Tests were performed to measure airflow characteristics of properly installed multibranch duct systems composed of wire-wound flexible ducts and rigid sheet metal ducts.When flexible duct systems were not sized per applicable standards required to design residential duct systems, the total system pressure loss penalty was as much as 44% when expressed in terms of the measured system loss coefficients. However, the difference in pressure loss for typical residential duct systems constructed from properly sized wire-wound flexible ducts and similar systems constructed from rigid steel ducts was negligible at design conditions.