This paper presents the experimental results of the performance of various types of HVAC system airflow control dampers over a wide range of types, installations, and operating conditions. Three hundred and sixty-eight tests have been conducted with the data and results discussed herein. The main fundamental performance parameter for predicting the flow rate through a partially opened damper is the pressure loss coefficient as a function of the degree of damper opening. Major variables included the type of damper, blade movement, manufacturer, installation, approach velocity, and total system pressure drop. Results should prove very useful for HVAC system designers in the proper selection of airflow modulating dampers.