Definition
Tonnage is the cooling capacity of an air conditioner or heat pump, where one ton equals 12,000 BTU/h (British thermal units per hour) of heat moved. Residential systems commonly range from 1.5 to 5 tons. Tonnage measures capacity, not efficiency, and the right size for a home comes from a Manual J load calculation.
Also called: Tons, BTU/h, Capacity
Why it matters to you.
In Central Kentucky's humid summers, an oversized air conditioner cools the house fast and shuts off before it has removed much moisture, leaving rooms cool but clammy. Tonnage also sets which Kentucky SEER2 minimum applies: 14.3 below 45,000 BTU/h and 13.8 at or above for split air conditioners.
Practical guidance.
Ask every bidder how they arrived at the tonnage on their quote; "same as the old unit" or a square-foot rule of thumb is not a load calculation. For a heat pump, also ask what heating capacity the unit delivers at your area's winter design temperature, not just its nominal tons.
How Tonnage (BTU/h) options compare.
| Nominal tonnage | Nominal cooling capacity | Kentucky split-AC SEER2 minimum |
|---|---|---|
| 1.5 tons | 18,000 BTU/h | 14.3 |
| 2 tons | 24,000 BTU/h | 14.3 |
| 2.5 tons | 30,000 BTU/h | 14.3 |
| 3 tons | 36,000 BTU/h | 14.3 |
| 3.5 tons | 42,000 BTU/h | 14.3 |
| 4 tons | 48,000 BTU/h | 13.8 |
| 5 tons | 60,000 BTU/h | 13.8 |
How many tons of cooling does my house need?
The answer comes from a Manual J load calculation, which accounts for insulation, windows, orientation, air leakage, duct location and local summer and winter design conditions, including humidity. Square-footage rules of thumb ignore those factors and often lead to oversizing. Two same-size homes in Danville or Harrodsburg can need different tonnage.
Why is an oversized system a problem in Kentucky?
Much of a Kentucky summer's cooling load is moisture, and an air conditioner removes moisture only while it runs long enough for water to condense on the coil. An oversized unit satisfies the thermostat quickly and shuts off, so humidity stays high. The short cycling also causes temperature swings and adds wear from frequent starts.
Should I replace my system with the same tonnage as the old one?
Not automatically. Copying the old unit's size repeats any original sizing mistake, and new windows, insulation or duct sealing may have changed the load. Switching from a furnace with a separate AC to a heat pump also changes the question, because the heat pump must be sized for heating too.
Does tonnage change the SEER2 minimum in Kentucky?
Yes, for split air conditioners. In the DOE Southeast region, which includes Kentucky, a split AC below 45,000 BTU/h of certified cooling capacity must reach 14.3 SEER2, and one at 45,000 BTU/h or more must reach 13.8 SEER2. Split heat pumps must reach 14.3 SEER2 and 7.5 HSPF2 regardless of size.
About this definition
Written by the Feistritzer Heating & Air Conditioning team — family owned and operated in Danville since 1976, Kentucky HVAC license M02584. Definitions inform; a whole-system diagnosis in your home decides.