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Don’t Be Confused by the A/C Capacity Measured in Tons — Here’s an Explanation

post img If you’re confused by the way A/C capacity is measured, you’re not alone. Though a system’s capacity is measured in tons, this isn’t a reference to the weight of your HVAC system.

Why do we rate A/C capacity in tons?

Back before air conditioning, people cooled buildings in the summer with ice harvested from lakes and rivers in the winter, and the crop of ice extracted from a river was measured in tons. Somewhere along the line, this term was institutionalized and remains in use today.

But we don’t use ice anymore, so how do you explain A/C capacity?

A 4-ton A/C unit doesn’t weigh 8,000 pounds. Though A/C capacity is measured in tons, this number is actually referring to how much heat the air conditioner can remove from your home in an hour. A 4-ton air conditioner can remove 48,000 British thermal units (BTUs) of heat per hour from your house.

Actual versus nominal capacity

There are two different ways to look at capacity.
  • Nominal capacity: To avoid confusion, fixed room and outdoor temperatures are used when rating A/C systems for the sake of uniformity. Using these specific test conditions, nominal capacity ratings are obtained. For instance, a typical 3-ton system has a nominal capacity of 36,000 BTU/hour.
  • Actual capacity: Actual indoor and outdoor temperatures will vary from the nominal testing conditions. Factors such as higher outdoor temperatures can result in lower actual capacity ratings when this same system is used in your home, lowering nominal ratings to an actual capacity of 2.8 tons.
What does this mean for me? The nominal and actual ratings are rarely the same, and since AHRI’s operating conditions often aren’t comparable to the conditions your A/C system is operating under, its capacity may be even lower than it’s rated. In addition, issues with ductwork, dirty air filters or coils, poor airflow and improper refrigerant charge can also effect capacity. Still confused about A/C capacity and how much you need to cool your home? Contact the experts at Apollo Home Heating, Cooling and Plumbing. Family-owned and operated since 1910, we offer the best customer service in the greater Cincinnati area. Image via
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