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Since most ductwork is located in non-conditioned space like attics, basements, garages, or crawl spaces, the HVAC system becomes an open system instead of a closed one. Leaking supply ducts can lose large amounts of cooled/heated air to these unconditioned areas. Leaking return ducts suck hot/cold unconditioned air into the conditioned space.

Duct leakage significantly increases cooling and heating loads, sometimes beyond what the HVAC system can handle.

The increased energy cost—because the HVAC system has to work harder—isn't the only effect of leaking ducts. Indoor humidity can increase when unconditioned air is introduced, leading to mold and mildew problems. If the air handler is in the garage and improperly sealed, return or supply leaks can introduce poor quality outdoor air or hazardous vapors from the garage (from cleaning supplies, pesticides, gasoline, paints, car exhaust, etc.) into the home

source: Wendell A. Porter, Hyun-Jeong Lee, and Kathleen C. Ruppert, The Duct System, UF/IFAS FCS3263, 2008.

Additional Resources

EnergyStar®  - http://www.energystar.gov/

Progress Energy  Home Energy Check-  http://progress-energy.com/

UF IFAS Energy Efficient Homes: The Duct System - http://edis.ifas.ufl.edu/FY1024     

Florida Building Code - http://www.floridabuilding.org

U.S. Department of Energy – Better Duct Systems for Home Heating and Cooling - http://www.nrel.gov/docs/fy05osti/30506.pdf

U.S. Department of Energy – Improving the Efficiency of Your Duct System - http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/27630.pdf

U.S. Department of Energy – Minimizing Energy Losses in Ducts – http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12730