Ducts may be obstructed by foreign objects dropped into registers. Collapsed ducts may be the result of animals or people crawling through attics or crawlspaces.
Ducts buried in concrete floor slabs may be partially collapsed during original construction, when the concrete is poured. These ducts may also be obstructed by debris or water that accumulates in the ducts. Water accumulating in the ducts may rust metal ducts causing them to collapse, creating a substantial obstruction.
Obstructed or collapsed ducts will reduce the airflow, lowering comfort levels and making the system work harder.
Trace the ducts and look for evidence of irregularities. In some cases, it is possible to remove registers and put a light and a mirror down to look at the condition of the duct supplying that register. This is particularly helpful for ducts buried in slabs.