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Round or rectangular supply duct?

1718 Views 48 Replies 9 Participants Last post by  supers05
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.
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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 .
 

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Discussion Starter · #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.
 

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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?
 

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The rectangular duct is more space efficient - space taken up relative to the amount of air being moved.
 

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The rectangular duct is more space efficient - space taken up relative to the amount of air being moved.
I suppose that depends on how you look at it. A round duct uses less cross sectional AREA, and less metal but it's hard to use the free space around the circular duct. So in the end a square duct is much more practical.
 

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I assume it is due to round facilitating vortex movement and thereby reducing drag? Do you know of actual research?
It's more about surface area vs. volume, and the relative remaining cross-section at any given distance from the wall. Not much air moves through the corners of a rectangular duct.

Anyway, as others have said, round is most efficient on the basis of material vs. volume, but rectangular may be better when fitting a duct into the smallest space, because those spaces are typically rectangular. Rectangular ducts that are closer to square are more efficient for their cross-sectional area than wide, flat rectangular ones. According to this chart, 6x8 is 60% better than 3.5x14, even though they have nearly the same cross-section area.
 

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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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Discussion Starter · #14 ·
Adequate air flow from basement to upper floors in the cooling season is also rare . We read it on this forum nearly every day .:)
Good to know but I won't have basement. This build will be stacked duplex, ie. One unit on each floor each with its separate ac unit. No connection between the two units.
But I hear you. I have never seen or felt a two-story house with good performance as far as equal temps on both floors. Especially when using one unit for 2 floors makes it worse.
Can't change the science : hot air rises and cold air settles down every time.
 

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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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Discussion Starter · #17 · (Edited)
It requires designing the ductwork well and following the plan. I usually suggest people pay a designer.
My observation is that there is a very big open staircase that lets hot air travel easily to the second floor irregardless of the ductwork. In our hot climate, the second floor is usually unbearable. Heat rises...everytime...and it goes up the staircase in addition to the fact there is invariably a big open room (usually encircled with stair pickets) at the top of the staircase and an attic above that that is 140 degrees.
I have been looking at houses under construction and the attic looks like a spaghetti bowl of silver ducts draping everywhere. Hundreds of feet of flexible silver ductwork. It appears to me that none of those designers have a clue. Same story on all the TV shows regarding house building. Maybe where the TV shows are filmed in cold climate it works. Maybe heating is cheaper and more effective than cooling.
But where I am building, it is a heatwave. Uncomfortable and expensive and energy busting to build that way.
I haven't seen an efficient design.
I have tried to use some apps but none even have energy efficient options in the app like shingle color, all ductwork in air conditioned space, etc.
Two recent discussions with shingle contractor and shingle suppliers were pushing and promoting black shingles. They just swooned when they suggested black shingles. I was just flabbergasted by their lack of understanding regarding black shingles in our climate. There are out-of-pocket costs for electricity and environmental costs for inefficient design.
 

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Discussion Starter · #18 ·
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 .
Yes. I think a restaurant would have a lot of humidity due to many people, all the cooking, dish washing, door openings, floor mopping, etc. I think I will use bare ductwork.
 

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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.
Round is more efficient for fluid flow because the friction comes primarily from the surface of the material. Square ducts will have more "wetted perimeter" than round ducts of equivalent sectional area, causing more friction. Example: 2x2' duct is 4 sq ft sectional area, 8' perimeter. Equivalent round is 2.26' diameter, which is 7.1' perimeter.
 

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Discussion Starter · #20 · (Edited)
Round is more efficient for fluid flow because the friction comes primarily from the surface of the material. Square ducts will have more "wetted perimeter" than round ducts of equivalent sectional area, causing more friction. Example: 2x2' duct is 4 sq ft sectional area, 8' perimeter. Equivalent round is 2.26' diameter, which is 7.1' perimeter.
OK. That gives the math I wanted to know. Thank you for explanation.
I will also consider the actual room size and configuration to determine whether I want a little less everyday efficiency in exchange for appearance and fit.
I have 9 foot ceilings so I have the space for round if that also works well with vent connection design.
My best efficiency is derived from ducts in living space, short ductwork, white shingles, windows with 3 layers of glass, window placement, additional insulation and design of duplex.
 
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