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How do I tune the response of a reheat temperature control loop?

Issue

Have used tuning parameters that have worked good other places but at this site the loop output hunts continuously.  How do I properly tune this PI loop?

Environment

MNL-V2 or other temperature controller

Proportional hot water valve for the reheat control.

Cause

Loop was tuned too tight causing continuous hunting that resulted in premature failure of the actuator.

Resolution

The following information is paraphrased from a number of previously published HVAC controls documents.

  1. Using only proportional control, adjust the Throttling range (TR) until the loop control is stable with no oscillation. The desired control range x 2.5 is a good TR to start with.  When the loop is still oscillating, note the period of oscillation (in minutes). You may need to increase the TR to 25 or more to achieve stability in the loop output.  You must achieve stable control before continuing.  If proportional control cannot stabilize the loop, adding integral and/or derivative will cause further instability.
  2. Make a step change (5 to 10%) in the setpoint.  If the system does not stabilize after a few minutes, repeat step 1.
  3. Increase the TR by an additional 5% to 30%.
  4. Use the following formula to calculate the initial value to be used for the integral value:
          i = 1 / [ ( Loop period in Minutes ) x 2 ]
  5. Monitor loop to evaluate response,  If the response is slow with integral action, increase the "i" value slightly.  It may be necessary to upset the loop, by changing the setpoint, to get a good test of the loop response.  The final integral value is typically in the range of 0.05 to 0.5.
  6. In most cases HVAC control loops do not require derivative action.  Improperly tuning the derivative action can result in poorer control than without any derivative action.  The following formula can be used to determine a starting value for the derivative value:
         d = (Loop period in Minutes) / 8
    This value may need adjustment to optimize the control.
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