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02-05-2010, 06:09 PM   #1
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## Split Air Returns

How is split air return calculated?

I have a 20"x25" air return that is connected to two 12" flex ducts. The flex ducts are squeezed between two floor joists making them more like 10"x13".

The information I have states the air handler is supplying 1100 cfm. To keep the 20"x25" filter efficient, I understand the velocity should not exceed 500 fps.

Since the return air is split between two ducts, how do you know the cfm and fps for each duct? Is the total cfm split in half and the fps remains constant?

I read that a 20" diameter duct at 1100 cfm gives a velocity of 504 fps. Splitting the 20" duct into two ducts does what to the cfm and fps?

Thanks.

02-05-2010, 09:18 PM   #2

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The air velocity through the air filter, and the duct are not always the same.

Is your return filter in a return air filter grille, or at the air handler/furnace?

As for our question of splitting a 20" into 2 ducts. What happens to CFM and FPM.
Depends on the size duct it was split into.

A 20" round duct has a cross section of 2.18 sq ft.
Splitting it into 2-14" round ducts. At the same total 1100 CFM, they would have a velocity of 514FPM.

BUT. The 20" would have had a friction ate of .019. The 2-14" would have a static pressure of .03".

Take your 20" example to a CFM of 1900CFM, and a FPM of 871.
Run 2-14" at the same CFM, and the FPM is 889.

But. The 20" had a friction rate of .055", the 2-14" runs have a friction rate of .08".

The smaller ducts have more surface area. So they are more resistive to air flow.

The higher the friction rate. The less the air filter can load up before needing changed.

 02-05-2010, 10:06 PM #3 Member   Join Date: Jan 2008 Location: N.C. Posts: 425 Rewards Points: 264 The filter is in a 20x25x1 grill/frame located in a wall, not in the air handler. The air filter box has two 12" flex ducts running 7 feet to the air handlers return box. The point to my question is, since the flex ducts are squeezed out of shape by the floor joists, could/should the ducts be reduced in size to 10" or 11" and changed to rigid pipe? A 10" or 11" pipe would easily fit between the joists and possibly provide a less restrictive path for the return air, providing they could provide enough CFM of return air.
 02-05-2010, 10:37 PM #4 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,584 Rewards Points: 6,132 They don't make 11" pipe. 2-10" pipes may be big enough, may not be. What size is your unit, or what CFM do you want to pull through 2-10" pipes?
 02-05-2010, 11:13 PM #5 Member   Join Date: Jan 2008 Location: N.C. Posts: 425 Rewards Points: 264 3 ton. I would like to see at least 500 CFM through each 10" pipe but I don't see that happening. Especially if I want to keep the 20x25 filter below 500 FPS. I may just have to except the installation as is since right now I don't see a way to make the space between the joists larger. I guess if I have to squeeze a pipe, squeezing a rigid pipe would be better than squeezing flex.
 02-05-2010, 11:21 PM #6 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,584 Rewards Points: 6,132 The velocity through the pipe won't effect the velocity through the air filter. If the wall frame was built out. Yep. Won't get 500 CFM through a 10" without increasing the static pressure above a level that your blower is going to be able to handle. Without making it that you need to change your air filter every 2 weeks. Can an additional return be added some where else?
02-06-2010, 08:29 AM   #7
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At this point, I'll see if there is a way to enlarge a small section between the joists to allow the flex ducts to expand to their correct diameter. If not possible I'll hunt for a location to add another return.

Thanks BT.

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