Glossary
Manual J
Manual J is the industry-standard procedure (published by ACCA, the air conditioning contractors' association) for calculating a home's cooling and heating load from its size, insulation, windows, orientation, air infiltration, occupants and local climate. Its result, in BTU per hour, is what defines the correct equipment size.
What Manual J is
It is a calculation, not a rule of thumb. The technician measures or takes from the plans every wall, window, door and ceiling; records the orientation, the insulation R-value, the glass type, exterior shade, how airtight the house is, and the number of people. With climate data for your city, the software calculates how many BTU per hour must be removed on the design day (one of the hottest of the year, not the all-time record).
Tonnage comes from that number. Then come Manual S (selecting the equipment that meets that load) and Manual D (designing the ducts). Together, the three are what separate an engineered installation from a catalog one.
Why it matters in Florida
The “so many square feet per ton” rule many contractors still use was created for homes with no insulation and single-pane windows. In a Florida home with decent insulation and impact windows, that rule oversizes. And oversizing here has a concrete cost: short cycles, moisture left in the air, mold, more kWh, and a compressor that dies early.
Florida’s code requires a load calculation for new equipment. Many contractors sign off on it without doing it. Ask for it.
Common mistake
Accepting a replacement quote that says “3.5 tons, same as the current one” with no Manual J attached. Ask for the full report (several pages, room by room) and check that your home’s data is real: current insulation, current windows. If they tell you “it is not necessary,” you already have your answer about that contractor. More in the right size air conditioner.
Full guide: Right-sizing an air conditioner in Florida: Manual J, tons and humidity →
Updated