In this paper, an advanced PID auto-tuner for both single- and multi-variable processes is described and its application to HVAC systems is presented. The auto-tuner exploits two different process identification methods to suit a wide range of processes. It incorporates two recent PID design rules to achieve high control performances for processes with different dynamic characteristics. Decoupling control is used for multivariable processes. The auto-tuner was implemented on a distributed control system and applied in industry. Experimental results demonstrate the effectiveness and superior performance of the implemented auto-tuner over the manually tuned PID controller and the standard relay auto-tuner.