In [3] the study characterized the field performance of residential
air distribution systems. The findings were summarized by the fact
that older homes (before 1979) tend to show an increase in leakage
rates as opposed to newer ones and about 20% of heat lost from
furnace installations in a residence tend to be due to duct leakage.
Adding to that the location of duct returns was found to have a large
influence on duct-system efficiency during the cooling system.