# How to calculate size of air intake for air conditioner

We have replaced our air conditioner and it has a higher capacity than the old one. We were told to increase the size of our air intake, but I am not sure how they calculated the square footage (they told us to double the size). How do you calculate the proper size of the air intake? Any reference or formula would be greatly appreciated. Thanks!

 NorthernHeat Posts: 1,457, Reputation: 699 Ultra Member #2 Aug 6, 2007, 02:38 PM
Are you talking about a return air grill or the return trunk? You need 400 cfm per ton of cooling, there are many ductulators that will tell you what the size needs to be, based on the static pressure in the duct work. These formulas take into effect lengths, elbows and bends etc.
KidChaos (Apr 12, 2011 11:50 AM): answering incorrectly to question! non related answer!   Source:
 acetc Posts: 1,006, Reputation: 402 Ultra Member #3 Aug 6, 2007, 07:56 PM
There is more here to consider then just the size of the return , the size of the supply duct, size of the refrigerant lines and size of the electrical service to the new condensing unit, all these are just as important .When the old unit was installed every thing was sized to that installation and if not taken into account then this system may cause you some problems. Mike
KidChaos (Apr 12, 2011 11:53 AM): read the question! its a 7 " to a bathroom, thats fine ! its huge! extra 66 ft so what? what you want a 10or 12 inch to the bathroom?   Source:
 KidChaos Posts: 31, Reputation: 10 Junior Member #4 Jun 30, 2012, 09:05 PM
Quote:
 Originally Posted by LeslieK We have replaced our air conditioner and it has a higher capacity than the old one. We were told to increase the size of our air intake, but I am not sure how they calculated the square footage (they told us to double the size). How do you calculate the proper size of the air intake? Any reference or formula would be greatly appreciated. Thanks!
1st off, almost all existing returns were undersized for the existing equipment, and the newer stuff indeed uses much more air, in fact replacing equipment with out replacing the ducts is a wate of money and a regular efficincy unit will ssave you money, better to have R8 ducts all sealed up tightly with a 80% efficiency unit! existing ducting is junk generally, unless hard piped. Tract homes have sags and dips and crush points etc that and the R vqalues terrible, all the cans leak and supply grilles are open to the attic too! lol! there is alot more to it than just the equipment, nobody is doing you a service by ignoring the ducts and bigger the better smaller louder and never good!

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