Within an air conditioner a specific substance known as a refrigerant is changed between gaseous and liquid states by applying pressure changes within a sealed system. This process is called refrigeration. Refrigeration capacity is measured by how much heat can be moved within a given period of time. One unit of measure for this capacity is British thermal units (Btu) and another is tons of refrigeration (ton).
The Btu rating of all AC units or systems is stated on a label or data plate on the machine. It should be easy to find.
Once the Btu rating is known, it can be converted to tons of refrigeration.
Locate the outdoor part of the AC unit.
If it is a split system, with an air handler within the building and a condenser unit outside, the data plate or label displaying the Btu rating will be on the condenser unit, outside.
If it is a package unit, all in one housing, that will be outside.
It should be close to the an outside wall, in a painted metal housing. Unless it is water-cooled, it should have an obvious electric fan in it.
Locate the data plate or label, which will be about 3 inches by 4 inches in size, applied flatly to the surface of the housing.
It will be a metal tag, riveted to the outside of the housing, with information stamped onto it, or a stick-on label, with information printed on it.
Find the Btu rating. The label will state the manufacturer, model number and serial number. It will also state the voltage, amperage and wattage ratings of the AC, as well as the Btu rating. The Btu rating is the figure that you need.
Convert the Btu rating to tons of refrigeration.
One ton of refrigeration is equal to 12,000 Btu of cooling capacity. Therefore, the Btu rating divided by 12,000 equals the ton rating.
Formula: Btu ÷ 12,000 = tons
Example A: The label states that the cooling capacity is 36,000 Btu. Divide the 36,000 by 12,000. The answer is 3. The ton rating is 3 tons.
Example B: The label states that the cooling capacity is 18,000 Btu. 18,000 divided by 12,000 equals 1.5. The ton rating is 1.5 tons.