Duct Leakage: Method

Leakage Classes

Ducts leak at their seams, transverse joints and penetrations. SMACNA's test manual expresses the leakage of a duct system as Q = C_L p^0.65, the flow per 100 ft² of duct surface at the static pressure p in inches of water, with the leakage class C_L 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.