Although (Direct Digital Control) DDC-based air conditioning systems can potentially carry out efficiency checks, the instrumentation installed in these systems is not always suitable for making reliable plant status estimations. In order to verify the metrological performances of these systems and to define how reliable measurements can be made the authors set up a small-sized HVAC (Heating Ventilating and Air Conditioning) system provided with standard HVAC type sensors. The measurement instrumentation was verified by means of calibration and field tests, and the operating conditions in which it is possible to estimate in a sufficiently reliable manner the performances of each system component using the same instrumentation were determined experimentally. This analysis has shown that the normally used sensors are often highly dependent on operating and installation conditions and, consequently, the efficiency estimation of each component is very unreliable using uncalibrated sensors.