What is the difference between direct acting and reverse acting controllers?

Direct acting controller increases output when process variable increases (e.g., cooling system - temperature rises, increase cooling). Reverse acting controller increases output when process variable decreases (e.g., heating system - temperature falls, increase heating). Selection depends on the process and final control element: consider if the valve fails open or closed and the effect on the process. The combination of controller and valve action must provide negative feedback for stable control.