The following excerpt is an overview of the information contained in the manual.)
Ducts must be sufficiently airtight to ensure economical and quiet performance of the system. It must be recognized that airtightness in ducts as a practical matter cannot, and need not, be absolute (as it must be in a water piping system.) Codes normally requite that ducts be reasonably airtight. Concerns for energy conservation, humidity control, space temperature control, room air movement, ventilation and 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 assured by:
A. Selecting a static pressure construction class suitable for the actual operating condition.
B. Sealing the ductwork properly.
The designer is responsible for carefully determining the pressure class of classes required for duct construction and evaluating the amount of sealing necessary to achieve system performance objectives. It is recommended that all duct constructed for one- and 1/2-in. pressure class meet Seal Class C. However ? Small systems, residential occupancies, location of ducts directly in the zones they serve, short runs of duct from volume control boxes to diffusers, certain return air ceiling plenum applications, etc., have at times been exempted by designers from sealing requirements.
Leakage testsThe need to verify leakage control by field-testing is not present 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-in. w.g. class or lower be tested since it is generally recognized as not being cost effective. For duct systems constructed to 4-in. w.g. class and higher, the designer must determine if justification for testing exists. If it does, he or she must clearly designate in the contract documents the portions of the system(s) to be tested and must specify appropriate test methods.
Duct sealingLong-standing industry acceptance of so-called low pressure duct systems without the addition of sealants may have left some contractors (and designers) with little or no previous experience with sealing. The contractor should carefully select construction detail consistent with sealing obligations - the direction of the air pressure and the sealing methods employees have become familiar with. Costs related to the restoration of systems not receiving the required sealing or of those haphazardly sealed can greatly exceed the modest cost of a proper initial application. However, with proper attention to joint selection, workmanship and sealant application, almost any joint can achieve low leakage.
Sealant choicesThe selection of the most appropriate sealant will depend primarily on the basic joint design and also on application conditions such as joint position, clearances, direction of air pressure in service, etc. Available sealants include:
cLiquids. Many manufacturers market liquid sealants specifically for ducts. They have the consistency of heavy syrup and can be applied either by heavy brush or with a cartridge gun or power pump.
cMastics. Heavy mastic-type sealants are more suitable for applications as a fillet, in grooves or between flanges. Mastics must have excellent adhesion and elasticity.
- Gaskets. Durable materials such as soft elastomer butyl or extruded forms of sealants should be used in flanged joints.
General testing procedures1. Conventional leak testing is based on positive pressure mode analysis. It involves inserting temporary plugs (plates, sheets, balloons, bags, etc.) in openings in a section of duct and connecting a blower and a flowmeter to the specimen in such a manner that pressurizing the specimen will cause air escaping from the specimen to pass through the flowmeter.
2. Select a test pressure not in excess of the pressure class rating of the duct.
3. Calculate the allowable or allocated leakage using leakage factors related to the duct surface area.
4. Select a limited section of duct for which the estimated leakage of duct will not exceed the capacity of the test apparatus.
5. Connect the blower and flowmeter to the duct section and provide temporary seals at all open ends of the ductwork.
6. To prevent over-pressurizing of the ducts, start the blower with the variable inlet damper closed. Controlling pressure carefully, pressurize the duct section to the required level.
7. Read the flowmeter and compare the leakage in cfm per square foot with the allowable rate determined in step 3.
8. Complete test reports and if required, obtain witness' signature.
9. Remove temporary blanks and seals.
(More detailed information on duct leakage and testing is available in the SMACNA Hvac Air Duct Leakage Test Manual. For information on ordering and prices, write the Sheet Metal and Air Conditioning Contractors' National Association, P.O. Box 221230, Chantilly, VA 20153-1230; call 703/803-2980; fax 703/803-3732; see www.smacna.org on the Internet.)
Leakage is largely
a function of static pressure and the amount of leakage in
a system is
to system size.