A "good" duct system design is always checked for its pressure balance. It incorporates correctly sized ducts, terminal boxes, and fittings to equalize the pressure losses through the duct branches at each node. This is necessary to produce the design airflow rate at all terminal boxes. A good design effort also minimizes first costs and offers safety and performance while minimizing noise levels in the duct system. Total pressure should be used when calculating the system design. Using static pressure only is inadequate.
If the estimated fan pressure is lower than the actual pressure loss in a branch of the system, air flow will be lower than the design rate; consequently, a larger fan must be installed or duct cross sections have to be enlarged.
If the estimated fan pressure is higher than the actual pressure loss in a system branch, actual air flow will exceed design air flow, requiring that balancing dampers be partially closed to reduce the air flow down to the design rate. Such systems are usually costly and noisy. Excess pressure at terminal inlets not only raises the general sound level but also increases the tendency toward control instability.
