Reducing Size of Cold Air Return

I'm finishing my basement and I have a run of 18 inch by 8 inch rectangular cold air trunk that runs an additional 15 feet after the last feed into the wall cavity to the floor above. That additional 15 feet is used exclusively for a bedroom two floors above and has a six inch takeoff at the end from which a six in diameter duct runs another five or so feet and connects to the wall cavity to the bedroom 18 or so feet up (sorry if this is confusing).
Question: I am boxing in the ductwork and would really like to minimize the lost headroom area wise. Can I get rid of the 15 feet of 18 inch by 8 inch rectangular and simply run a 6 inch round in its place and connect it to the exisiting 6 inch takeoff at the end of the current run or will this totally screw up the balance of the system??
Thanks - D. Cervoni
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duh.................
Dario Cervoni wrote:

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Nope..
Nope.
Yup. You are messing with a duct calculation called a manual D... Your current duct size is aprox 150sq inches in area, and the round equiv to that is 13 inch round. Since you cant get 13 inch round in most markets, 14 inch would be the alternative. Depending on the static pressure of the unit and lets use .5 as a figure...you are talking about taking a 1575CFM duct that can handle about 1590FPM and going to a duct that will at the same static, handle maybe 190CFM at just under 900FPM.... Guess what that will do? Since we do not know the particulars of the system, one needs to assume, that currently, the manual D was correct, and if you do what you plan, you can kiss your airflow to the upper level goodbye.

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Appreciate the responses and I am hearing what I feared I would hear and that is that I really shouldn't be screwing with the existing "professionally" done setup. But being the stubborn guy that I am and coming in at 6'4", I would really like to do something about that extra 15 feet of cold air return duct that does nothing but "one" bedroom and take a shitload of headroom away from me. Any other alternatives or could I just throw caution to the wind and do what I intended and keep my fingers crossed?
Thanks - D. Cervoni
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Have a professional come in and evaluate the duct system.
Then they will know rather they can build custom ductwork, so you can raise your ceiling.

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This is turtle.
With no more info than we got here it would be just a guess as to what to do. Now what is needed to do is to get a manual D calculations and then it will tell you exactly as to what you can do. Doing something with out the Manual D to look at , is like becoming a hack and start guessing.
OK Let me put my Hack Hat on here for a minute and give you a guess but I'm not responciable for anything because I told you already what this advice is and treat it as so.
I would not run a 6" return for that 15 feet run but use a 8" or 9" flex return line to replace the 18" X 8" return line extenion. You have a fair chance of making it with this bigger size. I don't think 6" will ever do it.
Now I've taken my Hack Hat off Now. Don't Do it.
TURTLE
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forced air systems are pretty simple. The returns are low pressure and the supply is higher pressure, relatively. So if you reduce the low pressure area your going to reduce the high, causing a lower output and higher costs. The unit will have to run longer.
I would really like to do something about that extra 15 feet of cold air return duct that does nothing but "one" bedroom and take a shitload of headroom away from me.
Are you really sure this is a return? Sounds more like a supply duct to me.
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OK well you never told us you were *that* big. So OK - you can do whatever you want!

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