Besides optimization of pipe network, the HVAC water systems also exhibit significant hysteresis and nonlinear characteristics when undergoing optimized control, which is of important reference significance for improving the decoupling optimization effect of the system. The hysteresis feature refers to the time-delay response characteristics exhibited by the parameters when propagating in the pipeline network, while the nonlinear feature refers to the spatial coupling characteristics of interactions between devices. This chapter will conduct a model study on the hysteresis and nonlinear characteristics of HVAC water systems, constructing the time-delay response characteristics of parameters in the pipeline network and the nonlinear characteristics among various devices. The chapter will first analyze the dynamic response characteristics of water temperature and flow in the HVAC water system pipeline network, exploring the time-delay characteristics of the quality–quantity parameters in the control process. Secondly, it will analyze the influence scale of constitutive parameters on the parameter time-delay response characteristics, verifying the applicability of the time-delay characteristics model in systems of different volumes and structures. Finally, by considering the coupling characteristics among operating parameters of various devices in the system, the chapter will conduct a model study on the spatial characteristics of the system, clarifying the spatial coupling relationships among various devices and control loops. The characteristic model of the HVAC water system established in this chapter will lay the theoretical foundation for subsequent chapters’ research on global optimization methods.