Sizing return vent in floor?


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Old 02-26-07, 09:44 AM
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Sizing return vent in floor?

If, at the smallest part, I had a return that was 8" round duct, what are the calculations for determing how large the return vent needs to be in the floor so as to not choke the air down (and prevent that whistling I have heard about)? Would 6x10 work, 6x12 or bigger?! The vent covering will not be the original egg crate type as well, but the louvered type if that makes any difference. Thanks for the info!

Nick
 
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Old 02-26-07, 01:02 PM
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Not sure about the calculations but you will for sure need at least a 12"X10" floor return grille with 8" duct. It is not advisable to use a louvered return grille on the floor. Here is a site that has a varying number of floor return grilles to meet your needs when size is determined.

http://heatac.com/index.php?cPath=23_36&osCsid=2acf39264de81eda1000760f6ed6f99a
 
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Old 02-28-07, 09:31 AM
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Thats odd. I asked a contractor and he said just make sure it has the same (preferabbly bigger) free area and you will be fine. The area of a 8" pipe is about 50 inches (pie multiplied by the radius squared) so technically a 6x10 or a 6x12 vent grille would work fine. And he said you could use louvered but you would just want to make sure it had a bigger free area (like 6x12) because it might choke down air a little more then then the egg crate open style grille.

So, i'm a little confused because i am hearing different things, what do other guys think?
 
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Old 02-28-07, 10:10 AM
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I would say that in the very least, return air should match the supply air in calculations. In other words the amount of air supplied to a room should have an equal amount of return air going back to the air handler.
 
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Old 03-01-07, 04:29 AM
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I agree with mjb, regarding matching return to supply. I have never seen a single 8" return for an entire system. Usually there are several 8"ers collecting at a return plenum. Do you have more than one return for the system? And don't try to exactly match the area size of the pipe with your grate. You can have a larger collection chamber, but not smaller than the area of an 8" pipe.
 
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Old 03-01-07, 07:38 AM
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Originally Posted by chandler View Post
I agree with mjb, regarding matching return to supply. I have never seen a single 8" return for an entire system. Usually there are several 8"ers collecting at a return plenum. Do you have more than one return for the system? And don't try to exactly match the area size of the pipe with your grate. You can have a larger collection chamber, but not smaller than the area of an 8" pipe.
There are going to be multiple 8" return runs (collecting at the main return trunk) to match the return cfm to the supply cfm as closely as possible but I am just wondering on each individual run how big the grille opening should be. Judging by your comments about the collection chamber, a 6x10 or 6x12 grille would work because the area of them is larger then the area of an 8" pipe, correct?

Nick
 
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Old 03-01-07, 11:54 AM
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Try this approach. determine how many supply registers there are in the room. Lets say you have two 4"x12" supply registers, that would equal 96 sq" supply to that room. You would at least need a minimum 10'x10"(100sq") floor return grille connected to the 8" duct.
 
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Old 03-01-07, 12:09 PM
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Wink

Can also work by the book. A 8" pipe is 200 cfm .Should have a 8"X14" grill.
1 ton of AC should have 400cfm. So you need 2 --8" return pipes for a ton of AC.

 
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Old 03-01-07, 02:56 PM
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Then we agree that a 6"x12" floor grille attached to 8" return duct is to small for his application. Then again we have not received information as it relates to the number of supply registers to the room.
 
 

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