Why is Air Conditioning Measured in Tons?

Tons of Ice

In the early days of cooling, ice harvested from lakes in the winter was stored and used as the medium to absorb the summer heat from the air. Blow a fan over ice, the heat of the air is absorbed by the ice (the cool of the ice is not added to the air). The ice melts as it absorbs the heat. Back in the day, a movie theater would order stored ice to be delivered daily. The theater would receive ice in a quantity to get them through to the next day. The business would order “tons” of ice to make through until they needed ice the next day. This means a 3 ton system in an average home would need 3 tons of ice per day to get the equivalent cooling of a modern air conditioner. A cubic foot of ice weighs about 57 pounds, so one just one ton of it is about 35 square feet of ice!

Heat BTUs

BTU (British Thermal Unit) is a measure of heat. Specifically, 1 BTU is the amount of heat required to raise the temperature of one pound of pure water one degree Fahrenheit (about 16 fluid ounces, since water is the bridging medium between volume and weight measurement: i.e. one fluid ounce of water = one weight ounce of water) . Heat is the equivalent of energy. Just like you can’t produce negative energy, you also can’t produce negative heat (one might call this concept cooling). Because of these limitations, we are left with our only possible method of cooling. AC systems had to be made in a way to push heat BTUs from the air inside, into the already hot air outside. We have to face the fact that if air conditioning was easy, we wouldn’t need all these complicated machines.

The Science of Cooling

Why is this all about heat? one might say, I want to cool my home! Well, science (currently) doesn’t allow you to add cooling, it only allows your to add heat. But, one might protest, it’s hot in my home, it’s really hot outside, where does the heat go? Air conditioners use an evaporation and condensing cycle to move heat (energy) from inside your home to the outside. Using the properties of refrigerant under pressure, an AC unit pumps refrigerant gas to reject heat to the outside air which then subcools the refrigerant to allow the indoor air passing over it to move it’s heat to the now cooler gas (superheat). Refrigerants are chosen by the ease at which they can be pressurized into a liquid and depressurized into a gas with a temperature change.

Convert HVAC Tons to BTUs

Tons is the weight of ice, a BTU is the amount of energy it takes to raise one pound of water by one degree Fahrenheit.