Abstract: This invention relates to an apparatus and method for monitoring and controlling a heating and/or cooling system which controls the temperature of air in a conditioned space within a building and to such an apparatus and method capable of indicating the performance of such a system and effectuating control over the system for maximizing the operating efficiency of the system. As compared with a conventional control system, additional temperature sensors are provided to sense the temperatures of air drawn into and discharged from a temperature modifying unit such as an air handling unit having a heat exchange coil. The sensed temperatures are then used as a basis for determining the differential attained by flowing air through the unit, and compared with a desired differential taken as indicating an optimal condition, with the percentage of attainment of the optimal differential being visually displayed for an operator of the system.