PID (Proportional-Integral-Derivative) control is an automatic feedback algorithm that continuously adjusts HVAC equipment output to maintain system variables at desired setpoints. The controller calculates three components: proportional response to current error, integral correction for cumulative offset, and derivative prediction of future trends. PID is the industry standard for temperature, pressure, and flow control in modern HVAC systems.
Control Components and Tuning
The proportional band (P) is typically 3-4°F for temperature control, producing linear output from 0-100%. Integral time (I) ranges from 300-600 seconds to eliminate offset errors. Derivative time (D) of 60-120 seconds improves response to rapid load changes. Proper tuning prevents oscillation (hunting) and overshoot beyond setpoint by 2-3°F.
System Applications
PID controls modulate chilled water valve openings, hot water mixing, variable frequency drive speeds, and damper positions. Response times under 30 seconds maintain temperatures within ±0.5°F of setpoint. Direct digital control systems typically implement multiple cascaded PID loops: outer loops controlling space temperature and inner loops regulating supply air conditions for faster, more stable operation.