Heating, Ventilating, and HVAC Systems
Design sequence
HVAC design starts with an ACCA Manual J load calculation, then Manual S equipment selection, then Manual D duct design. Use actual envelope, orientation, windows, and infiltration-not square-footage rules of thumb. Oregon energy code provisions reinforce sizing and duct sealing. Select equipment that matches the calculated load, not the old unit's size.
Airflow and charge
Total external static pressure must stay within the manufacturer's rating; high static reduces airflow, raises supply temperature, and can freeze coils. Verify temperature rise for furnaces and superheat/subcooling for air conditioners. Duct leakage and kinked flex duct are common airflow killers. Record final readings on the startup sheet.
Worked example
R-410A system, suction pressure 120 psig, saturation about 41°F, suction line temperature 55°F. Superheat = 55 − 41 = 14°F, within a typical 10–15°F target. If it were 3°F, look for overcharge or an overfeeding metering device; if 25°F, check for undercharge or a restriction.
Traps
Sizing by square footage; charging by “beer can cold” suction line; ignoring static pressure; and failing to record final readings.