A working knowledge of physics may be the key to helping you get to the bottom of what ails your furnace blower or air handler. Air flow or rising energy usage are generally the result of two root issues: friction and turbulence. Both must be reduced to increase the airflow through ducts. Friction creates slow moving, low- pressure airflow. If the duct is improperly installed, dirty, or made from rough material, friction will be a problem. Turbulence is a specific friction type. It occurs when air rubs against air. This happens as air turns corners. It is important to choose duct fittings that reduce turbulence. Curved, rather than square, is the better option.
Read the full article here: The Two Main Reasons Your Ducts Don’t Move Enough Air | GreenBuildingAdvisor.com