Load Calculations and System Sizing
Loads, Design Conditions, and Equipment Match
Sizing begins with an ACCA Manual J (eighth edition) room-by-room load calculation using Florida design conditions, typically an outdoor summer design dry bulb near 91 to 93 degrees Fahrenheit, with indoor conditions of about 75 degrees dry bulb and 50 percent relative humidity. Load components are transmission through envelope surfaces, infiltration, internal gains from people, lighting, and appliances, duct gains or losses, and ventilation air. Because Florida is latent-load dominant, pay attention to the sensible heat ratio; a system sized only for sensible load will run cold and clammy.
Manual S then selects equipment using the manufacturer's capacity at the actual design conditions, not the nominal tonnage on the box. Confirm the indoor and outdoor units are an AHRI-rated match and that both total and sensible capacities meet the load. Oversizing causes short cycling, poor dehumidification, humidity complaints, and premature compressor failure; undersizing causes long runtimes and loss of setpoint on design days. The old 400 to 500 square feet per ton shortcut is not an approved method and appears in questions as a tempting wrong answer.
Worked example: a calculated total load of 31,500 BTU/h divided by 12,000 BTU/h per ton equals 2.63 tons, so the next nominal size is 3 tons. Before accepting it, verify the AHRI match produces at least 31,500 BTU/h total and covers the sensible load at design conditions, and confirm airflow of roughly 400 cfm per ton, about 1,200 cfm here.