A weatherproof louver is selected by the combination of free area, water-defense class, and pressure drop your wall can tolerate; a Class A (best) louver typically stops driving rain up to 1,100 mm/h while adding 30–90 Pa at 2.5–4 m/s face velocity. Choose the class from your local rainfall intensity and the penalty your fan can absorb, because a louver that keeps water out but starves the fan is as bad as one that leaks.
What a Louver Actually Does
A louver admits air and bars rain, birds, and debris. Its free area—the open percentage of the frame—decides how much metal the air must squeeze through. A 50% free-area louver at 3 m/s face velocity passes about 5,400 m³/h per m² of frame; a 45% unit passes closer to 4,900 m³/h for the same velocity, so a poor free area quietly forces a bigger wall opening or a higher-pressure fan. This hidden cost is why free area, not just the frame size, should drive the specification.
Beyond airflow, the louver is the first line of defense for the whole AHU. Water that passes the blades wets the filter, raising its resistance 20–40% and breeding biofilm, and can reach the coil and drain pan where it drives corrosion. A correctly specified drainable louver protects every downstream component and extends filter life by weeks or months in wet climates.
Rain Defense Classes
Louvers are graded by how much water they let through under test rain at set wind speed. Class A (best) passes under 1 L/m²/h of water at up to 1,100 mm/h rain and 13 m/s wind; Class B allows more; Class C is basic defense for sheltered walls. Removing the blade drains and gutters can multiply water carry-through by 3–5 times, so specify drainable blades for any outdoor intake exposed to wind-driven storms.
Comparing Louver Types
| Type | Free area (%) | Pressure drop at 3 m/s (Pa) | Rain class | Typical use |
|---|---|---|---|---|
| Stationary blade, drainable | 45–55 | 30–60 | A–B | Outdoor intake/relief |
| Blade-and-screen combo | 35–45 | 50–90 | A | Heavy rain, coastal |
| Non-drainable econoline | 50–60 | 20–40 | C | Sheltered, low rain |
| Operable/adjustable | 40–55 | 30–70 | B | Controlled ventilation |
Pressure Drop and Sizing
Pressure drop rises with velocity squared. At 2.5 m/s a drainable louver loses about 25 Pa; at 4 m/s it reaches 60–90 Pa. Because a fan also drives silencers and filters, keep louver loss under 15% of available fan static pressure—on a 1,500 Pa fan that means under roughly 225 Pa, easily met, but on a 250 Pa axial fan it is a real constraint. Size the opening so face velocity stays 2.5–4 m/s, and never let it exceed 5 m/s or rain defense collapses.
Worked Example: A Coastal Intake
A plant on the coast needs 20,000 m³/h with 1,100 mm/h storm exposure. We pick a Class A blade-and-screen louver at 3 m/s face velocity, free area 40%, so required frame area is 20,000 / (3 × 3600 × 0.40) ≈ 4.6 m². Loss at 3 m/s is about 70 Pa, under 15% of a 1,500 Pa fan. The screen adds defense against salt spray and birds, and the gutter drains to a trapped outlet, keeping the filter dry through the wet season.
Decision Checklist: Which Louver
- Choose Class A drainable blades for rooftop or wall intakes exposed to 800–1,100 mm/h storms and coastal EAC sites with salt spray.
- Choose a blade-and-screen combo when water must stay near zero and you can absorb 50–90 Pa of added resistance without starving the fan.
- Choose a non-drainable econoline only on sheltered walls with under 600 mm/h rain and no wind-driven exposure from adjacent structures.
- Choose operable louvers when the opening must modulate with season or integrate with fire and smoke control sequences.
Installation Details That Matter
Mount the louver with a 50–100 mm forward drip lip and a continuous internal gutter piped to drain; without it, captured water re-entrains at the next gust. Keep 300–500 mm of straight duct behind the louver before any elbow to even out velocity, and avoid placing the louver where roof runoff or adjacent discharges can splash the blades. A poorly located louver can admit 2–4 L/m²/h even when the blade class is good, defeating an otherwise correct selection.
Standards and Testing
Refer to AMCA 500-L for louver air-performance and water-penetration test methods, and to local building code for storm design rainfall intensity (often a 1-in-100-year 50–100 mm/h, but wind-driven coastal sites exceed 1,000 mm/h). Specify the test velocity and class explicitly so suppliers quote comparable data rather than best-case brochure numbers from a still-air lab.
Manufacturing Note
Keruite manufactures HVAC fans and air-handling equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment. Coordinating louver free area with the fan’s certified performance prevents the common mismatch where a small opening starves a large intake, and the in-house test capability means the fan curve you design against is the one you actually receive.
Wind Load, Screening, and Structural Fixings
A louver is also a structural opening in the building envelope, so it must resist wind pressure without deflecting the frame or loosening fixings. On a 30 m tall wall the local wind pressure can reach 0.8–1.2 kPa; specify a frame rated to at least 1.5 kPa with fixings at 300–450 mm centres and into a continuous backing angle, not just the cladding. A frame that racks under gust load opens gaps that admit rain exactly when the storm is worst, defeating the rain class the blade lab certified on a rigid test rig.
If birds or insects are a concern, add a fitted screen of 10×10 mm or 13×13 mm mesh behind the blades, but recognise it costs 10–30 Pa and roughly 5–10% free area, so upsize the opening to compensate. For coastal or corrosive sites, specify stainless or powder-coated frames and confirm the coating thickness is 60–80 µm so salt spray does not pit the blades within five years. Finally, keep the louver clear of adjacent roof edges and discharges; an inlet placed 1 m below a condensate or exhaust outlet will pull that moisture straight back through the very blades meant to keep it out, regardless of class.
FAQ
What rain class do I need?
Pick Class A for exposed walls facing 800–1,100 mm/h storms or coastal wind; Class B for moderate exposure; Class C only on sheltered walls under 600 mm/h. Specifying drainable blades cuts water carry-through by 3–5 times versus non-drainable, so class alone is not the whole story—the gutter and drain do the real work.
How much pressure drop is normal?
A drainable louver loses about 25 Pa at 2.5 m/s and 60–90 Pa at 4 m/s; blade-and-screen combos reach 50–90 Pa. Keep louver loss under 15% of fan static pressure. On a 250 Pa axial fan this is tight, so size the opening for 2.5–4 m/s face velocity and never exceed 5 m/s or rain defense fails.
Does free area really change sizing?
Yes. A 50% free-area louver passes about 5,400 m³/h per m² at 3 m/s, while a 45% unit passes near 4,900 m³/h. A 5% free-area drop can mean a 10% larger wall opening or a higher-pressure fan, so confirm the published free area before ordering and check it was measured, not estimated from blade geometry.
Where should the louver drain to?
Route captured water through a continuous internal gutter and 50–100 mm drip lip to a piped drain. Without gutters, re-entrainment at the next gust can admit 2–4 L/m²/h even with good blades. Keep 300–500 mm of straight duct behind the louver too, so velocity is even and the first elbow does not strip water off the blades back into the airstream.