Heating Ventilation and Air Conditioning (HVAC) system energy consumption on average accounts for 40percent of an industrial sites total energy consumption. Studies have indicated that 20 - 30 percent energy savings are achievable by re-commissioning HVAC systems to rectify faulty operation with savings of over 20 percent of total energy cost possible by continuously commissioning. Automated Fault Detection and Diagnosis (AFDD) is a process concerned with automating the detection of faults and their causes in physical systems. AFDD can be used to identify faults in HVAC systems with a view to reducing their energy consumption. An AFDD tool has been designed and developed to allow the performance analysis of AHU's by utilizing knowledge-based principles. Based on an initial alpha testing phase on 12 AHU's across four large industrial pilot sites, in excess of 120,000 euro of energy savings have been detected by the AFDD tool and verified by site survey.