Controls, DDC Interfaces, and System Verification
Core concepts
Know control fundamentals: sensors, controllers, actuators, and controlled devices; open vs closed loop; proportional, integral, derivative (PID) control; setpoint, deadband, throttling range, and offset. Understand DDC architecture: field controllers, network protocols (BACnet, Modbus, LonWorks), points (AI, AO, BI, BO), and trending/alarming. Verify sequences of operation against the design intent and confirm sensor calibration and point-to-point checkout.
Understand economizer control, static pressure reset, demand-controlled ventilation (CO₂), and valve/damper actuator stroke and fail position. Know how to verify a control sequence by forcing points and observing responses, and how to document results.
Traps
- Confusing analog and binary points or input vs output direction.
- Assuming a trend log proves correct operation-verify the sequence, not just the data.
- Ignoring sensor location errors (e.g., a temperature sensor in a dead zone or near a heat source).
- Misunderstanding fail-safe actuator position and its effect on freeze protection.
- Confusing proportional-only offset with integral action eliminating steady-state error.
- Overlooking network addressing and protocol gateway issues during point-to-point checkout.
Worked example
A discharge-air temperature loop has a setpoint of 55°F and a throttling range of 4°F. With proportional-only control, at 57°F the output is 50% (half the throttling range). If the measured value stabilizes at 56°F with 75% output, the proportional offset is 1°F; adding integral action drives the offset to zero over time.
Rule of thumb
Verify sequences by forcing inputs and observing outputs, then document point-to-point results and trends.