The main trunk is the primary large-diameter ductwork that carries conditioned air from the furnace or air handler to branch ducts throughout the building. It serves as the central distribution line for the entire HVAC system. Properly sized main trunks minimize pressure drop and ensure adequate airflow to all zones.
System Function
The main trunk receives 100 percent of the supply airflow before it splits into individual branch ducts serving different rooms and areas. Its diameter directly impacts system efficiency and noise levels. Undersized main trunks create excessive velocity and pressure drop, while oversized trunks waste material and space.
Installation Considerations
Main trunk placement in attics, basements, or crawl spaces requires careful routing to minimize duct runs. The trunk should remain as straight as possible to reduce turbulence and friction losses. Proper insulation and sealing of main trunk connections prevent energy loss and air leakage. Most residential systems use main trunks ranging from 12 to 24 inches in diameter.