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.