“That building is huge! It must take a ton of cooling to keep it comfortable!” Jokes aside, when most people hear “tons” they likely think of weight. However, we size air conditioners in tons. Confusingly enough, this has nothing to do with the weight of the unit. Here is the reason air conditioners are measured in tons.
Before air conditioners, buildings were cooled using blocks of ice. A ton of cooling is 12,000 Btuh, the amount of heat needed to melt 2,000 pounds of ice over 24 hours.
How were buildings cooled before air conditioning?
I have two daughters. Therefore, I have seen the movie Frozen more times than I can count. However, the best part is that the very first scene is a pretty accurate representation of people harvesting ice. That is also how Kristoff makes a living. Along with his trusty reindeer friend Sven, of course.
Humans have been harvesting ice for centuries. During the winter, ice harvesters would be out on rivers cutting ice to store and use throughout the year.
First, ice dealers would work to maintain frozen rivers to ensure that the ice was clean and thick enough to harvest. Then, they would cut large blocks of ice using saws and picks. The chunks would either be pulled out of the water using ice tongs or would float down the river and gathered using a chute.
They used ice houses to store the blocks year-round. Businesses and homes bought chunks of ice to keep meat, fish, and dairy products from spoiling and to cool their homes during the summer months. They also used the ice to keep food from spoiling during transport.
The air conditioner is born
The year is 1902 and the printing presses in Brooklyn have a problem. In the summer time the paper would expand due to heat and humidity. In the winter the paper would shrink. This caused poor quality images and equipment malfunctions which cost the printing companies time and money.
Willis Haviland Carrier invented the modern air conditioner to regulate the temperature and humidity in the print houses. His invention was later used to heat and cool buildings, preserve food, and to produce ice.
Given that buildings were first cooled using volumes of ice, engineers adopted the ton as the standard measurement for the cooling capacity of air conditioners.
How big is a ton of ice?
Most people think of a ton as a large measurement. After all, when was the last time you lifted something that was 2,000 pounds?
The density of ice is roughly 57.37 pounds per cubic foot. Therefore, 2000 lb of ice would be:
2000 lb ÷ 57.37 lb/ft³ = 34.86 ft³
If you were to make a ton of ice into a cube the sides would be about 3 ft and 3 in long. That is roughly a cube with sides a meter long!
Why a ton of cooling is 12,000 BTUh
12,000 BTUh is cooling seems like an arbitrary number. Why not make a ton of cooling 10,000 BTUh and make sizing easier? To understand why 12,000 BTUh is the number for a ton of cooling let’s first discuss the process of ice melting.
Melting ice is an endothermic process. This means that ice absorbs heat as it changes state from solid to liquid. Water melts at 32°F (or 0°C for our friends across the pond). However, something interesting happens at the melting point.
Let’s say you have a 20°F chunk of ice. As you apply heat the temperature increases. When the ice hits 32°F the temperature stops increasing and the ice starts melting. You continue to add heat to the ice, but it stays at 32°F.
Once the ice fully melts the temperature starts to increase again.
At the melting point, all of the energy added goes into changing state from solid to liquid. None of the energy goes into increasing the temperature. The energy needed to change state from solid to liquid and vice versa is called the latent heat of fusion.
Now to the fun part: the calculation
There is some debate over the latent heat of fusion of ice. However, most scientists use 143 BTU/lb. Since a ton of ice is 2000lb we can determine how much energy we need to melt it.
143 BTU/lb × 2,000 lb = 286,000 BTU
That’s a lot of energy! However, we are not done yet. Remember that a ton of cooling is measured in BTU per hour (BTUh). Also, a ton of cooling is 2000 lb of ice melted over 24 hours. Therefore:
286,000 BTU ÷ 24 hr = 11,917 BTUh
Hold up! We said that a ton of cooling is 12,000 BTUh, not 11,917 BTUh! You are correct, but we round the number up to make it easier to work with.
How to find the tonnage of your AC
Modern air conditioners for residential applications generally come in set half ton sizes:
- 1.5 Ton: 18,000 BTUh
- 2 Ton: 24,000 BTUh
- 2.5 Ton: 30,000 BTUh
- 3 Ton: 36,000 BTUh
- 3.5 Ton: 42,000 BTUh
- 4 Ton: 48,000 BTUh
- 5 Ton: 60,000 BTUh
To figure out the size of your air conditioner we first must find the label with the product information. Head outside and check on the exterior of your condenser. The label is usually somewhere near the line set hookups.
You are looking for the model number.
Once you find the number you could look up the nomenclature sheet for the manufacturer. However, there is an easier way. Remember how air conditioners come in half-ton increments? Look for an 18, 24, 30, 36, 42, 48, or 60 somewhere in the model number. This number indicates the cooling capacity of your unit in KBTUh. Simply convert it to tons and you are done!
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Anthony Marini, PE is a Mechanical Engineer licensed in the state of Colorado. He graduated from Colorado School of Mines in 2012 and has been designing residential HVAC systems ever since. He has designed the mechanical systems for tens of thousands of homes all over the continental United States.
