Automatic Control, Testing, Commissioning and Operation
These questions are the practical ones: can you sequence a plant so it does not trip, tune a loop so it does not hunt, and prove installed performance?
Control fundamentals
- Sensor location decides whether a loop can work: a discharge air sensor too close to the coil sees stratification; a room sensor in a dead corner controls the wrong space.
- Valve characteristics: equal-percentage for modulating coils and heat exchangers, linear for constant-drop paths. Valve authority = ΔP_valve/(ΔP_valve + ΔP_circuit), targeted at 0.3–0.5. Low authority turns a good characteristic into on/off behaviour.
- PID: too narrow a proportional band causes hunting; too short an integral time causes oscillation; derivative action is avoided on noisy flow or pressure signals.
Sequencing and interlocks
Chiller start: cooling tower fan → condenser water pump (flow proved) → chilled water pump (flow proved) → chiller. Stop in reverse with a run-on timer. Freeze protection on coils and chilled water lines; low-limit thermostats stop the fan and close the outdoor damper.
Testing and commissioning
- Duct air-tightness testing by pressure class per GB 50243, with test holes and temporary fans as specified.
- Hydronic balancing by the proportional method, adjusting the index circuit last.
- Capacity verification: coil capacity from measured flow × Δt × 4.187; chiller capacity from measured power; both compared with design and the equipment schedule.
Worked example
A control valve has 40 kPa drop in a circuit totalling 100 kPa. Authority = 40/100 = 0.40, which is acceptable. Resize the valve smaller and the drop becomes 10 kPa: authority 0.10. The valve now behaves nearly linearly at low lift, the coil loses controllability and overshoots at part load. The fix is to size on the design drop, not to fit a larger actuator.
Traps
Starting a chiller before proving condenser flow, causing a high-pressure trip; setting a VAV static-pressure setpoint without a reset schedule; reporting airflow from a single pitot traverse point; confusing design acceptance testing with seasonal performance testing; leaving plant without an operating log, which licensed operation requires.