I think you could get similar flow with a wye duct fitting, http://thesheetmetalkid.com/wye-branch/splitter-ductwork-wye-branch/5-x-5-x-5-splitter-duct-wye-branch/ or use a saddle tap like this one. https://www.ferguson.com/product/5-in-galvanized-steel-saddle-in-round-duct-shmst926s/_/R-4042515

Bruce

Bruce, In this case it is going from square duct to a circle branch to the register. It's not clear without the rest of the article. The one in the "Good" diagram would be something like this. https://www.google.com/shopping/product/17257520787006445761?q=high+efficient+duct+take+off&biw=1680&bih=949&prds=epd:8401493824797744913,prmr:3&sa=X&ved=0ahUKEwjp9bL3qMjrAhUpmuAKHfPYBdsQ8wIIhAQ

The easiest way to do this is to install a round duct takeoff. There are a few options on the web site where you make a hole in the main duct and attach the takeoff on the side. The seam can be sealed with Nashua foil tape.

It's argumentative what is best & what is not. It greatly depends on the design of the 'entire' duct system.

The more turns there are: this hinders air flow. If you split paths of ducts as in a wye type connection this will reduce air flow as well as have less options to control what air flow you do get.

Amateurs look at parts and pieces of what is said to be best, but only focus on one small part of the system of which is far from what is 'best'.

It's a science paradigm. One piece here or there described as best... does nothing for improper design.

There are more than a few ways to design a duct system, of which there can be pros & cons of those choices in relation to the structure. Because there are no two structures exactly alike is why you hire a professional.

I service the Katy, Texas area.

@ Austin Air Companie - I know it looks like I am just focusing on individual pieces but it is intended to be part of a full design. I did send out info to an HVAC design company to find out their price so I am looking to get a pro design done.

I just have not seen this particular part before and figure it might be something I could include for efficiency sake. I could do something somewhat similar if using a 90 degree adjustable elbow but mitered elbows aren't as good as radial and adjustable elbows aren't smooth to begin with.

There is a lot of information online about what is best based on testing and fluid dynamics. A radial turn with a high radius to width ratio is best.

My purpose in particular is not to split the duct into 2. This would be for the duct take-offs to the registers. Most likely going from the square supply duct to a circular flex duct. I know flex duct is not as efficient as other options, but I've seen a lot of good builders state they use flex duct at the ends to reduce noise from the system..

Most of my house was retrofit with A/C and they used insulated flex duct for most of the feeds. You can't even really hear the A/C running. However, in a new addition we installed all nice shiny metal duct with beautifully taped seams, and you can really hear the air flowing. Its not a big deal overall, because the addition is mostly bathrooms and closet space, but it never occured to me how much more the sound would transmit with uninsulated metal ducting.

Bruce

There is a lot of information online about what is best based on testing and fluid dynamics.

A good amount of it is untrue. They are merely attempting to sell an amateur something because 'they say' it's the best.

I see it all the time... garbage posing as the best thing since sliced bread. If you want a professional project, hire a professional. Someone selling you the latest piece of manufactured garbage?

There was a saying back in the day: There goes another Sears Sucker... Sears made it, the sucker bought it. Guess what? it eventually caught up to Sears. (Sears is a fraction of their former selves.)

The talk about flex duct as an example: I live in the over heated south. About 3 clicks or so west of the Gates of Hell. 99% of the homes that exist here are guess what?

Flex Duct.

If it's so bad, why in God's name would we use it in one of the hottest locations in the country?

It's bad, because someone is trying to sell you: something else. The oldest trick in the book, we are better than you, because....... It's the internet anyone can say whatever they want.

Knowledge in the right hands is power, knowledge with no understanding is worthless.

@ Austin Air Companie.

Yes, I agree there is a lot of untrue stuff out there. I try to stick to stuff that at least seams reasonable and not a specific product trying to be sold to me. Here is the link where I saw the idea of a radial take off. As you can see, they are not trying to sell me anything in particular. I was just curious if such an item currently exists without being custom made. You can see at the bottom of the article they list as references publications from SMACNA, ASHRAE, and ACCA.

https://buildingengineer.wordpress.com/category/mechanical/

I figure when putting something together,, make it efficient as can be within reasonable cost. Plus if I make the air to my main bedroom register efficient enough, I might be able to delete the second register. Also because of where I want to put a direct return vent from the bedroom area to the return plenum, the second supply register is mostly just going to be sucked away to the return instead of heating/cooling the room. So, an efficient higher flow register solves the problem.

