A few years ago, a HERS rater student in a class I taught told a funny story. He was an HVAC contractor and said he was installing a new air conditioner for an elderly woman. As he was explaining things to her, he mentioned that they would be installing a 4-ton unit. “Oh, my,” she said. “How are you going to get something so big into my back yard?”

The confusion here is completely natural. HVAC and home energy pros find this story funny because when you say an air conditioner is 4 tons, we know we’re not talking about the weight of the equipment. It’s a number that tells how much heat the air conditioner can remove from the house in an hour. (Fro now, let’s ignore the issues of nominal vs. actual capacity and AHRI derating.) A 4-ton air conditioner is one that can remove 48,000 BTUs of heat per hour from the house. (A BTU is a British Thermal Unit, approximately the amount of heat you get from burning one kitchen match all the way down.) For most people, though, 4 tons means 8,000 pounds.

It’s cold enough to start harvesting, so get out your ice saw

Most pros also know how such a common term as “ton” has turned into a bit of HVAC jargon. Before Willis Carrier invented the modern air conditioner, people used to cool buildings in the summertime with ice harvested from rivers and lakes in the wintertime. A Green Homes America article quotes ice production figures from a 19th-century journal, Ice and Refrigeration, indicating that the 1890 crop from the Hudson River was about 4 million tons.

OK, so people used to cool and refrigerate with ice. How does that equate to air conditioning capacity in BTUs per hour, you ask? Well, let’s get quantitative and find out.

The latent heat of fusion

When ice is below freezing and it absorbs heat, its temperature increases. When ice is at its melting point, 32°F, and it absorbs heat, its temperature doesn’t change. Instead, it melts. If you’ve had a physics or chemistry class, you may recall that the amount of heat needed to melt ice is called the latent heat of fusion. In Imperial units, that number is 143 BTUs per pound.

That’s actually a lot of heat to pump into a pound frozen water. Once the ice is melted into liquid water, it takes only 1 BTU per pound to raise the temperature 1 degree. So if you’ve got a pound of ice at 32°F, you put 143 BTUs into it to melt it completely. Then it takes only 180 more BTUs to raise the temperature of that pound of water from 32°F to 212°F, the boiling point.

Anyway, getting back to our main discussion: if you have a ton of ice, it takes (143 BTU/lb) x (2000 lbs) = 286,000 BTUs to melt it completely. You could do that in one hour or 10 hours or a year, depending on how quickly you pump heat into it. Somewhere along the line, though, someone decided to use 1 day — 24 hours — as the standard time reference here. If the ice melts uniformly over the 24 hours, it absorbs heat at the rate of 286,000 / 24 hrs = 11,917 BTU/hr.

Rounding that number up makes it a nice, round 12,000 BTU/hr. In air conditioning jargon, then, a ton of AC capacity is equal to 12,000 BTU/hr. There it is.

We’ve been talking about “tons of cooling” for a century

If you’re wondering how this term got institutionalized, it was probably the usual way. People in the industry start using it, and then the professional organizations make it official.

An architecture website has a quote from 1912 that claims the American Society of Mechanical Engineers standardized it. It sounds likely, but their numbers don’t work out, so I’m gonna go with Honest Abe (see image below) on this one and remain skeptical (until someone in the comments shows me what’s wrong with my thinking anyway).

For the fearless: If you want to read some funny HVAC banter on this topic, check out this thread in the HVAC-Talk forum. And if you figure out what “heat of zaporization” is, let me know!

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.