Comparison of direct digital control and pneumatic control systems in a large office building

Abstract

A distributed, microprocessor-based direct digital control (DDC) energy management and control system was developed and installed in an eleven story office building in Gaithersburg, Maryland. Over a period of one year the performance of this system under various control strategies was investigated along with the performance of a conventional pneumatic control system. The quality of control and performance characteristics for the two control systems were compared. Overall the DDC system was found to maintain better control of the supply air temperature and to follow planned reset schedules more closely than the pneumatic system. One particular air handling unit performed as well under pneumatic control as it did under DDC in maintaining a supply air setpoint, but it did not precisely follow the planned reset schedule. Each air handling unit had different performance characteristics while under pneumatic control, but all units behaved essentially the same under DDC. The results indicate that a pneumatic system can perform as well as a DDC system but more effort is required to maintain system performance. The DDC system was found to be flexible and it is easier to insure that multiple air handling units follow the same control strategy with the DDC system. The instrumentationmore » associated with the DDC system proved to be very useful in troubleshooting problems with the pneumatic system because of the information readily available to the operators. 9 refs., 27 figs.« less