Why Is an Air Conditioner’s Capacity Measured in Tons?

When we visit our customers for AC repair in St. Louis, MO it often confuses them to hear us refer to their air conditioner’s capacity using the word “tons.” They’ll look at a three-ton unit and say in disbelief, “That thing really weighs three tons?”

If the word “ton” here referred to weight, a three-ton AC would be too big for your back yard. Luckily it is not referring to weight at all, but rather to a measurement of how much heat it can remove from your house.

Answering this question is going to be fun, because it requires taking a little bit of a peek back in time.

The answer

The first thing to know is that people used to use ice to cool homes and refrigerate foods, and the measurement of an AC unit’s heat-removing capacity reflects this. A three-ton AC removes heat at a rate that a three-ton block of ice would, if it removed heat evenly and melted completely across 24 hours. Confusing? Here’s a simpler way to think about it. A three-ton AC unit is like putting a 6,000-pound block of ice in your home every day. Just without the mess, of course.

It may seem strange that ACs are measured using such an abstract standard, but it’s not strange at all when you think about how the power of other modern machines are measured. Just think about a car’s horsepower. ACs are more or less measured in ice power. So with both machines, their power is explained in terms of the things they were replacing: blocks of ice for the AC, and horse-drawn carriages for the cars.

The math

Here’s how that translates into math. A one-ton block of ice melting evenly across 24 hours would absorb heat at a rate of about 12,000 BTU/hour—one BTU can be understood as being about the same amount of heat caused by burning one match all the way down. So if you have a three-ton unit, it removes about 36,000 BTUs of heat from your home per hour, and about 864,000 BTUs per day. The heat of one match might not seem like much—but now picture the heat of 864,000 matches. Your AC unit’s pretty amazing, isn’t it?

The bottom line

What would your ancestors—after returning home from the market on their horse-drawn carriage, chipping new ice for their rooms, hour after hour, just trying to stay bearably cool—say if they could step into your home and enjoy the effortless cool that your AC provides, knowing it does not require moving three tons of ice into the house, bit by bit, in order to enjoy that cool feeling? They’d probably say, “Are you kidding me?” and then beg you to switch places. What do you think?

We hope you enjoyed this little history lesson. We know we really enjoyed answering your question! If you’ve got more questions, give us a call here at Ashley Brothers Heating & Cooling. We’re experts in AC repair in St. Louis, MO, and we’ve got the knowledge and expertise to get the job done right.