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 SW Washington a heat pump has to cover both the winter heating load and summer heat waves, and those two loads are rarely the same size, so the tonnage has to be chosen against both. An oversized system short cycles through the many mild days that make up most of a Pacific Northwest year.
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 |
|---|---|
| 1.5 tons | 18,000 BTU/h |
| 2 tons | 24,000 BTU/h |
| 2.5 tons | 30,000 BTU/h |
| 3 tons | 36,000 BTU/h |
| 3.5 tons | 42,000 BTU/h |
| 4 tons | 48,000 BTU/h |
| 5 tons | 60,000 BTU/h |
How many tons of heating and 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 winter and summer design temperatures. Square-footage rules of thumb ignore those factors and often lead to oversizing. Two homes of the same size in Vancouver or Portland can need different tonnage.
Why is an oversized system a problem in the Pacific Northwest?
Most heating and cooling hours here are mild, and an oversized system meets those small loads by switching on and off frequently. That short cycling causes temperature swings, adds wear and wastes energy on starts. Upsizing by a ton for the occasional heat wave trades comfort the rest of the year for a few extreme days.
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 small add-on 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 Washington or Oregon?
No. In the Southeast and Southwest regions the minimum for split air conditioners drops at 45,000 BTU/h, but in the North region, which includes WA and OR, split air conditioners must reach 13.4 SEER2 at every capacity. Split heat pumps must reach 14.3 SEER2 and 7.5 HSPF2 regardless of size.
About this definition
Written by the Home Energy Group team — a family-owned HVAC company celebrating 25 years of comfort & care across SW Washington and the Portland metro, licensed in Washington (WA LIC# HOMEEGI003J7) and Oregon (OR CCB# 184255). Definitions inform; an on-site diagnosis decides.
