The improvement of HVAC systems requires consideration of the work processes that take place in them. HVAC systems are complex dynamic systems with distributed parameters, and the work processes occurring in them are difficult to formalize clearly. Therefore, the task of identifying mathematical models of HVAC systems as object of automatic control is actual. The purpose of paper is to analyze the methods for structural and parametric identification of complex HVAC systems and to develop recommendations for the construction of mathematical models of HVS. It is shown that HVAC systems are inertial, in which the regulated variable is stabilized at a certain value, and transients have a non-oscillatory appearance and characteristic delay. The identification of mathematical models for working processes of HVAC systems by the transfer functions of the aperiodic links of the first and second orders with delay is recommended. The authors present the example of mathematical model for heating-ventilating system is presented in the form of the transfer function for the temperature at significant (working) zone of the room by the heat flow power of supply air.