Duct Leakage White Paper

October 4, 2010
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Lindab Inc. published a revised “Duct Leakage White Paper” in November 2009. Here are some of the highlights of that report.

Air duct leakage should be a concern to both the design engineer and the building owner because of its potential impact on the initial construction costs, energy usage, and system performance.

Every duct system requires power. Power drives the fan or blower unit needed to move the air over or through the resistance elements, and power operates the compressor or heating elements depending on the desired function. No matter the size, the total benefit of the duct system is based and measured on the peak fan efficiency for that system.

Small commercial buildings typically use a packaged air handling unit where all of the supply and return air ductwork is in the conditioned space. If the design airflows are not properly delivered to the building HVAC loads, the occupants will respond with corresponding higher or lower temperature settings to meet their comfort requirements. The end result is higher energy costs.

In large complex buildings, particularly those with variable air volume systems (VAV), recent research has found that a more complex interaction occurs which requires computer simulation to quantify the energy impact. The Public Interest Energy Research Program (PIER) reported that “When conditioned air leaks from the supply ducts, the heating or cooling energy associated with leakage heats or cools the return air and changes its temperature.”

Typically “one-third of the total annual energy consumption is related to HVAC. In addition, 39 percent of this HVAC consumption is associated with fan operation.”

The key elements influencing ductwork leakage start with the very basics - size, shape, and construction materials. Ductwork is made from a wide range of materials - galvanized steel, carbon steel, aluminum, stainless steel, fiberglass, polyvinyl chloride (PVC), polyvinyl steel (PVS), ductboard, and others. Perhaps the most common material used is galvanized steel.

Ductwork is available in rectangular, round, and flat oval geometric shapes. The particular shape that is selected for a specific system should adhere to minimizing the initial installed cost and annual operating costs, as well as conform to the constraints of the building envelope.

There are many reasons why SMACNA HVAC Systems Duct Design Manual recommends the following:

•  Use the minimum number of fittings.

•  Consider the use of semi-extended plenums.

•  Seal ductwork to minimize air leakage.

•  Consider using round duct.

•  When using rectangular ductwork, maintain an aspect ration as close to 1-to-1 as possible.

There are several reasons for specifying round ductwork versus rectangular ductwork:

•  Lowest possible duct friction loss for a given perimeter.

•  Lowest weight based upon the same airflows, pressures, and friction loss rates.

•  Requires less supports per running foot.

•  Handles negative pressures with less weight and reinforcement.

•  Handles higher air velocities than rectangular ductwork while achieving the same acoustic design criteria.

•  Least expensive to seal for air leakage.

As reported in the SMACNA 1990 HVAC Systems Duct Design Manual, duct leakage previously specified as an arbitrarily established percentage of the airflow was impossible to attain by the installing contractor. Joint research conducted by SMACNA and ASHRAE has since developed a methodology used to relate the amount of ductwork leakage to the ductwork surface area and the design static pressure independent of the actual airflow in the ductwork. In addition, the leakage class recognizes that under the best conditions, rectangular ductwork will leak air at a rate twice greater than round.

Various recognized methods of sealing ductwork also vary in degrees of cost, quality, visual appearance, and performance. Choices range from various types of flanges, to slip fit connections that require liquids, mastics, tapes, or heat-applied materials to seal the joints. In addition, several ductwork manufacturers offer a factory-applied gasket with self-sealing characteristics that do not require the field application of external sealants. Exposed ductwork remains in vogue with the architectural tastes of designers and building owners, a trend that requires more attention be paid to the ductwork sealing methods than in the past because of the visual aesthetics.

A typical response to unanticipated ductwork air leakage has:

•  Increased design airflows, which increase the initial construction costs for equipment and ductwork.

•  Increased fan energy.

•  Increased energy for heating, cooling, and dehumidifying the airstream.

•  Increased difficulty in air balancing the system airflows.

•  Impacts on the indoor air quality (IAQ).

•  Compromised occupant comfort.

Publication date: 10/04/2010

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