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In existing homes all over the country duct leakage typically wastes 10% to 30% of the heating or cooling energy in a home. Leaky ducts located in conditioned areas of a building lead to some inefficiency and temperature variations, but isn’t the major problem for duct leakage. Duct sealing is most effective when the ducting is located in unconditioned spaces (e.g. attic, crawlspace, etc.). Ducts leaking into unconditioned space waste energy in two ways:
- Conditioned air is lost (supply) or outdoor air enters (return) and needs to be conditioned, and
- Duct leakage will pressurize (supply) or depressurize (return) the home, which will in turn drive infiltration or exfiltration across the building shell.
Leaks on the supply side of the system can be the most severe energy problems found in homes because the leaky supply air can be anywhere from 20°F to 70°F warmer than indoor air in the winter and 15°F to 25°F cooler in the summer. Furnaces and packaged air conditioners located on roofs, the ground outdoors, crawl spaces, and attics are often major sources of air leakage.
Duct leakage is typically expressed as a percentage of the measured system airflow or nominal system airflow. For example, if the measured duct leakage is 100cfm and the system airflow under normal operation is 1000cfm, then the duct leakage is 10%. The Efficiency Standards require that all new duct systems demonstrate leakage at less than or equal to 5%.
See Calculating a Duct Leakage Target for specifics on Measured vs. Nominal Airflow.