From Cold Air Distribution: System Design Guide

4.7 Duct Leakage

Leakage of supply air from ducts reduces the cooling capacity delivered to the conditioned space. If the leakage is to an unconditioned space, it represents an increase in the total load and a loss of energy from the system. If the leakage is to a conditioned space such as a return air plenum, the cooling capacity is not entirely lost from the system, but additional supply air must be provided to meet the space loads.

Leakage can also cause condensation on adjacent surfaces if the supply air temperature is lower than the ambient dew point. This concern is heightened with cold-air distribution systems.

The HVAC Air Duct Leakage Test Manual (SMACNA 1985) defines leakage classifications and provides a method for rating, analyzing, and testing duct leakage. The leakage class, C L, is a number proportional to the leakage in cfm/100 ft 2 (L/s per m 2) of duct surface at 1 in. w.g. (250 Pa) static pressure.

ASHRAE/IESNA 90.1-1989 (ASHRAE 1989) recommends that ductwork designed to operate at static pressures above 3 in. w.g. (746 Pa) be constructed and leak-tested to leakage class 6 or better. A leakage class of 3 is attainable for all duct systems by careful selection of joints and sealing methods and by good workmanship (SMACNA 1990; ASHRAE 1993).

Although leakage as a percentage of total flow rate is a useful criterion for system analysis, the SMACNA leakage classifications and test method require evaluation in terms of leakage...

Copyright American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 1996 under license agreement with Books24x7

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