Light Commercial Air Conditioning: Rooftops, Three-Phase Power, Economizers
Rooftop units, three-phase power and economizers
Light commercial cooling adds scale and controls: packaged rooftop units, split systems with remote condensing units, two-stage or tandem compressors, belt-driven blowers, VAV boxes and economizers. Know three-phase basics: 208/230 V and 460 V systems, line versus control voltage, phase rotation and phase loss, contactors, overloads, and crankcase heaters.
Know economizer operation: on a call for cooling with outdoor air suitable (dry-bulb or enthalpy changeover), the damper opens and the compressor is staged off or modulated. A failed actuator, stuck damper or bad enthalpy control causes frozen coils, high energy use and comfort complaints. Know TESP, belt tension and sheave adjustment, blower performance tables, and the correct airflow for each refrigerant circuit.
Traps: starting a scroll compressor with reversed rotation (low amp draw, no pressure differential, loud noise). Verify phasing before startup. Also, ignoring single-phasing as a cause of compressor failure, diagnosing an economizer as a refrigerant problem, and charging one circuit using the other circuit's readings.
Worked example: a 10-ton rooftop with two circuits, each rated 5 tons, should move about 2,000 cfm per circuit at 400 cfm/ton. If TESP measures 0.9 in. wc. against a 0.5 in. wc. rating, correct the duct or filter restriction first; low airflow on both circuits is a distribution problem, not a charge problem.