Loss from reducing 8" flex to a 7" boot?

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  #1  
Old 07-17-15, 07:57 AM
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Loss from reducing 8" flex to a 7" boot?

1) General terms, how much CFM loss by reducing 8" flex duct to 7" connection at the register boot (12 x 6)? Quite a bit, or barely noticeable? Percent loss?

The duct connects to boot at about 60 - 70 deg angle (not full 90).

Replacing the old 7" duct with 8" to increase CFMs to a warmer room.

Wonder if replacing the boot w/ one for 8" duct is worth it? Can probably get the old boot out - w/o tearing up sheet rock.

2) And will a 14 x 6 insulated boot vs. 12 x 6 have much CFM increase?

Either size, I'll replace the restrictive register - 3 way, fixed fin (that's more for ceiling), w/ a sidewall 2 way, or one w/ straight, adjustable fins. It's on a wall, 8 ft off floor.

Thanks.
 
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Old 07-17-15, 09:00 AM
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8in flex will handle 200cfm 7in metal will handle 200 cfm so only at the transition will be the loss.
 
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Old 07-17-15, 09:40 AM
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Thanks. The loss is only at the angled section (transition) of the reducer.
True, but I wonder how much, ballpark?
But once the loss happens (at reducer transition), it doesn't "regain" it, just because 7" metal handles the same 200 CFM? Doesn't any loss become permanent?

If 15 - 20% CFM loss - out the wall boot - for 8" flex to 7" metal transition, neglecting the register (for the moment), I'd probably replace the boot.
If 5 - 8%, doubt I would.
 
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Old 07-17-15, 01:42 PM
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No it will not regain it. 7in flex is 150cfm.
 
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Old 07-17-15, 04:47 PM
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I don't think I wrote clearly. There's no more 7" flex involved. Going away.
I'm reducing (new) 8" flex, down to the old 7" diffuser boot (w/ 7"dia, 3" long, smooth connection pipe out the back of boot).

So I guess question remains, how much loss, just from the transition of 8x7 reducer?
The 7" smooth connector (~ 3" long) in the boot shouldn't create additional loss, per what you said? (that 7" smooth pipe & 8" flex have the same CFM rating).
 
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Old 07-17-15, 09:50 PM
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Any loss will be negligible.
 
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Old 07-18-15, 06:37 AM
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Velocity might increase some, but as long as static pressure is not too high I wouldn't worry about it.
 
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