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Round or rectangular supply duct?

3K views 48 replies 9 participants last post by  supers05 
#1 · (Edited)
I have designed new home build (2 story duplex, concrete slab) with AC and all ducts within air conditioned space. I plan to not have any insulation around the duct because it is all within air conditioned space. Total supply duct length is 48 feet in each unit.
Is there reasons to use round duct rather than rectangular duct? I think I might like the look of retangular better.
 
#2 ·
I saw some neat looking round spiral in a restaurant . They had a slight problem with condensation drips in some areas but that was in a restaurant . With round there is no chance of oil canning .

I would think one would need to be certain dew point temperature stayed below supply duct temperature in the cooling season with either rectangle or round non-insulated .
 
#4 ·
Yes. My research indicates that square have more drag. Statements were made about that but I did not find any actual research
I assume it is due to round facilitating vortex movement and thereby reducing drag? Do you know of actual research? I want ac to be efficient.
 
#7 ·
Yes. My research indicates that square have more drag. Statements were made about that but I did not find any actual research
I assume it is due to round facilitating vortex movement and thereby reducing drag? Do you know of actual research? I want ac to be efficient.
In the end it doesn't actually matter. You size the duct according to airflow requirements. (manual d) Pick what you like more, and temper it with what's possible with your situation.

Have you done your manual J, s, d already?
 
#12 ·
For duct sizing, use this: Residential Air Duct Calculator - EfficientComfort.net

The friction rate still has to be figured out - depends on equipment and total equivalent length including elbows.

The ideal total static to design around, if at all possible is 0.5" but that can be hard, it is not a lot to work with.
 
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#16 ·
2-story homes with furnace in basement on typical duct systems tend to do much better with heating than cooling - cooling really exposes duct deficiencies.
 
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#33 ·
Something is definitely wrong with the 3.5x14 number. By logic, you'd expect 3.5x14 to be closer to 4x14 (180) than it is to 2.5x14 (70). (Assuming those flow numbers are correct, of course.) But that chart has 3.5x14 at 100, which is only +30 from the 2.5" duct and -80 from 4" duct.
 
#34 ·
Something is definitely wrong with the 3.5x14 number. By logic, you'd expect 3.5x14 to be closer to 4x14 (180) than it is to 2.5x14 (70). (Assuming those flow numbers are correct, of course.) But that chart has 3.5x14 at 100, which is only +30 from the 2.5" duct and -80 from 4" duct.
This is what I get for rectangular, galvanized, at AC conditions, 0.1"wc/100ft:

2×14


3½×14


4×14
 
#37 ·
Yes. Say you have a 4"x12" duct. Compare it to a 1/4"x192" duct. It has the same sectional area, but 12x the perimeter. Since the perimeter is the largest function of friction, you have a lot more friction, especially when you consider that friction increases as you approach the surface. A round section is pretty much the most efficient from a flow perspective (although not usually from a packaging perspective).
 
#38 ·
Elmer
Check out my house build on the showcase part of this site under “partial earth sheltered home” I used exposed round duct and very happy with it. It is not your garden variety round spiral duct. A company run by a Iranian engineer with a factory in Trenton Tennessee makes a patented product called Spiramir. It uses large orings to seal so no duct tape required. It is an excellent product and not overly expensive. Mine is unpainted standard galvanized and looks fine. You will see several photos on my build site. People said I would have condensation problems but did not. Never a drop of moisture.
Picture frame Window Lighting Wood Interior design
 
#42 ·
No, but with it being in conditioned space, I'd only tape the seems in concealed areas. The leakage can be compensated by closing some of the vents in the same space, if needed. It still is an intriguing concept that I might look into if I have a situation like this.
 
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