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Introduction to Allowable Leakage Specification

Before performing a duct leakage test, you must know the allowable leakage for the duct system you are testing. Usually, the limit will be expressed as a percentage of total system volume flow, or it will have to be calculated from a specified leakage class. Knowing the allowable leakage and the test pressure will allow you to choose the proper tester and orifice plate combination.

Specification using Leakage Class
The idea behind leakage classification is that the assignment of allowable leakage should be based on the surface area and operating pressure of the duct system.  Leakage class is defined as:

 

where

   

F  is the leakage factor, and is equal to the leakage rate in units of cfm/100 sq.ft., and
P  is the system static pressure in units of in.wg.

To determine the actual amount of allowable leakage, you must know the following:

  1. The leakage class specified for the duct system (if not specified, use leakage class of 4 since that is
    what ANSI/ASHRAE 90.1 specifies for both round and rectangular ductwork.)
  2. The design system static pressure (specified by Engineer, if not, use the fan ESP listed on the fan
    schedule)
  3. Duct system surface area (obtain data from your duct supplier or estimate)

Once you have the data, calculate F and then multiply by the total duct surface area.  Or you can use Oriflow’s simple FREE online program.

To learn more about leakage class, refer to SMACNA publication, “HVAC Air Duct Leakage Test Manual”. SMACNA gives recommended leakage classes (Table 4-1) of their HVAC Air Duct Leakage Test Manual.

Percentage of System Volume Flow
The idea behind the percentage method is that the percentage of leakage is what the building owner (or renter/lessee) pays for in heating and cooling costs that is wasted. Telling the building owner that you are allowing a leakage class equal to 4 may be the equivalent of 2% or 4% of the system volume flow. Specifying a percentage makes it very clear.

A percentage of volume flow is also a simple calculation for the testing contractor. For example, suppose you have a duct system where the duct system is handling 14,000 cfm. If the allowable leakage is specified as 2%, then the allowable leakage is 0.02 x 14,000, or 280 cfm. Use a simple calculator to perform this multiplication, or use Oriflow’s free online program.

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