How to Size an Air Conditioner Return Air Duct
- 1). Look for the data tag on the outside of your condenser (outside unit) to determine the size of your air conditioner (example: 3 ton).
- 2). Decide if you'll be using round or rectangular duct.
- 3). Calculate the return duct size based on the air conditioner's size and the type of duct you are using. Use the following formulas:
For rectangular duct:
Size of air conditioner (example: 4 tons) x 144 square inches = return air duct in square inches
For round duct:
Diameter of duct x 3.14 = return air duct in square inches - 4). Calculate the square root of the total. (Example: 4 (ton unit) x 144 square inches = 576 squared. The square root of 576 is 24. Therefore, your return air duct and grill size will be 24 by 24 inches.)
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