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What’s the Deal with Duct Leakage?

Nationwide, about 85% of homes use a central forced air duct system to deliver heating and cooling to the home. Unfortunately, according to US Department of Energy, “In a typical house, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes and poorly connected ducts.” The National Renewable Energy Laboratory (NREL) estimates that “typical systems with ducts in attics or crawl spaces lose from 25% to 40% of the heating or cooling energy that passes through them.”

In fact, NREL goes further to break down all the various losses, concluding that nearly half of the energy input into a typical heating system is wasted:

Image courtesy of NREL

The implication of this article is that more potential gains can be made by improving the duct system than replacing the furnace! These losses can come from a variety of locations in the duct system:

Duct leakage can occur at every location where there is a connection between two or more solid pieces:

Where the main duct plenums connect to the furnace
Where individual trunks connect to the plenums
“Y” and “T” connections between trunks and individual ducts
Connections between ducts and register boots
Gaps around the rough hole where the duct enters the inside of the home
Rigid metal duct seams (radial and linear)

These leaks are prevalent on both the supply and return sides of the duct system. On the return side, air is being sucked into the system from inside the home. When duct leaks occur on the return duct, they suck air into the forced air system from wherever the duct is located. This has major implications for moisture and air quality in the home. Return ducts are typically located in a home’ crawl space, which is also usually the largest source of moisture in the home. When return leaks draw in moisture, humidity levels increase throughout the house as the moist air is distributed through the supply ducts. On the supply side, ducts dump warm air into the crawl space or attic, and as this warm air cools, it becomes more moisture-laden, and eventually causes condensation to collect on roof decks, foundation walls, and other cold surfaces.

In addition to the mold and mildew caused by moisture associated with duct leakage, return leaks also pull in contaminants associated with construction materials, including formaldehyde from fiberglass and asbestos fibers. Warm attics and crawl spaces (warmed by duct leakage) make better homes for rodents, and as rat feces build up, these, too, will be drawn into the house through return duct leaks. At this point, we should be very thankful that our duct systems are equipped with filters to catch all these particles before we breathe them…. except that most filters are located at the return grille, which is upstream of the rest of the return system. So, in actuality, most systems do absolutely nothing to keep drawn-in contaminants from passing through the furnace and ending up in our living areas. Other potential contaminants include carbon monoxide from furnaces, water heaters, and car exhaust, and even chemicals from gasoline, paint, and solvents that are often stored in the crawl space or garage.

Duct Leakage Can Affect Health, Safety, and Comfort

Image courtesy of NREL

Considering all the potential hazards of duct leakage, along with the major efficiency losses, it comes as no surprise that sealing leaks in ductwork are often top priority when planning home performance improvement projects. For a relatively small investment, the homeowner can achieve energy savings equivalent to a furnace replacement while eliminating sources of poor indoor air quality. Home performance contractors frequently seal and re-insulate existing ductwork, but for the best results, duct replacement will yield the best results in terms of both efficiency and air quality gains, and it is often almost as expensive to “tune-up” the ducts as to replace them entirely.

Sources:

NREL http://www.nrel.gov/ , http://www.nrel.gov/docs/fy05osti/30506.pdf

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