Energy Management and Control Systems (EMCS) are used to manage Heating, Ventilation, Air Conditioning (HVAC), sometimes also lighting and security systems in single or multiple facilities. Although EMCS control algorithms are designed to be energy efficient, inefficiencies can result from: (1) equipment or sensor or controller malfunction, (2) operator error (building operators can override control settings without understanding the implications), and, (3) facility use inconsistent with design assumptions (e.g., the maximum design cooling load is exceeded in everyday use). Such problems can go unnoticed until they result in a significant deterioration in performance resulting in either an alarm condition or complaints from occupants. Identifying the problems well in advance of the alarm or complaint stage can result in substantial savings and occupant satisfaction. This paper proposes a model-based approach to detect and diagnose these problems.