Load Calculation, System Sizing, and Design Principles
What a load calculation includes
A load calculation, commonly Manual J for residential, totals sensible and latent gains and losses: conduction through walls, ceilings, floors, and windows (U times area times temperature difference), solar gain through glass (a function of orientation, shading, and solar heat gain coefficient), internal gains from people, lights, and appliances, and infiltration plus mechanical ventilation. Then equipment is selected with Manual S and ducts designed with Manual D. Arizona designs are dominated by attic duct gains, west and south glass, and a large latent load during monsoon season.
Design conditions and altitude
Use published outdoor design temperatures rather than peak records, and use indoor design of roughly 75°F dry bulb. Altitude matters across the state: airflow formulas assume standard air density, so at elevation the same cfm carries less heat, and gas equipment must be derated. Contractors who skip altitude corrections oversize or undersize equipment and misread airflow.
Sizing discipline
Oversizing is a defect, not a safety margin. Excess capacity shortens run times, which harms humidity control, increases cycling wear, and can create cold, drafty rooms. Undersizing loses design temperature on the hottest afternoon. Match the indoor coil and metering device to the outdoor unit, and confirm the selected equipment capacity at the actual entering conditions.
Rule of thumb and worked example
Rule of thumb: never size from square footage alone. Worked example: a 1,000 square foot ceiling with R-30 insulation (U about 0.033) at a 33°F design difference: 0.033 x 1,000 x 33 = about 1,089 Btu/h through that assembly. Add every assembly, then windows, infiltration, and internal gains together before selecting equipment.