Duct systems leak. HVAC construction contractors, duct manufacturers and sheet metal products manufacturers have always acknowledged that duct systems are not 100 percent airtight.

Common opinion was that “this is just air.” A little bit of leakage from an HVAC market  system wouldn’t harm anyone. From a health and safety standpoint it’s a bit hard to argue with that logic. But the industry was overlooking two important things: Most people didn’t realize how much air actually leaked from a typical system. And this leakage was causing harm — to the building owner’s pocketbook.

Almost 25 years ago, reports were published that said the typical commercial HVAC market systems were leaking 10 percent to 30 percent of the air produced by fans. Many of these same reports hinted that leakage in residential systems could be much higher. At the same time, ducts for universities and hospitals were being installed and tested to perform at 0.5 percent leakage and at 1.5 times the system operating pressure.

It seemed logical to pass the reports of leaky ducts off as the work of unscrupulous or poorly trained HVAC sales installers. After all, leakage does come down to component choices and workmanship. The surprising fact is even those “half of 1 percent” leakage ducts were part of systems that leaked at much higher rates. If you look at how a leakage test is typically performed you can see how the majority of duct system leakage remains unmonitored.

A duct system to be tested will usually be installed using higher-quality components — spiral duct or rectangular ducts with flanged transverse connectors. Duct sealants are used liberally on the joints and side seams. The ducts would be capped off and tested. But what was not being tested? A typical duct-leakage test never included any components. Access doors were excluded. Duct sections were always capped off prior to fire dampers so they were not part of the test. Sound attenuators were not tested. Typically, the trunk ducts were tested but the caps were placed on the T taps, so round run-outs to variable-air-volume boxes were not included.

In the mix

Flexible ducts and most importantly, their connections to metal ducts and mixing box connections, were not tested. The mixing boxes themselves were not checked for leakage, nor were any components connected to them like coils and heaters. Downstream of the mixing box was a whole different type of duct — unmonitored and often dismissed as irrelevant in leakage.

Industry groups and contractors would say, “It doesn’t leak much. It’s low pressure” and that testing for leakage in these systems would not be cost-effective.

Research performed at Lawrence Berkeley National Laboratory on installations just like a typical “low pressure” duct system show that these leakages are not irrelevant. These systems often use no sealants at all. The ducts are smaller so you see rectangular ducts with S-and-drive connections instead of flanged ends. You would see components like spin-in taps used with dampers that pivoted on a peg stuck through a 3/8-inch open hole. Adjustable elbows seldom had seams sealed. Longitudinal seams of rectangular ducts and snap-lock pipes were not sealed.

Flexible ducts were installed on crimped duct ends and too-short grille collars — with no profile to prevent pull-off —with, if anything, duct tape used to seal the connection. An unsealed “low pressure” duct system can easily leak 10 percent of the supply fan flow.

One case study by Semco titled “Efficient Duct System Design” illustrates the problem. In a very typical VAV duct system, there was almost 50 percent more square footage of surface area in the “low pressure” duct system versus the “medium pressure” duct system. We classify leakage as “Cubic feet per minute per square footage of duct surface area.” There were eight times as many transverse connections in the low-pressure duct. There was more than three times the length of seams that should have been — but often were not — sealed.


Looking at the total duct system, you could see that even a tightly sealed medium-pressure duct system would only partially affect “system” leakage. Any air that leaves the “duct system” between the fan and the grille is leakage — and is wasted. All system components have their own leakage and contribute to what usually becomes a substantial amount.


SOURCE: Snips Magazine

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