Duct leakage is always uncontrolled, so it should be avoided. Building owners pay to have conditioned air delivered to diffuser outlets in the quantities called for by the building design. Although leakage may not cause an air distribution system to perform improperly, it drives up operating costs. Even when supply ducts are inside a building's envelope and air leakage goes into a plenum space, leakage diminishes the supply air's effect on control sensors, thus increasing system operation.
Studies [Durfee ref173, 1972; Swim, 1984 ref206; ASHRAE/SMACNA/TIMA, 1985 ref158; AABC, 1983 ref148] have shown that duct leakage depends on the method of duct fabrication and of sealing as well as on workmanship and static pressure differential. Research shows that leakage in an assembled duct can be estimated by an exponential equation. According to the Fundamentals Handbook, Chapter 32 [ASHRAE, 1997], duct leakage can be determined as follows:
Leakage = (Leakage class) (Static pressure)0.65
Where the leakage class reflects the quality of duct construction and sealing method. This class is based on experimental data and ranges from zero for welded ducts to 110 for rectangular unsealed ducts. The average leakage class for rectangular unsealed ducts is 48 [ASHRAE, 1997].
For typical duct construction and sealing, the leakage equation is the same in negative or positive pressure modes. The leakage equation can be used to calculate leakage at any system point for a known internal static pressure and given duct construction. However, internal static pressure is constantly changing from the fan to the terminal inlets/outlets because of control devices. Testing per ANSI/ASHRAE/SMACNA Standard 126-2000 is advised. [ASHRAE, 2000]
Because the amount of duct leakage depends on internal static pressure, lower pressure in a system reduces leakage. For a single straight duct static pressure changes uniformly. However, local resistance causes sudden changes of static pressure. Therefore, in general, static pressure and duct leakage are not equally distributed through a duct's length and should be calculated locally between fittings [Tsal, et. al., 1998].
Duct Leakage Calculations