Nonlinear devices have different gains at different load conditions. Depending on the severity of the nonlinearity, PRAC+ may not tune to optimal parameters, and you may need to manually tune the device. Prior to the release of the lead compensator, PRAC+ could suboptimally tune a loop with a ball valve actuator. The valve allows almost no flow for the first 15% of rotation, opens quickly for the next 50 to 60% of rotation, and has almost no change for the last 25% of rotation. During normal operation, the ball valve is in the high gain region for which PRAC+ tunes accordingly. However, after shutdown, the dead zone is in the first 15% of rotation, which causes a slow response. Control strategies are in place to overcome these problems for zone control (such as implementation of discharge air temperature setpoint reset and the lead compensator). However, this behavior is still possible on highly nonlinear control loops.