Model calibration remains a significant topic for both researchers and practitioners in the HVAC field although calibrated building and air handling unit (AHU) models have been used for “measuring” building energy retrofit/commissioning savings, identifying operational problems, and optimizing system operation and control schedules for more than decades. This paper discusses the modeling and simplification of building and AHU systems used in the simplified model calibration process. The paper presents a two-level calibration procedure integrated with energy signatures for simplified building and AHU heating and cooling energy consumption models. The procedures are demonstrated using a detailed example. This procedure can be implemented into computer simulation programs to allow automatic calibration and fault detection in many HVAC engineering processes.