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Chapter 2 PID settings
18 CNT-APG002-EN
Determining the action
Table 3 shows the action settings for several applications. These settings
are a good starting place for most applications.
To find the action for other applications, determine whether the actuator
and measured variable move in the same direction. If so, then the action
is direct. If not, then the PID loop is reverse acting.
Example 1
An exhaust fan controls the static pressure in a building. The exhaust fan
operates at its highest speed when the building pressure is too high.
When the pressure goes above the setpoint, the exhaust fan should speed
up to blow air from the building. So when the measured variable (the
building pressure) increases, the actuator increases the fan speed.
Because the measured variable and the actuator move in the same direc-
tion, the PID loop is direct acting.
Example 2
A supply fan controls the static pressure in ducts supplying variable-air-
volume (VAV) boxes. The supply fan operates at its highest speed when
the pressure is too low. When the pressure goes above the setpoint, the
supply fan should slow down to blow less air to the VAV boxes. So when
the measured variable (the pressure) increases, the variable-frequency
drive (VFD) decreases the fan speed. Because the measured variable and
the control signal to the VFD move in opposite directions, the PID loop is
reverse acting.
Table 3: Action settings
Application Output
Actuator setting at 100%
PID output Direct or reverse acting
Discharge-air cooling Valve position 0–100% Completely open* Direct
Discharge-air heating Valve position 0–100% Completely open* Reverse
Duct static pressure Inlet guide vane position
0–100%
Completely open Reverse
Duct static pressure variable-frequency drive
0–100%
Completely open Reverse
Building static pressure Supply fan control Maximum fan speed Reverse
Building static pressure Exhaust fan control Maximum fan speed Direct
Discharge-air cooling Electric/pneumatic
5.0–15.0 psi (34–103 kPa)
15.0 psi (103 kPa) Direct
Space temperature Discharge setpoint
50–100°F (10–37.8°C)
N/A (calculates a setpoint) Reverse
* These settings may vary by region