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Chapter 6 Frequently asked questions
52 CNT-APG002-EN
I tried the 4:1 ratio for proportional and integral gains, but this
did not optimize my system. Can I try another ratio?
We recommend maintaining a 4:1 ratio between the proportional and
integral gains. Changing the gains may slightly improve the speed and
stability of a system, but the 4:1 ratio has proven to work effectively. See
“Calculating the gains” on page 11 for more information.
Why is the derivative gain usually zero?
Derivative control is rarely used in HVAC applications because the large
error deadbands used in HVAC applications make derivative control inef-
fective. Also, derivative control can react to noise in the input signal, and
the lag in derivative control makes tuning difficult. Setting the derivative
gain to zero prevents derivative control from being used. See “Derivative
calculation” on page 5 for more information.
What is the difference between direct acting and reverse acting?
Reverse acting loops decrease the output as the measured variable
increases. Discharge-air heating, duct static pressure control, and space
temperature control applications are typically reverse acting. Direct act-
ing PID loops increase the output as the measured variable increases.
Discharge-air cooling and building static pressure (with exhaust fans)
applications are typically direct acting. See “Action” on page 17 for more
information.
When should I use proportional-only control?
Proportional-only control works well in staging applications because it
can be simpler to manage than full PID control. Proportional-only control
is also used in pneumatic controllers. To use proportional-only control, set
the integral and derivative gains to zero. See “Proportional calculation”
on page 3 for more information.
Why should I use PID control in staging applications?
Most Trane controllers have built-in PID functions, making settings easy
to enter and adjust. See “Staging cooling-tower fans” on page 37 for more
information.
How can I tell if a PID loop is working well?
Graph the measured variable, setpoint, and valve position over time,
especially during periods of change, such as when changing from unoccu-
pied to occupied mode. The setpoint should be achieved in a reasonable
time and the valve position should remain fairly stable once setpoint has
been reached. If the measured variable oscillates around the setpoint,
fails to reach setpoint, or takes too long to reach setpoint, follow the
guidelines in Chapter 5, “Troubleshooting.”