How to Figure Tonnage for AC Units
The capacity of an air conditioning unit can be measured in British thermal units or tons. One Btu is the amount of energy needed to raise 1 pound of water 1 degree Fahrenheit. One ton is equal to 12,000 Btu. The term "ton" is used because 12,000 Btu is the amount of energy needed to melt 1 ton of ice. The tonnage needed for a central air conditioner is determined by the size of the home, the exposure to sunlight or lack thereof, and the usage of the space. Does this Spark an idea?
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Instructions
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1
Determine the square footage of the home. If you do not have a floor plan that gives this information, you can measure the length and width of each room, multiply the length times the width of each room to find the area, and add the areas of each room together to get the total square footage.
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2
Use an AC size chart to approximate the size of the AC unit that you need. These charts can be found online (see Resources). For example, a 1,000 square foot home would need approximately 19,500 Btu.
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3
Modify your estimate to reflect the amount of sunlight your rooms receive. For each room that is very sunny or very shady, calculate Btu requirements by using a room sizing chart (see Resources). Add 10 percent to this value if the room is very sunny and subtract 10 percent if the room is very shady. For example, if a room is very sunny and has a square footage of 125, it would need 5,000 Btu and you would add an additional 500 Btu because of the sunlight.
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4
Add to your estimate from step two for the purpose of the rooms. For each kitchen, add 4,000 Btu to the approximation in step two. For each room with three or more people living in it, add 600 Btu for each person beyond the second.
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5
Add 4,000 Btu to the estimate in step two for each room that is not insulated.
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6
Divide the total Btu capacity found from steps two through five by 12,000 to convert the number to tons. For example, if your total was 30,000 Btu, you would need a 2.5 ton AC unit.
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