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Duct Leakage

Leakage airflow of a duct system from its surface area, the static pressure in it and the SMACNA leakage class of its seal level: Q = CL p0.65 per unit of surface, the class being 3 to 48 cfm per 100 ft² at 1 in. w.g. depending on the duct shape and the seal class (A, B, C or unsealed). The tool gives the leakage flow, its share of the system airflow, the fan power and energy it costs over the operating hours, and compares the seal classes so the value of sealing can be weighed against its labor.

  • Leakage class from the duct shape and the seal class (SMACNA / ASHRAE), or entered from a test

  • Leakage flow at the operating pressure, as a fraction of the system airflow

  • Fan power and annual energy spent moving the leaked air; comparison of the seal classes

Duct Leakage

Unit System: /

Duct System







Energy





Leakage Classes | Method page
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Duct Leakage

Leakage Classes

Ducts leak at their seams, transverse joints and penetrations. SMACNA's test manual expresses the leakage of a duct system as Q = CL p0.65, the flow per 100 ft² of duct surface at the static pressure p in inches of water, with the leakage class CL the flow at 1 in. w.g. The exponent 0.65 is the measured behavior of leaks between orifice flow (0.5) and laminar flow (1.0).

The classes used here are the ASHRAE Handbook values by duct type and seal class: rectangular ducts 6 (class A, everything sealed), 12 (class B, seams and joints), 24 (class C, transverse joints only) and 48 unsealed; round and flat-oval ducts half those, 3, 6, 12 and 30. A tested system can enter its measured class.

The energy cost is the fan power on the leaked flow at the fan's total pressure over the operating hours; the heating and cooling lost with conditioned air leaking into unconditioned spaces is not included and often exceeds it. ASHRAE 90.1 requires seal class A for ducts above 3 in. w.g. and sets a leakage limit; local codes may require testing.



References

Sheet Metal and Air Conditioning Contractors' National Association (2012). HVAC Air Duct Leakage Test Manual, 2nd ed. SMACNA, Chantilly, VA.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

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