Why is A/C Capacity Measured in Tons?

Originally published in June 2013, updated in September 2016
A/C capacity can be a confusing concept for many Charlotte, NC homeowners, and this is understandable because it uses an abstract unit of measurement. If you heard that your HVAC unit was 4 tons, what would you think? Would you assume that it meant your A/C weighed 4 tons? Most people might think this is the case, when in actuality it really represents the amount of heat your A/C can remove from your home in an hour. This is known as your A/C capacity.

So, if your A/C unit has a capacity of 4 tons, it can remove 48,000 British Thermal Units (BTUs) of heat from your home every hour. But how did the term tons become commonplace when referring to your A/C’s capacity?

A/C Capacity and Tonnage: A Brief History

Before there was A/C, people used ice to keep cool. As the ice absorbed the heat in the air, the temperature of the ice increased until it reached its melting point of 32 degrees F. The measurement used to track the amount of heat needed to melt the ice is know at BTUs.

Therefore, the amount of BTUs needed to melt a pound of ice is 143. If you have a ton of ice, you would take 286,000 BTUs to melt it. This just became the accepted terminology used in the air conditioning and heating industry over the years.

A/C Capacity and the Importance of Manual S Calculations

It is important to note that the capacity given in tons may not be the actual capacity of the unit. For this reason, a Manual S calculation and a load calculation are also used.

The Manual S calculation helps determine the actual A/C capacity by factoring in not just the indoor and outdoor temperature, but also the humidity in the air. If these calculations are done, then you may end up choosing an A/C unit that is not able to cool your home properly.

For more expert advice about measuring A/C capacity and other issues related to home comfort, please contact us at Ross & Witmer. We’ve been serving the Charlotte, North Carolina area since 1945.