What factor is most critical when determining duct size for HVAC systems?

The most critical factor when determining duct size for HVAC systems is manufacturer recommendations. Manufacturers provide specific guidelines and calculations that are based on the design and performance characteristics of their systems. These recommendations take into account various factors such as airflow requirements, static pressure, and the intended purpose of the system, allowing for optimal performance and efficiency. Following these guidelines ensures that the HVAC system operates effectively, delivering the right amount of air to each area, maintaining comfort levels, and preventing issues such as noise or inadequate heating and cooling.

While the other factors may influence duct design in certain circumstances, they do not carry the same weight as manufacturer recommendations. For example, the shape of duct fittings can affect airflow and pressure loss, and external temperature plays a role in system load calculations, but they should align with the parameters set by the manufacturer. Additionally, the color of the duct material is largely a cosmetic consideration and does not impact the functional aspects of duct sizing. Therefore, adhering to manufacturer recommendations is vital for achieving an efficient and well-functioning HVAC system.