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Old 03-16-2014, 08:31 PM   #1
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How can I tell what size flex duct I currently have installed?


I looked all over both of my flexible HVAC return ducts. I found one that said about every foot what the 'R' values were. I couldn't find out the inside diameter.
I know one is 22 feet long, and am trying to calculate how many CFM it flows. I am replacing the return register, with a larger (calculated) size, but don't want to replace the flex tubing if it is adequate. Do I just cut a hole and stick a tape measure in it if nothing else appears?

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Old 03-16-2014, 08:36 PM   #2
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How can I tell what size flex duct I currently have installed?


Measuring the outer circumference and divide by 3.14 is as close as need be.

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Old 03-16-2014, 08:41 PM   #3
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How can I tell what size flex duct I currently have installed?


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Originally Posted by bdmeyer View Post
I looked all over both of my flexible HVAC return ducts. I found one that said about every foot what the 'R' values were. I couldn't find out the inside diameter.
I know one is 22 feet long, and am trying to calculate how many CFM it flows. I am replacing the return register, with a larger (calculated) size, but don't want to replace the flex tubing if it is adequate. Do I just cut a hole and stick a tape measure in it if nothing else appears?
flex duct...6" design air flow about 75 cfm
7" """"""""""""""""""""""""""'110 cfm
8""""""""""""""""""""""""""""'160 cfm
9" """""""""""""""""""""""225 cfm
10" """""""""""""""""""""""300 cfm


see if you can find where it connects at main trunk should have a connector you can measure...
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Old 03-16-2014, 09:24 PM   #4
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How can I tell what size flex duct I currently have installed?


Take off a register.
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Old 03-17-2014, 04:18 AM   #5
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How can I tell what size flex duct I currently have installed?


Yep, remove return grille and measure it.
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Old 03-17-2014, 06:19 AM   #6
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How can I tell what size flex duct I currently have installed?


I feel like a total maroon. Each of your answers were total common sense. I also appreciate the CFM chart. That is going to help me on determining the total cfm for both returns combined. Thank you again.
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Old 03-17-2014, 02:41 PM   #7
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How can I tell what size flex duct I currently have installed?


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Originally Posted by bdmeyer View Post
I feel like a total maroon. Each of your answers were total common sense. I also appreciate the CFM chart. That is going to help me on determining the total cfm for both returns combined. Thank you again.
Common sense is something we learn. Anything you have doubts about. just come back and ask. We'll help if we can.
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Old 03-19-2014, 08:49 PM   #8
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How can I tell what size flex duct I currently have installed?


I need to increase the cfm in my house by 600 or .6 ton. The easiest return to work on is in a FROG (Hot southern state)

Currently it is a 14 x 20. I think that is 280 sq " / 200 = 1.4 ton
If I change it to a 14 x 30 that gives me 420 sq " / 200 = 2.1 ton.

This will exceed my return requirement by .1 ton.
(I also have a second return that is a 20x20) which I would prefer not to mess with as it will be difficult.

So next I am looking at the flexible hose.
Currently it is 10" or 300/315 cfm.

Since I need to increase the airflow from the return grille (with filter) through the flex hose which is currently 10", I believe I can go to a 12" flex hose for 510 cfm.

I understand that there are many variable I may not be taking into account like resistance of the air through 22' of flex hose, and other things I just don't know.

Does anyone see anything wrong with my guesses here?
Are there any other tips I should consider before starting to shop for some flex hose and a new return grille?

Thank you folks,
-= Bruce D. Meyer
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Old 03-19-2014, 11:08 PM   #9
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How can I tell what size flex duct I currently have installed?


12" flex duct design air flow 480 cfm
14" """""""""""""""""""""""""""""''700 cfm
16" """"""""""""""""""""""""""""'1000 cfm
18" """"""""""""""""""""""""'''''1300 cfm
20" """"""""""""""""""""""""''''''1700 cfm
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Old 03-20-2014, 04:40 AM   #10
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How can I tell what size flex duct I currently have installed?


1 ton of air is generally considered 400 CFM. So to increase air flow by/for .6 ton would be 240 CFM.

A 14X20 grille(non filter) is generally okay for 525 CFM at 300 FPM.
A 14X30 grille(non filter) is generally okay for 787 CFM at 300 FPM.
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Old 03-20-2014, 07:31 PM   #11
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How can I tell what size flex duct I currently have installed?


Thank you. I am getting there. I went to a local HVAC supply, and he wanted to sell me 25' of 12" flex duct for $99, and a 14x30 return grill for filter for $120. I went to Home Depot online and found them for $36 and $63.

The only two things I think I have left to identify are the piece that attaches the flex duct to the return grill, and the piece that attaches the other end of the flex duct to the unit itself. I have searched for pictures, diagrams anything to show what these are and what they look like. All I have found are for sheet metal ducts. Can anyone one point me in the correct direction? I guess I should use some sort of mastick as opposed to the alumin tape. The tape in my attack has come apart in several places.

Thank you for all help with this. You folks are great.

-= Bruce
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Old 03-20-2014, 07:50 PM   #12
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How can I tell what size flex duct I currently have installed?


Probably just start collars. They make them to be installed in sheet metal or duct board.
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Old 03-20-2014, 08:22 PM   #13
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How can I tell what size flex duct I currently have installed?


Thank you. I found a start collar that I 'think is correct.
http://www.globalindustrial.com/p/hv...w-no-damper-12

I have two questions Sir:
Do I purchase one for each end of the flex hose?
Do I need to purchase Duct Board for the return grille filter item? I ma not clear what connects to the back of the return grille filter. I looked at one at the store, and the back had a large open rectangle. It looked like I needed a rectangle that popped in with a circular hole in it, that the collar would attach to. I guess. Also, I assume I'll need to replace the current 10" 'thing' that is on the air handler with a 12" c'thing' for the collar to join the flex hose with. What would these items be called, so I may shop for them?

Thank you,
-= Bruce
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Old 03-21-2014, 04:12 AM   #14
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How can I tell what size flex duct I currently have installed?


What is behind the current grille? Duct board, sheet metal, thermalpan? Sheet metal and thermalpan, you can use a start collar for sheetmetal. Duct board, only one for duct board.

You make up the box for the grille yourself, out of your preferred approved material.

If the air handler itself tapped, or is there a plenum that the 10" attaches to. Need to see what its actually attached to. And then get a start collar, or take off for it.

Take off, and collar and the names of the items you will need/ask for.

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