Manual J is the residential load calculation methodology published by ACCA (Air Conditioning Contractors of America). It computes a home's heating and cooling loads (in BTU/hr) based on dozens of inputs: square footage, ceiling height, insulation R-values, window type and orientation, exposed wall and roof area, infiltration rate, internal heat gains, and the climate zone's design temperatures.
The output is the actual BTU/hr load your home demands — separately for heating and cooling. The HVAC equipment is then sized to match (or slightly exceed) those loads.
Rule-of-thumb sizing ("500 sq ft per ton") is essentially always wrong. Most older Charleston homes that have rule-of-thumb-sized equipment are oversized — which produces short cycles, poor humidity control, and shortened equipment life. A real Manual J takes 30-60 minutes of in-home measurement plus the software calc.
Any quality replacement quote includes a Manual J. If a contractor doesn't run one, they're guessing.
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