What is a key detail Manual D considers when sizing ducts?

The key detail that Manual D considers when sizing ducts is airflow requirements. This is essential because proper duct sizing is crucial for ensuring that the HVAC system can deliver the necessary amount of conditioned air to each space in a building. Adequate airflow is vital for maintaining comfort, efficiency, and indoor air quality. If the ducts are too small, they can restrict airflow, leading to inadequate heating or cooling and potentially causing the system to work harder, which can increase energy costs and wear on the equipment. By focusing on airflow requirements, Manual D helps ensure that the duct system is appropriately designed to meet the specific needs of the space it serves.