Argues that system trend analysis is essential for the proper performance of an air handling system. Sets out a step-by-step study of system analysis and advises that this can even be used for preventive maintenance trends. Notes that an engineer cannot tell what is going on merely by watching a system's instantaneous operation. It is necessary to see the simultaneous interaction over an extended period of time and under a variety of conditions. In this way the analysis can be used for solving operating problems, increasing energy efficiency, locating defective hardware, identifying out-of-calibration devices, improving occupant comfort and extending equipment life.