On many projects, the test & balance (TAB) firm is responsible for witnessing and certifying duct leakage testing as performed by the installing contractor. There have been hundreds of technical papers written on how to properly perform duct leakage testing. Unfortunately, some project specifications can be confusing for both the installing contractor and the witnessing team. Many times it seems, the larger the specification, the more convoluted it can become. In its simplest form, only two things need to be determined: "what is the test pressure?" and "what is the allowable leakage rate?" This is difficult to determine in some specifications because several standards are sometimes referenced.

On a recent project, the specification stated: "Comply with Sheet Metal and Air Conditioning Contractor's National Association's (SMACNA) HVAC Air Duct Leakage Testing Manual, and submit a test report for each test." This sentence was interpreted by the installer to also mean using SMACNA's duct leakage class which, in the case of low-pressure rectangular ductwork, is a leakage class of 16 as per the latest (2012) SMACNA HVAC Air Duct Leakage Testing Manual.

All test sheets and duct leakage pre-testing were conducted under this premise.

The leakage pre-testing reports were not available to the TAB professional prior to conducting the witnessing and certifying. However, after reviewing the specifications onsite and reading several pages into the specifications, the TAB professional interpreted the allowable leakage class to be 6 (which has a lower leakage rate compared to static pressure) instead of the 16 recommended by SMACNA.

It is common for the specifications to require a lower leakage class than what is suggested by SMACNA, since a leakage class of 16 is quite high. The leakage class of 6 was shown in the duct construction schedule several pages removed from the leakage testing section of the specification. This became a point of contention between the installing contractor and the TAB professional as the pre-testing was conducted under the premise that the allowable leakage was based on a duct leakage class of 16, and would not pass when a leakage class of 6 was used.

Further exacerbating the issue was the usual construction sequencing where the insulation contractor was following the testing team, wanting to start installing duct insulation immediately following the leakage testing.

After several phone calls from the sheet metal foreman to his supervisor, and eventually the mechanical engineer, it was determined that the TAB professional was correct in his reading and interpretation of the specifications. After a week or so of additional duct sealing, the TAB professional returned to the project to witness the tests using the correct criteria and all was well. It is extremely important to thoroughly read the specifications and consult with whoever is ultimately responsible for compliance if there is any confusion.