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There are two ratings used for air conditioners in the US. Air conditioners are rated in either BTUs or Tons. Here is some information on what these ratings mean and how you can use them to make a reasonable decision on a choice of cooling system for home, office, or business.

BTU, which stands for British Thermal Units, is a measure of the amount of heat it takes to change the temperature of 1 pound of water one degree Fahrenheit. To give some idea of what that means, it would take 180 BTUs to take 1 pound of water from its freezing point at 32 to its boiling point of 212 degrees Fahrenheit. Every substance has a quantity of heat it takes to change its temperature one degree. This amount of BTUs is called the specific heat of the substance. Water has one of the highest specific heats of regular substances. Air, for example, requires about 1/4 BTU per pound to change its temperature one degree Fahrenheit; therefore it has a specific heat of about 1/4. Combine this with the fact that a cubic foot of air weighs about 0.08 pounds, and you can calculate the amount of heat needed to be removed to cool a specific space.

Take any room 20 ft. by 20 ft. with a 10-ft. high ceiling. This room takes up 4000 cubic ft. (20 x 20 x 10), or about 320 pounds of air. For each degree of change, you would need about 80 BTUs. Now remember that an air conditioner in order to be useful would have to be able to cool a room as much as 35 to 40 degrees quickly. It is not unusual to want to cool a room from 115 to 75 degrees in less than a third of an hour (20 minutes). This is where the BTU rating of air conditioners comes into play.

Here is an example of the air conditioning process regarding BTUs: the temperature outside is 105, and the room needs to be cooled to 75 degrees. The heat needed to be removed would be 80 BTUs times 30 degrees or 2,400 BTUs. This is not all, however; remember that heat from outside is flowing in to warm the room, and some warmer outside air is coming in and replacing the air inside the room. It's not unlikely to have an air conditioner designed to handle two complete replacements of air an hour in a room, or a total of three rooms of air an hour or a requirement of 3 X 2,400 BTUs or 7,200 BTUs. Air Conditioners are rated by the number of BTUs they can remove per hour of constant operation; in this example, a 15,000-BTU window air conditioner would take about 1/2 hour to cool the room and be able to maintain that cool with constant operation comfortably if necessary due to constant air replacement. Of course, this air conditioner (15,000 BTU unit) would easily cool a well-insulated room up to 8,000 cubic feet, one without a high rate of air replacement, in 1/2 hour. At the same time, this maintains the cool of the room without constant running.

Have a bigger room, want faster cooling, have a bigger need due to air replacement? Remember that how much you expect to have to lower the temperature, how fast, and the rate of air replacement all go into your rating need. For a room with a slow replace rate, a figure of about 2,000 BTUs per 1,000 cubic feet of space is a good rule of thumb estimate. 4,000 BTUs per 1,000 cubic feet would be used for a room with a lot of air replacement or poorer insulation. The larger figure would apply for most business and light industrial uses.

Air conditioners are sometimes rated in TONS. This rating system is used more in industrial and business uses. It is not always consistent, and BTUs are often used as a frame of reference. TONS are convertible to BTUs; the rating in TON is easily gotten by dividing the BTU rating of the air conditioner by 12,000. For example the 15,000 BTU unit used in this example would be a 1 1/4 TON unit.

Before you purchase an air conditioner, consider your room size, the traffic you expect, the insulation of the room, and the maximum number of degrees you expect to have to lower your room's temperature. There are other factors, but you should figure between 2,000-4,000 BTUs rating (1/6 to 1/3 TON rating) per 1,000 cubic feet for cooling. In locations where the temperatures consistently require over 35 degrees cooling, multiply these rating figures by 1 1/3.