Manual J (ACCA residential load calculation) determines the actual heating and cooling capacity your building needs based on its specific insulation, windows, orientation, air leakage, and local outdoor design conditions. For Utah’s Wasatch Front: outdoor winter design temperature at -4°F in Sandy/SLC (far colder than national averages), outdoor summer design at 98°F, and altitude at 4,200 feet (requiring equipment derate of ~4% per 1,000 feet). IECC 2021 requires Manual J for new construction HVAC design. Oversized systems (from rule-of-thumb sizing) are short-cycle, can’t control humidity effectively, and cost more to operate throughout the system’s life. Manual J produces the calculated loads; Manual D sizes duct systems to deliver correct airflow room-by-room; Manual S selects equipment matched to calculated loads at Utah’s altitude.