Why Are Air Conditioners Measured In Tons?
As anyone who has ever installed an air conditioner can attest to, AC units can be bulky, awkward, and very heavy – but they certainly don’t weigh tons. Nevertheless, air conditioners are sold by the “ton,” and if you don’t know how many tons you need, you can end up with an undersized or oversized unit, both of which will leave your home uncomfortable and which may decrease the unit’s efficiency and lifespan, costing you hundreds of dollars over the life of the unit.
But if not a reference to weight, to what do these “tons” refer and how can you determine what size unit you need?
The History of the “Ton”
Air conditioners are sold by the “ton.” You may hear them referred to as “one-ton” or “five-ton” units, all the way up to 100-ton commercial behemoths. However, “ton” in such a context is not a reference to its weight, but to its cooling capacity.
This odd use of a measurement of weight harkens back to the days before air conditioners when ice was used to cool rooms. Big blocks of ice were sold off wagons by the pound and used to pull heat out of the air. When hot air would come into contact with the ice blocks, it would do exactly as physics demanded – it would transfer some of its energy to the ice, resulting in cooler air and slightly melted ice. The amount of energy needed to melt one pound of ice completely was approximately 144 British Thermal Units (BTUs), which meant that one ton of ice (2,000 lbs) would pull 288,000 BTUs out of the air.
To make life convenient, someone along the way decided to use one day (24 hours) as a standard measure of time during which the ice could melt, resulting in 12,000 BTU/hr. The convention stuck, and even today, engineers measure the ability of an air conditioner to cool a building by its equivalent in tons of ice melting over the course of a day.
In short, when talking about air conditioners, 1 ton means the unit can pull 12,000 BTUs of heat out of the air in an hour. A 2-ton unit can pull 24,000 BTUs of heat per hour, and so on. The more tons you have, the more cooling power you receive.
Is Bigger Better?
If larger cooling capacity means that more heat is removed from your home every hour, it logically follows that you can reach a comfortable indoor temperature faster the more capacity you have. But does that mean a larger air conditioner is better? Far from it.
While it is true that an oversized air conditioner will cool your house faster, it’s also a common cause of many other uncomfortable and expensive problems. Most of these problems originate with the unit turning on and off frequently, since it takes so little time to cool the room. These short but frequent bursts of AC are called “short cycles” and do not allow the unit to be active long enough for adequate dehumidification. When this occurs, you feel warmer at any given temperature than you would if the air was at a lower humidity. Not only does this cause many people to turn the thermostat down farther, using more energy, but high humidity levels can also contribute to mold growth, causing health problems and expensive damage to your home.
Short cycling also uses energy inefficiently, raising your electric bill, and can cause premature damage and wear to the equipment, resulting in more frequent maintenance and a shorter lifespan.
For these reasons, an oversized unit is not a good idea. It’s much better to choose a size hat’s suited to your home and which can keep you comfortable while maintaining a reasonable cycle length.
How to Determine Your Needs
There are a couple ways to determine the actual needs of your home. One way (assuming your last AC contractor installed the correct model) is to look at your existing air conditioner. Find the model number and look for a two or three-digit section containing one of the following numbers:
*Note: not all manufacturers construct their serial numbers the same way. When in doubt, contact a certified HVAC contractor
These numbers represent BTUs per hour. 018 means 18,000 BTUs per hour or 1.5 tons; 024 means 24,000 BTUs per hour or 2 tons, etc. Once you know what kind of unit you have, this can give you a baseline for what kind of unit you need.
However, oversized installs are common. The only way to be certain you buy the right size is to perform a complex equation (often using specialized software) taking in to account the square footage of your house, the number of windows and doors, the materials used to make your walls, windows, insulation, roof, and more. A quality HVAC contractor can visit your location and make this calculation for you.
If you’re looking for such a contractor, we can help! Our experienced crews are intimately acquainted with cooling capacity as it relates to the specific needs of your home.