Fiber board duct to metal equivalent questions

[WillysJeep]

Hi, first time post here. I'm trying to figure out if you measure the ID of the fiber duct to get the equivalent of metal ducting. If the fiber duct has an exterior of 8x18" and each side has approximately 1" of material that should equate to a 6x14" metal duct, right?

[BNME8EZ]

Hi, first time post here. I'm trying to figure out if you measure the ID of the fiber duct to get the equivalent of metal ducting. If the fiber duct has an exterior of 8x18" and each side has approximately 1" of material that should equate to a 6x14" metal duct, right?

6X16

[kdean1]

6X16

Math show-off.

[WillysJeep]

Thank you. I'm trying to figure out this quote I'm looking at and that part just struck me as odd.

[WillysJeep]

6X16

I did not know that there was a 6x16 available

[kdean1]

He was correcting your subtraction. 8x18 duct, subtract 2" from from each dimension becomes 8x16, not 8x14 as you stated in your first post.

Duct can be fabricated in any size you specify for a metal shop.

[kdean1]

I even messed it up.

Obviously it is 6x 14. Not 8x14.

[WillysJeep]

I even messed it up.

Obviously it is 6x 14. Not 8x14.

I figured as much. I did the post in a rush and wound up with a new math answer. I was trying to figure out if keeping the inside measurements of the existing fiber junk the same with new metal was kosher.

[kdean1]

The inside dimension is what determines how much air the duct can deliver. Outer insulation does not affect it but a duct liner on metal duct will. Duct board has a slightly higher friction rate compared to metal duct but a good contractor takes that into consideration.

[BNME8EZ]

I figured as much. I did the post in a rush and wound up with a new math answer. I was trying to figure out if keeping the inside measurements of the existing fiber junk the same with new metal was kosher.

If I understand what you are saying it would depend on how much air you are trying to move through it. If replacing fiber with metal the metal should move slightly more air but if the fiber was not sized correctly in the first place the metal will be wrong also.

[WillysJeep]

That's why I was scratching my head. I figured there's bound to be calculations to be run, and maybe he did them and I didn't know, but it's bound to be more than a simple one to one swap. Cool.

I'll hope the guy is on top of it. Thanks

[garyed]

Actually the 6x14 inside dimension would be the equivalent size for metal to a 6x16 inside ductboard dimension if you want the same friction rate.
http://www.oceanhvac.com/ductulator/