# Thread: 6 inch to rectangular duct conversion

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Hi,

I've got two 6" round ducts in a closet that are taking up too much room. I'd like to convert to a flatter rectangular duct (in the closet portion only) to save some room. Two 6" ducts are (2 x pi r squared) 56.52 square ins of area. Would a 3"x20" rectangular duct (giving me 60 sq ins of area) be the same thing in terms of air flow?

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i would get oval pipe in 6" or go with 4x10 it will be easy to do with out custom sheet metal work done.

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Thanks rasmussen! What is the width of the oval duct? is it 4"? I thought that in order to fit inside of 2x4 studs ductwork came in a 3-1/2" width standard?

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A 4X7 oval= a 5.7 round ,4X9 oval = a 6.2 round.

6" round = 9X3.5 or a 3 X11

All based on equal friction(resistance),there's more to it then just equal area,as the duct gets narrower in one dimension,the surface are of the inside of the duct increases,as does it's friction(resistance to air flow).

Another thing to consider is the fittings to go from round to "what you choose" and back again,these will also reduce air flow.So you may want to use an even larger duct like a 10X4,as mentioned above it's equal to 6.7" round.

[Edited by dash on 08-15-2005 at 05:50 PM]

Hi,

I've got two 6" round ducts in a closet that are taking up too much room. I'd like to convert to a flatter rectangular duct (in the closet portion only) to save some room. Two 6" ducts are (2 x pi r squared) 56.52 square ins of area. Would a 3"x20" rectangular duct (giving me 60 sq ins of area) be the same thing in terms of air flow?
I was always told that you multiply the round duct diameter by its self and then multiply by .7854.

Here is the dilemma:6x6 = 36x.7854 = 28.2744x2 = 55.54.

If you were to add the two 6" ducts to equal one 12" round duct you get: 12x12 = 144x.7854 = 113-sq.ins. or a 20x6 or 10x12 duct.

In order to connect to a 6" duct you will need more than a 3" depth, or static pressures will rise from the squeeze and sharp change in airflow direction. I would go with a 20x6 for 120-sq.ins.; you might get by with a 20 x 4 for 80-sq.ins. Any significant increase in static pressures will reduce the amount of airflow.

Examine your existing duct sizing for your A/C system and get all of it right; also check fior leaks and zeal them.

http://www.udarrell.com/proper_cfm_b...g_systems.html

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## Convert

6" round area = 28.3 sq.in.

I would recommned each 6" diameter be changed to
3.5" x 12" (42 sq.in)
(or 3.5 x 10" [35 sq. in] may even be adequate depending on the smoothness and number of transitions).
& definitely NO Problem with the resultant Static Pressure.

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Thank you all very much for your responses and willingness to share your expertise! Although there are some conflicts in the responses (especially in the area of static pressure) this is all very informative.

I like Dan's answer the best!

[Edited by adastra on 08-15-2005 at 04:17 PM]

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Udarrel; since when did 2 - 6 " diam pipes equal a 12" diam area?
6"DIAM AREA= 28"
therefore ,2- 6 "diam =56"
A 12" diam =113"

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Each tapered square to round fitting is equal to 10 feet of duct.So two on each run is the equivalent of adding 20 feet of duct to each run.

Without the proper tapered fitting ,it gets a lot worse.

You are likely to get less air ,to wherever these runs go.

10. Originally posted by deejoe
Udarrel; since when did 2 - 6 " diam pipes equal a 12" diam area? {Never of course. - udarrell]
6"DIAM AREA= 28"
therefore ,2- 6 "diam =56"
A 12" diam =113"
Right deejoe, and thanks; after reading what I wrote, I can't believe I didn't edit that piece of ambiguous nonsense, that could have confused the poster. I was in a hurry and did not properly review my post.

Well, "that is what I also illustrated," two 6 inchers' do not equal a 12-incher by a wide margin. I was trying to make a point for all to see, but my extraneous rhetoric confused the obvious point.

However, were I in his situation, I would probably go with somewhat larger duct area in an attempt to reduce the increased static pressure that will probably occur when he makes the change.

I thought he was going to feed both 6" ducts through one rectangular duct, which would call for some adjustments to make it work without a loss in CFM.

For all we know most of the existing duct system could be undersized.

11. Personally, if the 2 6" ducts are currently working, I would leave them alone and find some place else to gain storage space.

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