What is the relative size difference between AC supply and return ducts?

The correct choice indicates that return ducts should be 20% larger than supply ducts. This principle is based on the need for adequate airflow and pressure balance within the system. Supply ducts are responsible for delivering conditioned air into the space, whereas return ducts collect air from the space and direct it back to the air handling unit for reconditioning.

Having larger return ducts compared to supply ducts helps facilitate the efficient movement of air back into the system. When return ducts are proportionally larger, it aids in reducing resistance and allows for less friction loss within the system, which is crucial for maintaining proper airflow and ensuring that the HVAC system operates efficiently. This sizing characteristic ensures that the system can handle the volume of air being circulated, thus optimizing performance and comfort.

Unequal duct sizing can lead to imbalances in system pressure and airflow issues, potentially causing noisy operation, inefficient heating or cooling, and increased energy consumption. Thus, the guideline of return ducts being larger than supply ducts focuses on achieving effective air circulation and maintaining system efficiency.