In addition to creating exciting and distinct exteriors, louvers also act as the first line of defense to protect HVAC systems and ductwork from water damage. Basic louver performance criteria—free area, air volumetric flow rate, air pressure drop, and the beginning point of water penetration—are measured by the Air Movement and Control Association International Inc. (AMCA) and reported in their certified ratings program. Understanding these criteria will help maximize the benefits of louver form and function, ensure maximum HVAC system operation, and reduce the need for costly repairs from damage to air handling equipment. While none of these criteria offers enough information for louver selection alone, when combined, the results will provide direction for selecting the best louver for the application.
Free Area
The free area of a louver is defined as the total louver face area, minus the airflow restrictions (blades and frame) through which air can freely pass within the openings. Free area is a major determinant of a louver’s performance capabilities. The goal is to allow as much air as possible to pass through the louver while keeping out unwanted water or debris. Too great of a free area and the louver cannot reject water. Too little free area and the louver becomes an unwanted restriction to airflow. Free area is often represented in square feet or as a percentage of total louver face area. Specifiers should pay close attention to manufacturers’ product data sheets as even similar models within the same performance category can have noticeably different free areas (Example 1).
Example 1: Similar models within the same performance category can have noticeably different free areas.
Air Volume (CFM)
Air volume flow rate is the measurement of air flowing into or out of a space measured in cubic feet per minute (CFM). Louvers allow air to pass through the building envelope and into the HVAC system while keeping water, birds and debris out of the intake airstream. Additionally, louvers must provide the required system CFM per square foot of louver free area to the air handling equipment to ensure operation at peak performance. Louver CFM is calculated easily by multiplying the proposed air velocity (FPM) by the free area (sq. ft.) of the louver (Example 2).
Example 2: Air volumetric flow rate calculation.
The Beginning Point of Water Penetration
The beginning point of water penetration is the moment at which 0.01-ounce of water will pass through a louver at a given intake velocity measured in feet per minute. The ability to prevent water penetration through a louver is an important criterion for louver selection. As previously discussed, louver design allows air into buildings while keeping out unwanted water and debris. A complex arrangement of specific louver blade profiles, the angles of the blades, blade position, spacing, or a combination of blade position and spacing all contribute to the degree of minimizing water penetration.
Pressure Drop
Pressure drop is the resistance to airflow across a louver stated in inches of water. The level of airflow resistance is a significant factor in louver selection. Factors affecting pressure drop are the same for the beginning point of water penetration. A negligible pressure drop allows a high degree of airflow but may also allow water and debris through the louver opening. Conversely, a high-pressure drop may not allow an acceptable airflow. The latter may place a strain on air-handling equipment, causing it to work harder, resulting in increased energy cost and greater acoustic noise or the need to invest in more expensive air-handling equipment. Most louvers have a pressure drop in a range between 0.03-0.50 in. wg (Example 3).
Example 3: Louver A’s simple blade profile does not cause a higher pressure drop when compared to a complex blade profile (Louver B).
Conclusion
Louvers are essential components of a building’s HVAC system and proper selection and application can be complex as many variables govern their performance characteristics and potential applications. This makes selecting the best louver a challenge as manufacturers offer many different louver styles. The best approach to louver selection combines the importance of understanding the value of independent third-party certified testing and the applicable test standards, with the basics of free area, pressure drop, air volume, and the beginning point of water penetration. This information helps designers and their clients specify the best aesthetically pleasing louvers that meet the demands of today’s high-performance HVAC systems.
Anthony Jackson, CSI, CDT, LEED Green Associate, is product manager at Airolite, Schofield, Wis. For more information, visit www.airolite.com.