I am working on black box model of chiller which is 5 input and 3 output model. For simplicity I have considered them as three set of 5 input and 1 output model. From the identification toolbox, I tried applying many techniques to identify the model using training data and got fit ranging from 70-80% with validation data. But the problem is most of the time the Auto-correlation plot of the output goes out of the confidence limits. Is it ok to work with this kind of model for Predictive control?