In HVAC systems, efficient control of water-air heat exchangers is essential for energy efficiency. Conventional fixed-gain PID controllers often struggle with varying operating conditions, leading to suboptimal performance or component wear. The issue originates from the nonlinear behaviour of heat exchangers and valves. This contribution presents a model investigation of a gain-scheduling control strategy for heat exchangers in HVAC systems. The PID controller parameters (gains) kp, ki and kd are dynamically adjusted based on system load/airflow rate, inlet air temperature, and supply water temperature. A gain-scheduling approach can be easily integrated into existing HVAC systems, offering a practical and cost-effective method to enhance performance and sustainability. Improvements are observed in temperature tracking and system stability, particularly during rapid changes and part-load operations. The adaptive nature of the gain-scheduling strategy enables more responsive and efficient climate control across a wide range of operating conditions. The approach was tested on a model of a reheater of an HVAC system. A grey box model was used that described time series of measured temperatures of a reheater with deviations better than 1–2 K. Based on the model, the tuned PID parameters varied considerably in the full operation range from 2%/K to 38%/K for kp, 0.03%/(K s) to 3%/(K s) for ki and 23%s/K to 143%s/K for kd. The most influential factors were the current mixing valve position and the heater inlet air temperature. Limitations: The control parameter influence could thus far only be shown for the model system. A comparison of the used model with a conventional empirical system parameter identification showed significant differences that require further experimentation for clarification.