Why ac rating in ton?

Before electric air conditioners existed, people used large blocks of ice to cool their homes and businesses. The ice absorbed heat and slowly melted as it warmed up. The use of tons to measure cooling capacity comes from this time. A ton refers to the amount of heat it takes to completely melt a ton of ice.

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 it’s not weight. It’s a number that tells how much heat the air conditioner can remove from the house in an hour. (Let’s ignore the issues of nominal vs. actual capacity and AHRI de-rating.) A 4 ton air conditioner is one that can remove 48,000 BTUs of heat per hour from the house. For most people, though, 4 tons means 8000 pounds. (A BTU is a British Thermal Unit, approximately the amount of heat you get from burning one kitchen match all the way down.)

Most pros also know how such a common term as ‘ton’ 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 the 19th century Ice and Refrigeration journal, 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.

When ice is below freezing and it absorbs heat, the 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.

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 on this one and remain skeptical.

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!

Related Articles

The Magic of Cold, Part 1 – How Your Air Conditioner Works

In the HVAC field, a ton, or tonnage, refers to the cooling capacity of an air conditioner. Tonnage is measured in BTUs or “British Thermal Units.” (A BTU is equivalent to the amount of energy (heat) needed to raise one pound of water 1°F at sea level.) One ton of air conditioning can remove 12,000 BTUs of heat each hour.

Tonnage is most important to consumers as it can be used to determine the size of air conditioning unit you will need to adequately cool your home or business.

When choosing a new air conditioner, you'll need to calculate the amount of tonnage your unit will need to adequately cool your property. You can estimate this by using a simple calculation:

To estimate your AC tonnage needs, multiply the number of square feet you’re cooling times 25. This equals the total number of BTUs you need to adequately cool your space. Next, divide that number by 12,000 to determine the tonnage capability you need in your new air conditioning unit.

This calculation will give you an estimate of your needs. There are many other factors that could impact your home air conditioner tonnage needs, including:

How well insulated is your home? Homes with exceptional insulation will hold cold air more effectively and be easier, less expensive and require less tonnage to cool.

Homes shaded by large trees will stay cooler than homes in wide-open spaces, in turn, requiring less tonnage.

Typically, it's more difficult to cool the second (or third) story of a home. Ranch-style homes are usually more affordable to cool. The fewer the floors, the fewer the tonnage needed to cool a home.

Commercial air conditioners are typically larger than those used in homes, but not always by a lot. A simple office or small store may not need much more cooling capacity than a large home, whereas a restaurant or manufacturer may require a much larger system.

Variables to consider for commercial AC tonnage include:

If you've answered “yes” to any of the above questions, you may need an air conditioner or system of air conditioners with a larger tonnage to cool your business.

Whether you're looking to better cool your home or replace an air conditioner at your place of business, an HVAC technician will be able to give you a much more accurate estimate of your AC tonnage needs.

Residential air conditioners are available in 1.5, 2, 2.5, 3, 3.5, 4, 4.5 and 5-ton units. If you have an exceptionally large home and need a larger unit based on the calculation detailed above, you may need multiple units installed at your home. It's not uncommon to install two central air conditioners or an air conditioner and a secondary cooling source in a large home.

Did you know that you can determine AC tonnage from model numbers on your unit? Here's how: