If you look in the return air registers and see an accumulation of debris in the duct below, you may want to have it removed. CMHC research showed that return air ducts, which are relatively big and have slow moving air, are generally far dirtier than the supply ducts.You should be able to accomplish much of the return air duct cleaning with a household vacuum cleaner after removing the register grill. For example, households with hairy pets or young children may find it necessary to inspect their return air ducts more frequently. Do not expect any difference in airflow or house air quality after this cleaning.