The following is taken from HVAC Duct Construction Standards: Metal and Flexible, published by the Sheet Metal and Air Conditioning Contractors' National Association.

Ducts must be sufficiently airtight to ensure economical and quiet performance of the system.

It must be recognized that airtightness in ducts cannot, and need not, be absolute (as it must be in a water-piping system).

Codes normally require that ducts be reasonably airtight. Concerns for energy conservation, humidity control, space-temperature control, room-air movement, ventilation, maintenance, etc., necessitate regulating leakage by prescriptive measures in construction standards. Leakage is largely a function of static pressure and the amount of leakage in a system is significantly related to system size. Adequate airtightness can normally be ensured by:

A. Selecting a static pressure construction class suitable for the operating condition.
B. Sealing the ductwork properly.

The designer is responsible for determining the pressure class or classes required for duct construction and for evaluating the amount of sealing necessary to achieve system-performance objectives. It is recommended that all duct constructed for the 1-inch (250 Pascal) and half-inch (125 Pascal) pressure class meet Seal Class C.

However, because designers sometimes deem leakage in unsealed ducts not to have adverse effects, the sealing of all ducts in the 1-inch and half-inch pressure class is not required by this construction manual. Designers occasionally exempt the following from sealing requirements: small systems, residential occupancies, ducts located directly in the zones they serve, ducts that have shot runs from volume-control boxes to diffusers, certain return-air ceiling plenum applications, etc.

When Seal Class C is to apply to all 1-inch and half-inch pressure-class duct, the designer must require this in the project specification. The designer should review the HVAC Air Duct Leakage Test Manual for estimated and practical leakage allowances.

Seven pressure classes exist: half-inch, 1 inch, 2 inch (500 Pascal), 3 inch (750 Pascal), 4 inch (1,000 Pascal), 6 inch (1,500 Pascal) and 10 inch (2,500 Pascal) water gauge. If the designer does not designate pressure class for duct construction on the contract drawings, the basis of compliance with the SMACNA HVAC duct-construction standards is as follows: 2-inch w.g. for all ducts between supply fan and variable-volume control boxes and 1-inch w.g. for all other ducts of any application.

Some sealants can adversely affect the release function of breakaway connections to fire dampers; consult the damper manufacturer for installation restrictions.

Leakage tests

There is no need to verify leakage control by field-testing when adequate methods of assembly and sealing are used. Leakage tests are an added expense in system installation. It is not recommended that duct systems constructed to 3-inch w.g. class or lower be tested because this is generally not cost-effective.

For duct systems constructed to 4-inch w.g. class and higher, the designer must determine if any justification for testing exists. If it does, the contract documents must clearly designate the portions of the system(s) to be tested and the appropriate test methods. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) energy conservation standards Series 90 text on leakage control generally requires tests only for pressures in excess of 3 inch.

The HVAC Air Duct Leakage Test Manual provides practical and detailed procedures for conducting leakage tests.

Apparent differences of about 10 percent between fan delivery and sum of airflow measurements at terminals do not necessarily mean poor sealing and excess leakage. Potential accuracy of flow measurements should be evaluated.

Otherwise, open access doors, unmade connections, missing end caps or other oversights contribute to such discrepancies. When air terminals are at a great distance from fans (over 500 feet), more effective sealing is probably required to avoid diminished system performance.

Schools, shopping centers, airports and other buildings may use exposed ductwork. Selecting sealing systems for such ducts may involve more attention to the final appearance of the duct system than with ducts in concealed spaces.

Long-standing industry acceptance of so-called low-pressure duct systems without sealants may have left some contractors (and designers) with little or no experience with sealing. The contractor should carefully select construction details consistent with sealing requirements, the direction of the air pressure and familiar sealing methods. The cost of restoring systems not receiving the required sealing or not being properly sealed can greatly exceed the modest cost of a proper application.

The 2005 edition of SMACNA's HVAC Duct Construction Standards: Metal and Flexible is available as a book, CD-ROM and PDF file. Discounts are available for SMACNA members, architects and engineers. For ordering information and prices, contact the association's publications department at (703) 803-2989 or see on the Internet.