Hvac duct construction: myths vs. reality
Navas’ presentation coincided with a recent joint meeting of SMACNA Metro Detroit and Detroit’s ASHRAE chapter. The unusual joint meeting format was in place to allow contractors to present their “real world” view of duct fabrication and construction to hvac engineers— in most cases, those who are ordering and specifying their work.
Often, it was pointed out, the contractors performing this work are unable to follow specifications exactly as written — sometimes because the specifications are unclear, poorly written or contradictory. Occasionally, they conflict with work that other trades are performing. Sometimes a duct run must be moved to accommodate cables or other building infrastructure already in place, or a ceiling height may not meet the requirements to install the ductwork. For example, sometimes it is unclear from engineering drawings which ducts require an external lining. It can become even more problematic in cases where the insulating contractor is not the duct installation contractor.
“We want to leave here as friends,” joked Metro Detroit SMACNA president Ray Schemanske, Ventcon, Allen Park, Mich. There was an occasional chuckle as the engineers were shown examples of flawed or impossible-to-follow specifications — but the implications were serious, and hopefully a bit clearer by the end of the night.
Delivering more for lessIn tough economic times particularly, “We’re all pressured to deliver more for less,” Schemanske said. Communication on a project becomes more important than ever so the job is done correctly the first time, without unnecessary expense later. He termed this “expectations versus reality.” As an example, he said access doors are sometimes specified in places where they are redundant, or unnecessary.
Schemanske urged engineers writing specifications to use caution in using what he called “catch phrases” including terms that are often misunderstood, such as duct leakage.
Navas in an earlier presentation that night cautioned engineers not to specify duct leakage rates that are economically unfeasible, or “overspecified.” If properly fabricated and installed, duct leakage is seldom a problem. But it is generally not possible to specify ducts that are totally leak free, because of the cost involved.
Too often, he said, a figure of, for example, no more than 5% duct leakage is written into the specifications for a project. But this must relate to the type of duct and the type of installation, he said, and refers back to SMACNA’s duct construction standards manual:
Class A: 4” w.g. and above; all transverse joints, longitudinal seams, and duct wall penetrations.
Class B: 3” w.g.; all transverse joints and longitudinal seams only.
Class C: 2” w.g.; transverse joints only.
“Don’t overspecify,” Navas cautioned. A minor amount of duct leakage may be acceptable, or even preferable, to the costs incurred in providing a totally leak-free system —again, depending on the application. Remember, too, he said, that other factors also contribute to the amount of duct leakage: type of sealants used (liquids, mastics, gaskets, tapes); length of duct; number and type of connectors; static pressure, etc.
More information on duct leakage is available in SMACNA’s Hvac Air Duct Leakage Test Manual, ASHRAE Fundamentals Handbook (chapter 32), and SMACNA/ASHRAE/TIMA Testing, 1972, 1985.
The Duct Construction Standards manual was recently issued, incorporating numerous changes. In general, the text was edited to be more user-friendly.