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78 ASHRAE Journa l Sep tem ber 2012

Classifying Duct LeakageBy Lee Feigenbaum, Student Member ASHRAE

As an ever-increasing number ofbuildings seek LEED certifica-tion, energy-conserving initia-

tives call for HVAC systems that deliverhigh levels of performance at relativelylow pressures.1To that end, engineersroutinely specify air duct leakagetesting to verify that their f inishedsystems meet acceptable standards forair conveyance.

Based on the SMACNA standard forleakage testing, leakage classificationhelps relate leakage to duct surfacearea.1To avoid unnecessary costs orlost productivity resulting from failedleakage tests in the field, it is importantto understand how leakage classification

impacts the leakage factor, or permis-sible leakage.

Leakage classification is the value thatties static pressure (in. w.g.) to acceptableleakage rates. At any given pressure, leak-age classification identifies a permissibleleakage rate measured in cfm/100 ft2(L/sper 20 m2) of duct surface area at 1 in.w.g. (249 Pa).1To simplify the procedure of calcu-

lating acceptable leakage, SMACNAprovides leakage factors that correspondto Leakage Classifications 2, 4, 8, and 16in Table E-1.1Although leakage classifi-cation can be expressed by any number(including decimals and/or fractions), itshould not be selected arbitrarily.

SMACNA provides suggested valuesin its leakage standards as a generalguideline. Other values are also accept-able given that they are achievable us-ing the correct fabrication and sealingtechniques. ASHRAE/IES Standard90.1-2010 recommends Leakage Class6 for ductwork at 1 in. w.g. (249 Pa).

To maintain efficiency while conduct-ing leakage testing, it is important tounderstand how to properly calculate thespecified allowable leakage.

One example would be when thespecified leakage class is 5. Manyengineers and contractors in this situ-ation quickly jump to the next mostrestrictive classification as set forth inSMACNA Table E-1. In doing so theyexceed the specifications, which is un-

This article was published in ASHRAE Journal, September 2012. Copyright 2012 ASHRAE. Posted at www.ashrae.org. This article may not be copied

and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

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80 ASHRAE Journa l Sep tem ber 2012

necessary and potentially expensive ona first cost basis.The proper formula to calculate allow-

able leakage is:F= CL P

N

whereF =Maximum leakage rate per unitof duct surface area as cfm/100ft2(L/s per 20 m2) (Lmaxin Standard 90.1-2010)

CL=Leakage Class (given)P =Static Pressure (given)N =0.65 (constant value unless specified otherwise)1Using this formula on a duct system constructed to 4 in. w.g. (996

Pa) and Leakage Class 6, the leakage factor (F) is 14.8 cfm/100 ft2(15 L/s per 20 m2). The same system at Leakage Class 5 yields aslightly more restrictive factor of 12.3 cfm/100 ft2(12 L/s per 20m2). At Leakage Class 4 this system only allows a stingy factor of9.8 cfm/100 ft2(10 L/s per 20 m2).

When applied over a large test area, this difference becomessignificant. For instance, assume a test area of 2,000 ft2(186 m2)at Leakage Class 5. If an engineer or contractor follows Table E-1and jumps to the next most restrictive Leakage Class 4, rather thancalculating the leakage factor for Leakage Class 5, he essentiallyforfeits 50 cfm (24 L/s) of allowable leakage (Table 1). That isroughly equivalent to testing the system with an undampered air

Static

Pressure (P)Leakage Class

(CL)

Maximum

Leakage Rate (F)Test Area

(ft2)Allowable Leakage

4 in. w.g. 4 9.8 2,000 1964 in. w.g. 5 12.3 2,000 2464 in. w.g. 6 14.8 2,000 296

Table 1: Allowable leakage of sample section of duct tested to 4 in. w.g. (996 Pa)and various leakage classifcations.

outlet fully open. The impact to a contractors productivity and/orprofitability under these undesirable circumstances is unacceptable.

Leakage classification is an important component of leakagecalculations. When multiplied by the surface area of a test sec-tion the leakage classification can become a powerful and costlyhindrance if not given proper consideration and care. Diligencewhile calculating leakage factors is important, as unexpectedconsequences of erroneous calculations may include unrealisticleakage goals and lost productivity. To that end, SMACNA statesthat prudent contractors must take testing seriously; work se-quence, work duration and costs can be significantly affected.1

References1. SMACNA. 2012.HVAC Air Duct Leakage Test Manual(2nded).

Lee Feigenbaum is a project manager at Heri tage Mechanical

Services: An EMCOR Co. in Farmingdale, N.Y. He is studying for

an MBA degree at Hofstra Uni versity.

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