"First of all you need to consider the age of the ductwork. For example, if the ductwork of your house is more than 10 or 15 years old it may be time for replacement because the normal life span of ductwork is around 10 to 15 years. It is important to check the exposed ducts in areas like the crawl space or the attic. You should look for holes or tears. One should also look for disconnected joints or eroded duct shapes. It is also important that you visit each of the rooms and check the airflow. The moment you feel that the airflow in any of the room is not proper, you should become aware. This is because it may be a sign that the ductwork has eroded and needs a replacement. Another effective way to check whether you duct is functioning well is to check the duct blower test. This is generally conducted by the HVAC company specialists. This is a test in which the technicians test the amount of air that is leaking through the ducts. It needs to be mentioned here that replacement of ductwork is a job best suited for the HVAC professionals because they have all that is necessary to replace and check the condition of an existing ductwork"