"Tonnage" is HVAC slang for cooling capacity expressed in tons. One ton = 12,000 BTU/hr of cooling. Residential systems typically range from 1.5 tons (small condo) to 5 tons (large home), with 2.5-4 ton being most common in Charleston.
The term comes from the cooling required to melt one ton of ice in 24 hours (a quaint vestige of pre-refrigeration thinking). It's still used because round numbers are easy.
Critical: tonnage should be determined by Manual J load calculation, not by square footage rules of thumb. "500 square feet per ton" is a guess that's usually wrong — a 2,000 sq ft home in Charleston might actually need 3 tons, not 4, depending on insulation, windows, orientation, and shade. Oversizing is the #1 cause of comfort complaints in our humid climate.
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