Running proper duct work is extremely important to maximizing your HVAC's efficiency and ability to cool and heat a property. The proper size of the ducts, the placement of the ducts, and the installation of the ducts are all very important. The duct size can greatly affect your airflow. The supply vents should be on the outer wall, often by a window, and the returns should be located more towards the center of the building. When installing duct work, you also want to ensure that the system is properly sealed to prevent air from escaping where it shouldn't.