Low airflow, replace flex duct?

We have a one-story house, about 1750 sq ft. The whole house gets good AC flow except one of the two small bedrooms. They are right next to each other. Up in the attic, the cool bedroom has 6 inch flex ducting from the main unit, while the warm room has only 5 inch flex ducting.
The difference in airflow (one seems like a fan is on, the other you have to kind of concentrate to feel the cold air) is substantial and I can't believe it's just from an inch difference in the ducting diameter. I've checked the ducting for kinks and holes, but it seems pretty solid. My only thought is that it's clogged or blocked somewhere. The vent in the room is firmly painted to the ceiling and while I may end up doing so, I'd rather not rip it off just so I can see a foot into the 10 foot section of ducting.
I'm considering just buying some new flex ducting and replacing it. I haven't checked cost, but doing it myself would undoubtedly be hundreds of dollars cheaper than paying an AC technician. My concern is with sealing it to the metal round ducts. I've heard metal duct tape is good, but that there are also special sealants to use as well.
Thoughts on this fairly minor AC project?
Lymond
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The big thing with flex is to make sure it runs straight. It actually performs fairly well if there are not to many bends and curves in it.
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Not saying you don't have a problem but want to comment on your:

The difference in area between a 5" circle and a 6" circle is 44% more!
2 Area of a circle is Pi(radius) Pi=3.14159
3.14x5x5y sq in. 3.14x6x6-113 sq in
(113-79)/79*100D%
BTW, is the r-value the same on both pcs of ducting? I've seen cheap black flex that is like R2/3/maybe 4. The last quality residential flex I saw put in was insulated and foil covered (I think) but it had R6 stamped right on it. Could be a partial contributor.

5" duct from where? Not from the main duct? Has it been tapped from a duct going to another room. That would explain it. Air volume from that other room is being stolen and divided. You would not be getting a lot less air volume than if that 5" was connected to the main trunk.

Depends. I've seen cheap foil tape at WalMart that is some of the stickiest tape I've ever used...but...you are using it on duct work which means your local building code determines what is OK to use. For instance, they might require duct tape to be UL-723 fire rated. Sure you can put anything you want on it. Lets say you go to sell the place in 5 yrs and soem home inspector is trying to justify his services/fees by finding something that is broke or doesn't meet code. You really don't wanna go up there and pull off that sticky-ass WallyWorld tape.
All inspection depts have hours to take calls with questions even from homeowners. Part of their job.
For the record, I'm not an HVAC guy.
As far as the math part, that I guarantee but have never proven Fermat's Last Theorem of x**2 + y**2 = z**2. Left that one for Andrew Wiles :-)
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Al, you said you guarantee your math, but you are wrong. A 5" and 6" SQUARE are 25 and 36 square inches respectively, so your numbers are way off. The numbers you came up with are for 10" and 12". The RADIUS of the 5" will be 2.5", and the 6 =3". We actually had a guy a work that said two 9" ducts would equal one 18". That was about 5 years ago and we still laugh about it. BTW, an easy way to figure area of a circle is that it is about 21% less than that of a square the same size as the diameter of the circle, whether it is 1 inch or 1 mile. Larry
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lp13-30 wrote:

Hi, When he is talking about diameter, you're talking about square? Amazing! I am thinking of Pi, LOL.
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I love apple crisp :-)
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snipped-for-privacy@webtv.net (lp13-30) wrote in 3153.bay.webtv.net:

Duhhh!! Boy did I blow that part mixing up radius with diameter...well almost. Gotta stop posting so late. Yes you are right.
The percentage difference is still correct all being a function of pi which is what I was trying to get across. <== Note, Al is saving face here :-)

And the difference between a 10" and 12" is also 44% !!!
Area of a circle is Pi(radius)(radius) Pi=3.14159
3.14 x2.5 x 2.5 = 19.6 sq in. 3.14 x 3 x 3 = 28.3 sq in
(28.3-19.6)/19.6 *100 D%

Thanks. Good tip for roughing it.
And the percentage difference of area between two circles is regardless of whether it's diameter or radius as long as they are not mixed.
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does the room have a good return air register and pipe? sometimes they cheapd out not bothering for a return in small rooms...
my home has that problem... although the room is small and little used these days
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Electrical ties or "zip" ties. They make them up to 36" long and sell them at Home Depot, just for that purpose
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