# If it quacks like a duct...?

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• 02-07-2005, 11:41 PM
docholiday
Actually we answered at the same time, you just hit the enter button first. (both at 10:25).

I should claify an ealier comment, Flex can be used effectively as long as the added friction is considered. Just remember turbulance is part of the friction and as far as I know, there is just about nothing worse than flex for turbulance. To clarify, I meant that the 90 inside corner of an angle boot is probably less that 0.5% of the friction in those runs and probably not more than the resistance of the grille itself leaving 99% of the friction as a result of using flex.
• 02-07-2005, 11:50 PM
docholiday
Dallas, the 90 thing you may have read is primarily a trunk line thing.

If you have a rectangular duct and make a 90 degree turn and do not round the inside of the fitting, then rounding the outside of the fitting is virtually useless. What happens is the air along the inside of the turn maintains a straight heading and colides with the air being swept around the corner and basicly forms curtain of air that requires additional energy to overcome. By rounding the inside corner of a 90, you allow the air to flow more smoothly and evenly. The ideal 90 would have turning vanes installed.

But you are right in the sense that every fitting does have an equivelent length associated with it. I didnt look it up but for a single supply, I am not sure the differnece between a round 90 on a straight boot is equivlent to 40 feet less than a 90 boot. I'd almost bet the 90 boot isnt even worth that much to begin with.
• 02-08-2005, 12:09 AM
trane
Angle boot= 30 ft. duct
Straight Boot= 5 Ft. duct plus 90 Degree elbow= 10 For a total of 15 So its half.
• 02-08-2005, 12:42 AM
dallasbill
Thanks guys... you are great! I like a good discussion!

I won't worry about the angle boots! And the flex duct 'drag' itself was accounted for in design -- I'm just making sure that I get other opinions.

As others have stated "I woundn't do it thay way, but it looks OK from here" ...that's what I am looking for... reasoned opinions from others... and I thank you all~!

In the end, I can live with stuff IF I KNOW what other pros do or don't do.

And I will ask about the holes -- I think they probably look worse in a photo than they are and my GC is A-1 on not letting anything like that slip by if it will compromise this house quality. I walked through 3 other houses (not his) near here this past Sat. and they all have them in the 2nd floor subfloor for these large duct runs. Must be a Dallas thing.
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