I wasn't really saying flex duct is bad, just that it is less efficient for overall flow rates. My intention would be to have the square duct with the take offs leading to flex duct to the registers. I need the extra efficiency to make up for the fact that the original builders did not put any insulation in the walls. Even worse, most of the walls have airflow without much restriction to the attic. I live on the border of zones 4 & 5. So in the summer it can get into the upper 90's and winter it can get down to the single digits. Builders did not seal anything in the whole condo. (2 floor corner unit facing south west). Even the ductwork isn't sealed. The chase for the duct work that goes to the attic for the top floor limits me to square duct. Circular duct for the supply trunks would have too big a diameter for what I need. Flat oval is too expensive.

To give you an idea of the original builders, here is a picture of my return air. This duct section connects the return plenum to the filter box before the furnace. It is about 8X12 so undersized by maybe 60% or more. Notice how it is actually just leaning to one side. The return plenum is inside of what is supposed to be a fire rated chase and is technically supposed to be sealed by a 1 hr fire rated sealant for anything that penetrates it. 1 return air box is just as bad, the other is worse. It will take a long time to fix everything. Right now I have to have my damper to my downstairs zone partially blocked to get enough air to the 2nd floor due to all the holes and gaps in everything. If it is fixed and made efficient, I can open the damper. It will be a while longer to fix the first floor ductwork since I have to remove the whole ceiling to do it. Plus it will be a while before I can get to insulating the walls upstairs and down. And longer still to fix the return air. I have to see about moving the shared flue pipe over so I can put in a bigger and properly designed return since the flue pipe is put only an inch or 2 away from the duct. (flue pipe is shared with downstairs neighbor so flue pipe and neighbors MUA vents need to be moved to make more space for what would be my new return). My MUA vents need to be changed and moved before I can seal all the holes in my utility room or the hot water heater and furnace will backdraft when the dryer runs and would be starved for combustion air the rest of the time.

Hence the reason I am researching various concepts on how to make efficient ductwork with the constraints I have and considering the long term to a full solution. Do it well the first time since I won't go back to it again. I am looking into places that do HVAC design and prices. Considered doing it myself but too seems to be too much to learn from all the various manuals. Plus the software to do a proper design actually would cost more than having someone else do it for me. The various slide rules and stuff people use could get me close but not account for other constraints.

Looking at things like turning vanes since I need to have a square throat 90 degree turn to the attic, efficient takeoffs to get the proper air too the register and register options. Return air pathways to circumvent closed doors, Sizes and styles of return air intakes, sizing of air filters, fitler boxes, air restriction of filter levels. How to fix the jenga looking installation on top of my furnace. Looked at various custom duct connection systems but seem to be too expensive for the efficiency gains and some make things too wide. Plus the fact of living in northern NJ has made me jaded due to almost all contractors and vendors being either incompetent or worse, fraudulent and on the take. So I would be doing the work myself and would need to check the design as much as I could to be sure they didn't just half ass it and figure "good enough". So my original post is just 1 bit in a vast amount of research that I couldn't find an answer for myself.

I say you are wasting your time. Efficiency in an non insulated or poorly insulated building is not efficient. Nothing you gonna do without re-insulating to get around that.

If you are looking for a temporary solution until you can do it better, buy a window unit. Then you may have an HOA problem.

The alternative is get a home improvement loan, have the system redesigned completely.

Another alternative is sell the condo, then use the information you have learned from this experience and buy a home some where else, after you're able to test and verify the systems in place in that structure... what ever it turns out to be.

There are much better ways to accomplish a resolution to this than how you are trying to MacGyver this system into working.

It appears from the picture the duct work was badly installed and undersized. A lot of heating and cooling is being wasted. I suggest you spend the money on custom duct work on the main supply and return. The efficiency gains at the branches are smaller and not worth the money. There will be a significant improvement just by properly sizing and sealing.

Everyone is missing the point. I am in the process of getting a full redesign and will be fully replacing all my duct work. It will be properly sized, insulated, sealed, takeoffs will be properly distanced, etc.

All I wanted to know is if this particular type of radial takeoff exists to include as part of the redesign. If they don't already exist then I will just use a standard high-efficiency take off since I don't want to pay the extra to make this takeoff custom. I will only re-do my entire HVAC ductwork 1 time so doing it very well and efficient that one time is important to me. There are other factors as to why I want the high level of efficiency. But I won't list those again since I think I confused people too much when I mentioned them before.

So, one more time. Has anyone ever seen this type of radius duct takeoff? Going from square duct to circular duct.

@Stymie

Will you provide a link to the "original article" mentioned in your original post?

Thanks! And sorry you've yet to get an answer...