Some homes do not have return ductwork at all, a design that assumes the air
delivered to rooms will probably make its way back to the air handling unit.
In such systems, the air handler is usually in a cold basement or crawl-space
and has to do more work warming this cold air for delivery to the home
than it would have if warmer air from the living space were available
from return ducts. This type of system is also likely to be unbalanced.
