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The Reason Air Conditioner Capacity Is Measured in Tons

Posted on September 5, 2013

Interesting fact: Before our modern air conditioners, people harvested ice from lakes and rivers and used it for refrigeration. These “crops” of ice were measured in tons, which eventually became the vernacular for measuring today’s air conditioner capacity. 

Tonnage isn’t an air conditioner’s weight. Instead, it equals the heat, measured in British thermal units, that can be removed in an hour’s time. When a pound of ice has absorbed enough heat to reach 32 degrees, it begins melting. The amount of heat required to reach the melting point is 143 BTUs per pound. But once it’s completely melted, a single BTU per pound will raise the temperature an entire degree. In other words, although it takes 143 BTUs to raise a pound of ice to the melting point, it takes 180 BTUs thereafter to heat it to the boiling point.

Therefore, if you multiply the 143 BTUs required to melt a pound of ice by 2,000 pounds (one ton), you’ll find that you need 286,000 BTUs to cool (absorb heat from) one ton of air. Divided by 24 hours, the cooling load for this same ton of air is 11,917 BTUs per hour.

The air conditioner industry has rounded this figure up to 12,000 BTUs. Therefore, a one-ton air conditioner can remove 12,000 BTUs of heat per hour, a two-ton unit can remove 24,000 per hour, etc.

Tonnage estimates are often slightly off, although an expert HVAC technician can account for the differences by doing a Manual S calculation. Extenuating factors can include:

  • Indoor and/or outdoor operating conditions vs. actual conditions.
  • Inadequate ductwork.
  • Improperly charged refrigerant.
  • A dirty air filter.
  • Dirty coils or poor airflow through the condensing coil.
  • Actual vs. nominal capacity.

Have questions about air conditioner capacity? Contact the professionals at Comfort Systems in Wichita. We can supply you with all the answers, and if you’re in the market for a new air conditioner, we can help you find the system that’s ideally suited to your specific requirements.

Our goal is to help educate our customers in Wichita, Kansas about energy and home comfort issues (specific to HVAC systems).

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