Manual J calculates a building’s heating and cooling loads for accurate HVAC system sizing. First, it analyzes factors like insulation, windows, and climate conditions. Then, it considers occupancy, appliances, and air infiltration to determine heat gain and loss. Additionally, it accounts for humidity, ensuring proper dehumidification and indoor comfort. Moreover, it prevents oversizing, which can cause inefficiency, temperature swings, and excessive wear on equipment. Without Manual J, systems may struggle to maintain consistent temperatures and energy efficiency. Ultimately, it lays the foundation for a well-designed, high-performance HVAC system.