Air Conditioning and Cooling Equipment
Equipment and ratings
Split systems pair an outdoor condensing unit with an indoor coil; packaged units contain everything in one cabinet. Know SEER2 (seasonal efficiency), EER2 (peak-load efficiency), and that ratings assume matched components and specific airflow. A mismatched coil or undersized duct destroys rated capacity and efficiency.
Sensible and latent capacity
Total capacity splits into sensible heat (temperature change) and latent heat (moisture removal). Sensible heat ratio (SHR) falls as the coil gets wetter and the entering air becomes more humid. Trap: an oversized unit short-cycles, removes less moisture, and leaves the space cold but clammy. Evaporator freeze-up points to low airflow, low charge, or a restriction - not simply a bad thermostat.
Charging and airflow
Nominal airflow is about 400 CFM per ton (roughly 350 CFM per ton in humid climates). Verify airflow before charging: a system with low airflow cannot be charged correctly by superheat or subcooling. For TXV systems, set charge by subcooling; approach method (condenser saturation temperature minus outdoor ambient) is a useful cross-check. Condensate must drain by gravity with a properly trapped line; a missing trap on a draw-through coil pulls air and water.
Traps to watch
Do not judge charge from suction pressure alone. Do not confuse coil temperature drop with refrigerant superheat. Remember that a dirty condenser raises head pressure and discharge temperature and reduces capacity - it will not lower suction pressure the way a low charge does.
Worked example
A 3-ton system should move roughly 1,200 CFM. Measured external static of 0.9 in. w.c. against a 0.5 in. w.c. rating means the blower cannot deliver that airflow, and every charge reading downstream is suspect.