# Thread: How do I calculate return duct size?

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## How do I calculate return duct size?

I already learned from reading other posts my return grills are too small.

How do I calculate if the return ducts are too small, too?

I have a three ton downflow furnace a/c. I have two return ducts, hard metal round, one 14 inch and one 12 inch. How many square inches do I need to get the right airflow for 3 tons?

Both returns are above the ceiling, with lengths of 12 and 8 feet, two ninety bends, (one up, one down).

I want to find out if I need to tell whichever HVAC company I hire to replace my AC to just replace the return grills, or add another vent. I have not yet found anyone I trust to tell me what to do.

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14" equals 925 cfm @.08
12" equals 600 cfm " "
Total cfm 1,525
Needed for three ton: 1,200 cfm (min).
Now, what size are the grilles?

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## grill size

Thanks, Todd for the answer on the duct size.

Grills are 14 by 14 and 12 by 14. I am pretty sure they are too small.

But if the ducts are right sized, I can just have the grills replaced.

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Originally Posted by Maikerum
Thanks, Todd for the answer on the duct size.

Grills are 14 by 14 and 12 by 14. I am pretty sure they are too small.

But if the ducts are right sized, I can just have the grills replaced.
Hopefully they can be made larger. (2 cfm per sq. in.) 364 x 2= 728 cfm
Less than 2 ton of airflow.

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Originally Posted by Todd S 2
14" equals 925 cfm @.08
12" equals 600 cfm " "
Total cfm 1,525
Needed for three ton: 1,200 cfm (min).
Now, what size are the grilles?
That's fine if the friction rate is actually .08,which would be a guess without knowing.

However,chances are they are okay for three tons of air.

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Originally Posted by Maikerum
Thanks, Todd for the answer on the duct size.

Grills are 14 by 14 and 12 by 14. I am pretty sure they are too small.

But if the ducts are right sized, I can just have the grills replaced.
How are they connected to the ducts?

Aren't they noisey?

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Your return ducts are just right but the return grills are way to small. You have only 364 sq. inches. You need close to 600 sq. inches. We like to have 200/ton. This gets up down to around .05.

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## Thanks everyone

Thanks very much. I am glad my ducts are ok. Especially since I plan to install a smaller unit as a replacement, 2 or 2.5 ton. (I have significantly upped my insulation.) But still waiting on j-calcs.

I really want to replace my return grilles with a more standard size. Two 16 by 20's give me 640 sq. in., which should be plenty. I can't think of a downside to being oversized a little. Does not look like it will be too easy to do but should be doable. Just got to find an hvac guy who likes to bend metal and cut drywall.

They are in a hallway ceiling, and surprisingly, have not been too noisy. Not over the rumble of the blower motor, anyway.

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Not to burst your bubble, it's not the duct size but the length and how well installed the flex is that will determine if these 2 "hits are big enough. Is it pulled pretty tight or is it a loose solinky? Big differece in air flow and pressure drop. You need to take a monometer ( pressure meter) to see what the pressure readings are in the supply and return plenums to get an idea if the return is "choked"
Was this a concern because you weren't getting air in one or more of the outlets?

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Thanks, genduct for the input, but I think my bubble is still safe. It may have been in a side thread so you may have missed it, but I mentioned my returns are hard metal duct, not flex. I also gave an approximate length and # of bends.

I never had any problem with the system until my 21 year old ac gave up last week, but will be glad to enlarge my grills and hope to get slightly performance from it.

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Actually it was in the first post of this thead;

I have two return ducts, hard metal round

12. Are they just return grilles, or are they return air filter grilles?

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They are return air filter grills. I use 1 inch pleated filters in them, and they have always seemed small to me, and irritating because they are odd size anyway.

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