Gabriel Legorburu

How to size a Wall Louver

An HVAC wall intake louver is a type of vent that is installed on the exterior wall of a building and is used to bring fresh air, exhaust air or relieve air.  Louvers are common in mechanical spaces, warehouses or air-handler rooms.  When it comes to louvers, there are numerous options available to choose from, including sound-attenuating, sight-proof, hurricane-resistant, adjustable and more. The most important design condition to consider is Louver Sizing. 

There are a few terms and options critical for the selection of the appropriate louver.

  • Face Area
    • Size of louver opening in wall
  • Free Area
    • Free area is the available area for air to flow.  This would be the total area of the louver minus the area blocked by the blades.
    • Manufacturers publish the free area for louvers but it’s typically around 50%
  • Finish
    • Baked Enamel
    • Kynar
    • Anodized
  • Louver depth
    • Common depths are 4” or 6”
  • Drainable
    • Drainable refers to a feature in the design of a louver blade that captures rainwater that strikes it and directs it away. When a louver is installed in an outdoor location, it is advisable to use drainable blades to prevent water from entering the building. It’s important to note that drainable louvers require the addition of intermediate vertical mullions, which may impact the visual appearance of the louver.
  • Stationary or Operable
    • Stationary louvers, as the name suggest, have no moving parts.  In some application the designer may want the louver to only be open during certain situations.  For example, a louver may only be used during the summer to introduce fresh outdoor air.  Then in the winter the designer may want the louver to close to keep in heat.  In this case, a louver with a motorized damper section would be chosen.

Sizing Intake louvers

When selecting an intake louver, it is crucial to consider the free area velocity, which should not exceed 600 feet per minute to reduce the risk of rain carry-through. When pressure drop across the louver is critical, it is important to compare the pressure drop through the intake path with the pressure drop through the return path. If the pressure drop through the intake louver is not a critical factor (due to a high pressure drop in the return air path), consider using a louver designed specifically for wind-driven rain. These louvers, although more expensive at 2.5 times the cost per square foot, may offer architectural advantages and lower installation costs due to their smaller size and reduced need for downstream duct transitions.

Sizing Exhaust louvers

Velocity is not as critical for exhaust louvers because rain carry-through doesn’t apply.  The designer should select the louver for an acceptable pressure drop.  A good starting point is around 800 FPM.

Layout considerations

It is important to coordinate with the architect to ensure the proper size and location of louvers. Winter weather conditions, such as snow carry-through and hoarfrost, must be considered when installing intake louvers in areas such as laboratories or make-up air units. A heating coil can be installed after the louver to prevent hoarfrost, with a pre-filter bank in place during the summer. 

For buildings with brick or block construction, the louver size should be selected to coordinate with the coursing dimensions to avoid cutting the building material. The angle between the louver and the next item in the air path should not be greater than 45 degrees for optimal air flow and accuracy in free area calculations.

If a filter is provided for the louver consider sizing the louver in 2’ increments so that common filters can be used.

Placement of an intake louver is also critical.  Make sure that the louver isn’t installed near a source of pollution, such as traffic.  Or within 10’ of an exhaust location.

Selection software

There are many different louver manufacturers.  Some provide very easy to use on-line selection softwares to assist designers.  Here are links to some of my favorites websites for louver sizing.

Items to include in a louver schedule

  • Overall size
  • Minimum Free Area
  • Velocity
  • Allowable pressure drop
  • Material
  • Supply, Exhaust or relief condition
  • Tag
  • Color
  • Finish
  • Motorized Damper
  • Manufacturer and Model number

Design Summary

  • Size the louver for the appropriate velocity and pressure drop
  • Check that the louver location meets local codes
  • If the louver is an intake it has to be drainable
  • If louver is susceptible to frosting over consider electric heating coil
  • Coordinate louver location and size with architects and client

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