Microprocessor-based equipment using the velocity form of a proportional-integral (PI) algorithm has been used to implement direct digital control (DDC) of the leaving air temperature of a large air-handling unit in an office/laboratory building at the National Bureau of Standards in Gaithersburg, MD. Results using two types of interfaces between the digital controller and the existing pneumatic chilled water valve positioner are reported. The effects of air leakage from the pneumatic system and changes in controller parameters are examined. A simplified model of the controller/HVAC process is described and results compared with actual measurements during step changes in load.