Size of Central Air Units

BTUs and Tons

• When you hear central air “size” mentioned, it actually has nothing to do with its physical dimensions. It has to do with its cooling power and capacity. This size is measured in BTUs and tons. BTUs, or British Thermal Units, are a standard amount of energy that is equal to the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. Once you have 12,000 BTU of energy you have one “ton” of energy. This is equivalent to the amount of energy needed to melt one ton of ice in one day. “BTU” and “ton” are the terms you are going to find relating to air-conditioner size.

Square Footage Rule

• The size of your house is the most important factor involved in sizing a central air-conditioning unit. Although there are other important factors involved that alter the size needs of your particular home, you can always start with square footage. Although opinions vary slightly, you can reliably go with 5,000 BTUs per hour to cool 100 to 150 square feet of space. Twice as many BTUs per hour will adequately cool 400 to 450 square feet and it will take 34,000 BTUs per hour to adequately cool a 2,000- to 2,500-square-foot house, according to the U.S. Environmental Protection Agency’s Energy Star website.

Other Size Factors

• Square footage might be the starting point, but it is unwise to base the size of your new air-conditioner on this sole factor. There are other factors that can affect your air-conditioning needs. The local climate, the age of your house and how well it is insulated, number of windows and even the amount of shade or direction the home faces can all affect the size of the central unit you need.

Too Big or Too Small

• The size of central air-conditioning units is important, because you can get undesired results from installing a unit that is too small or too large. If you go with a central unit that is too small for your home, it will not be able to keep up with the demands of your house. It will run too often and might not be able to get your entire home to the temperature you desire. This near constant running will also raise your utility bills considerably. If the unit is too big, it can cool the home too quickly without adequately removing the humidity from the air. This will make the house feel less comfortable.