What a “ton” means
A ton is a measure of cooling, not weight: one ton removes 12,000 Btu of heat per hour. A 2-ton system is 24,000 Btu/h, a 3-ton is 36,000, and so on. Home systems are usually sold in half-ton steps, and the size is coded into the model number on the data plate.
Why square-foot rules miss
The Department of Energy warns against sizing estimates based on floor area or a contractor’s “experience.” Its own example: the same 2,000-square-foot house came out at 4 to 4.5 tons as conventionally built and 2 tons after insulation, window and duct upgrades. Same floor area, half the system.
Manual J: the load calculation
The method the DOE recommends is ACCA’s Manual J. It adds up the heat a house gains through walls, ceilings, windows and floors, with their insulation, plus air leakage, duct leakage, which way the house faces, roof color and how many people live there. Manual S then picks equipment to match that load, and Manual D sizes the ducts.
Why oversizing hurts in Houston
A bigger unit cools the air fast and shuts off before it has pulled much water out. The DOE says that in humid climates this short cycling can lead to poor humidity control, and that more starts and stops wear equipment faster. ENERGY STAR adds that a system too large won’t keep the house comfortable. The result is a house at 74°F that still feels clammy; the EPA says indoor humidity should stay below 60%.
Undersized has its own problem
A system too small runs all afternoon on the hottest days and still can’t reach the setpoint. Manual J is built to avoid both mistakes; the DOE notes ACCA methods already include enough safety margin for most homes, so padding the result “just in case” leads back to oversizing.
Ducts have to match
In slab homes across the South, ducts usually run through the attic. The DOE says that attic can reach 135°F at peak, and that a cooling system can lose 30 percent of its peak capacity to that heat gain alone. Ducts too small for the airflow, crushed or leaking, can make a right-sized system act undersized. That’s why Manual D, and a look at your ducts, belong in any sizing conversation; see duct repair.
One size boundary worth knowing
Federal minimums change at 45,000 Btu/h, just under 4 tons: below it, a new split AC in the Southeast must be at least 14.3 SEER2; at or above it, 13.8 SEER2.
Sources: U.S. DOE, Right-Size Heating and Cooling Equipment (PDF) · ENERGY STAR, Guide to Energy-Efficient Heating and Cooling (PDF) · ACCA Manual J · ACCA Manual S · ACCA Manual D · EPA, a brief guide to mold, moisture and your home · 10 CFR 430.32, DOE efficiency standards. Checked November 2026.
