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A feed forward is needed. Possibly a time delay and lead/lag block.
1) Do a step change with the valve in manual to graph out the temperature response over time.
2) Set up a time delay and lead/lag block to model the inverse of the temperature response. (MUST ALWAYS BLEED DOWN TO ZERO).
3) Add this to the PV of the controller.
In this way the controller will see the result of the CV change in the future. You can then be more aggressive in your tuning because the PV will predict what the new temperature will be.
Fred (no relation)