind-driven-rain test condition.
Model, size and test-source reference
Required: manufacturer/model, nominal size, accessory/installation condition, source revision, test air basis and rain/wind conditions where applicable. Source text does not authenticate user data.
Outdoor intake airflow (CFM)
Actual comparison airflow for the selected louver candidates. The calculator does not resize an opening or correct density/altitude.
Maximum available louver width (in.)
Nominal envelope screen only. It does not add installation clearance, flange, sill or structural allowance.
Maximum available louver height (in.)
Nominal envelope screen only; dimensions are not rotated automatically.
Maximum allowed pressure drop (in. w.g.)
Project-supplied pressure budget. A candidate without an in-range pressure point cannot pass.
Pressure curve coordinate
Use a supplied curve coordinate basis: actual airflow or free-area velocity. A coordinate curve is never converted between bases automatically. Choices: CFM; Free-area FPM.
Selection objective
Rank only candidates that pass every entered screen. The result is a comparison aid, not product approval. Choices: Lowest pressure drop; Smallest face area.
Project-selected still-air cap fraction (fraction)
Static-air mode: free-area velocity must be strictly below waterLimit × this explicit project fraction. This is not a universal louver standard or wind-driven rating. Applies to Performance method: Static-air pressure and water screen.
Test wind velocity (mph)
Wind-driven mode: must exactly match each candidate’s supplied wind condition. No milder-condition interpolation. Applies to Performance method: Wind-driven-rain test condition.
Test rainfall rate (in./h)
Wind-driven mode: must exactly match each candidate’s supplied rainfall condition. Applies to Performance method: Wind-driven-rain test condition.
Required water-rejection effectiveness (%)
Wind-driven mode: project criterion compared with the candidate’s tested effectiveness at the exact condition. Applies to Performance method: Wind-driven-rain test condition.
Rows: integer ID, nominal width/height in inches, free area in ft², beginning-water-penetration limit in FPM (or a clearly documented conservative tested cap). One row per same-condition candidate model/size. Applies to Performance method: Static-air pressure and water screen.
Wind-rain candidate rows: ID, width, height, free area, core area, wind, rain, min core FPM, max core FPM, min rejection (CSV)
Rows: integer ID, dimensions/free area/core area, tested wind/rain condition, tested core-velocity interval and minimum rejection. Use complete matching product/test data only. Applies to Performance method: Wind-driven-rain test condition.
Pressure rows must match candidate ID, selected coordinate basis, size/model and accessory condition. Coordinates must increase; no extrapolation. Applies to Performance method: Static-air pressure and water screen.
Rows must use the same candidate and selected pressure coordinate basis. Wind/rain support still comes from the candidate test rows; pressure is a separate supplied curve. Applies to Performance method: Wind-driven-rain test condition.
How to interpret the result
Read the selected ID as the best eligible candidate under the chosen objective. Keep its pressure margin, weather support and nominal envelope check together; installation clearances, drainage and accessory compatibility still need their own review.
Worked examples
Static-air pressure and water screen
Inputs: Published same-size source-point comparison: 6,000 CFM intake, 48×48 in maximum opening, 0.10 in. w.g. budget. Candidate 1: 48×48, 8.92 ft² free area, 989 FPM beginning water penetration, pressure .073 in. w.g. at 6,000 CFM. Candidate 2: 48×48, 6.80 ft², a conservative 1,250 FPM static cap (source onset is above this value), pressure .112 in. w.g.
Calculation: Candidate 1 free-area velocity=6,000/8.92=672.646 FPM; it is below its entered water screen and pressure budget. Candidate 2 pressure exceeds the budget.
Result: Candidate 1 selected. This is a source-coordinate comparison fixture; replace every row with the exact model, size, accessory and project condition.
Wind-driven-rain test condition
Inputs: Constructed wind-rain comparison at 29 mph wind, 3 in./h rain, 6,000 CFM intake. Candidate 1 core area12 ft² and tested core velocity450–550 FPM, free area8 ft², 99.5% minimum rejection. Candidate 2 has different wind/rain condition and is outside this exact comparison.
Calculation: Candidate 1 core velocity=6,000/12=500 FPM, inside its tested interval, with 99.5% rejection. Candidate 2 is not supported at 29 mph/3 in./h.
Result: Candidate 1 meets the exact entered wind/rain condition. This constructed example does not certify a real model; use the manufacturer’s complete AMCA-rated condition data.
Assumptions and limits
Free area, core area and gross face area are different quantities. The calculator does not infer one from another, rotate dimensions, add installation clearance or select a product from a generic free-area velocity rule.
Static-air waterLimit is not a wind-driven-rain rating. Wind mode requires exact matching wind/rain conditions, core-velocity support and rejection data; no milder-weather or cross-size interpolation is made.
Pressure curves are interpolated only inside supplied same-candidate support. Missing or out-of-range data withholds pressure and cannot pass. No density, altitude, screen, filter, flange, transition or accessory correction is added.
A candidate can pass the arithmetic screens while local structure, drainage, insect screen, acoustic, intake separation, weatherproofing or installation requirements remain unresolved.
The Greenheck ESD-435 static point and EHH-501 wind-rain table are source coordinates, not embedded product presets. Constructed worked examples must be replaced with the exact model/source data.
AMCA rating classes and manufacturer test setup are not recreated; this page compares user-supplied ratings and does not issue an AMCA certification or code approval.
A wind-test interval is usable only if its source explicitly supports at least the entered minimum rejection throughout that interval. Do not connect two favorable discrete tests into a certified range. For a lone test, enter identical minimum and maximum core velocities; it supports that point only.
The EHH-501 water and wind-driven ratings explicitly include a sill pan. A candidate source must match that drainage/accessory configuration. The wind table uses a 1 m × 1 m core; it is not automatically a tested 48 × 48 in assembly.
FAQs
Can I use a static water-penetration limit for a windy intake?
That limit does not establish wind-driven-rain performance. Use the wind method only with matching wind, rainfall, core area and rejection data for the actual assembly, including drainage accessories.
Why is a candidate unavailable for selection despite low calculated velocity?
Velocity alone does not supply an in-range pressure point or a matching weather rating. Review the support flags before changing the opening or pressure budget; an unsupported point is different from a documented failure.
Sources
Greenheck ESD-435 submittal, March 202548×48 point:8.92 ft² free area,989 FPM beginning water penetration and 0.073 in. w.g. at6,000 CFM; exact source coordinate, not a universal rule.
Greenheck EHH-501 submittal, March 202548×48 airflow resistance and wind-driven-rain table with3 in./h at 29 mph and8 in./h at 50 mph; table rated at1 m × 1 m core.
