Selecting Air Conditioner Size
There is only one correctly sized air conditioner for your home. You don’t want a unit that is too big, as it will turn on and off too frequently, be inefficient and improperly dehumidify the home. And you don’t want a unit that is too small since it will run too often, and on the hottest days of the year will not be able to cool the home sufficiently. You want just the right size. How do you determine which one is just right? There are several ways you might go about determining the correct air conditioning cooling capacity (or size) unit:
The cooling capacity (also referred to as size) of an air conditioner is measured in Tons or BTUs -- both are used interchangeably. BTU stands for British Thermal Unit, and Ton, although it sounds like a reference to weight, refers to the air conditioner's cooling capacity.
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There are 7 sizes of air conditioners available:
1.5 ton (18,000 BTU)
2 ton (24,000 BTU)
2.5 ton (30,000 BTU)
3 ton (36,000 BTU)
3.5 ton (42,000 BTU)
4 ton (48,000 BTU)
5 ton (60,000 BTU) |
If you selected a self-contained unit with gas heat and electric air conditioning, you will also need to determine the size of furnace you will require. Click here for more information on selecting furnace size.
The following sizing methods are available:
| Method 1: Manual J Load Calculation – This is the proper and scientific method for calculating AC size - it is also the most accurate. It is the method taught to HVAC technicians and recommended for use by professionals in the trade. It consists of taking information about your home’s constructions materials, insulation levels, number of windows, sizes of rooms, etc, and then makes a calculation based on those to determine the appropriate heating and cooling requirements needed.
You can purchase a copy of Manual J software for download from a software provider for $49 at this link if you wish to use this method.
It should be noted, that while this is the most accurate approach for determining cooling capacity, according to a study by the US Department of Energy, less than half of heating and air conditioning contractors actually use Manual J Calculations to determine the proper size for heating or cooling equipment.
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| Method 2: Use the Sizing Estimator. This is an online tool that will give you a rough estimate. It won’t be exact, but when used in connection with other information it can provide a fairly close approximation. |
Method 3: Compare your home to similar homes in your area. Does your neighbor have the same size home as you? If he has a properly sized air conditioner, then the same size unit may also work for you.
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| Method 4: Ask a contractor. Most air conditioning professionals give free in-home estimates for installing new air conditioning, during which they will recommend a unit size. While we don’t encourage using contractors for quotes if you don’t intend to hire them, a contractor familiar with the homes in your neighborhood will likely be able to give you an idea over the phone of what size you might need. |
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Method 5: If you are replacing an existing air conditioner, look at what size you have. If the unit you have now is the correct size for your home, replace it with the same size. How do you know what size you currently have? Look at the name plate on the condensing unit in the yard and locate the model number (not serial number). You are looking for 2 digits in the model number that match the numbers below to indicate Tons or BTUs.
18 = 1.5 ton (18,000 BTU)
24 = 2 ton (24,000 BTU)
30 = 2.5 ton (30,000 BTU)
36 = 3 ton (36,000 BTU)
42 = 3.5 ton (42,000 BTU)
48 = 4 ton (48,000 BTU)
60 = 5 ton (60,000 BTU)
For example: A model CKL24-1 is a 2 Ton (or 24,000 BTU) sized air conditioner.
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For more information about sizing air conditioning, check out these online articles:
Home Energy Magazine
Healthgoods.com
Energy Star / EPA
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