modifying basement ducts

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I'm about to do some basement remodeling. What I'd like to do is gain more headroom and was thinking about changing the shape of the basement ducts. They are currently 20" wide and 8" high. I'd like to take them down and bend them to change the width to 40" and the height to 4". They would contain the same amount of internal volume. Are there any problems with doing this?
Thanks.
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ermm... what I meant to say was 24" wide.
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No longer the same inches. Sounds like a bad idea.
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Christopher A. Young
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inches (20 x 8) to one with a cross-sectional area of 96 sq in (24 x 4).
Do you seriously expect that you will get anywhere near the same airflow, after cutting the size of the duct almost in half?
-- Doug Miller (alphageek at milmac dot com)
How come we choose from just two people to run for president and 50 for Miss America?
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wrote:

No, I corrected myself (again) changing it back to 40 x 4. It's possible your news server missed it, as mine misses posts all the time.
I was extremely tired wasn't functioning properly. :)
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hmm... I guess it would have to be 40" wide.
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I had this done at my place. It's not as simple as you make it and I suggest you get a heating contractor in to at least give you some measurements. The main problem lies in transfer points where you go from 8" ducting down to 4". As I said, get someone in.

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I had planned on making it a gradual change, like funneled over a distance of 4 feet. I'm getting a new furnace sometime this year so naturally I'll have an expert check things out. Who knows, maybe everthing will have to be ripped down anyway. I guess I was just wondering if something like this was able to be done, and It's encouraging to hear that someone else has done it. I realize it's not that simple and wasn't about to rip out the hacksaw and sledgehammer. :) That's why I posted here. Thanks for your help.

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Several. Main, and most important...while the area inside is the same, you have increased the amount of static pressure and will cut your airflow drastically. In other words.....bad idea.
The most efficient duct is round duct. You might want to get a contractor in, explain what you are looking for, and see if you have other options avaliable that you can look into. A manual D in a case like this will be VERY important and must be done.
Also...with most square duct, it is internally insulated and you dont just take it down and rebend it...you dont rebend it period...you take it down, throw it away and have new duct made.
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I'm rather curious as to why these particular ducts are so sensitive, since when they make a 90 degree turn they are just metal covering the floor joists. It's just metal nailed over wood. Also, these are old ducts (house built in early 50s) and don't appear to be insulated in the slightest.
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Bingo...a PERFECT case of "We cant see it from here". So you mean the duct is actually nothing more than a case of panned floor joists? God.....

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I guess I should have clearly stated "While grabbing a flashlight and removing a basement vent, I can't see any insulation on the inside." Just plain metal. Perhaps they did it wrong. I don't know. I'd like to correct it though, and understand how it works.

The 90 degree runs are sheets of metal nailed to the bare floor joists, yes. Not the main runs mind you. Those are long rectangular runs underneath the joists. These are the ones I had wanted to change.

It seems as though my question has upset you in some way. I'm not sure why. Everyone else has been more than courteous with their suggestions. As a network administrator and programmer I have people asking endless questions that to me seem "rather elementary". But I have an understanding that no matter how much you know, you don't know everything. And guess what, as much as I already know, I learn knew things everyday. Naturally this also includes home improvement projects, and I accept these challenges with an open mind and get as much advice as possible. But in doing so when I don't understand something I don't shrug it of as "well that's just the way it is". I like to know how things work and why they are done the way they are. I realize that everyone has to start somewhere, and have no illusions of expecting to learn everything overnight. HVAC is not my area of expertise, obviously, and that is why I am here.
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I dont follow you. Upset? Not even. What bothers me is that you take it upon yourself to try to read too much into a post. God, meaning, Panned joists...what garbage, or, why in hell did anyone do that to you. No..not upset, but at this point, quite amused.
I really dont understand your comment about being courteous with suggestions. If you suggest that I offer to come out and fix it for you for free, then I suggest you continue looking. I offered a post, that was quite informative. I dont coddle and I dont hold hands. I state facts. I wont sugar coat things. Never have, never will.
Now, its like this. What you suggest doing will result in results that you dont want. I again suggest that you get in a person versed in Manual D, and allow them to look it over. Perhaps you didnt get that post. Perhaps you took it wrong. No matter to me, since I am not here to win friends, but I am here to offer what I can, when I can, and not halfass guess at it.
Again..I cant see it from here. No one can, and to think that anyone that cant see your mechanical system can give you the dead on answer to your problem, other than to suggest that a professional versed in manual D look at it would be wrong. You can pop figures around all day, but until someone can see how its laid out, and what is, and what is not possible, then you are still shooting in the dark.
Hope THAT didnt offend you.

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Sorry, after reading the 2 previous sentences of your reply it looked more like "God what a stupid observation". But I guess not. (Plain text environments are a bitch.) Unfortunately, the house is old and I have know idea which owner did this.

Nope. I'll get someone local, but thanks for the suggestion. ;)

I should have took some pictures and posted a link. I took the "We can't see it from here" as though "I" didn't take the time to thoroughly check to see what the inside looked like. I didn't realize the "we" meant the newsgroup and thought it was directed at me. My mistake.

Nope, thanks for your help.
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Whew! That was close.
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Because of friction, the capacity of a 20x8 air duct is not the same as a 40x4 duct. The optimal shape is round. Search the web for Ductulator, then buy one at an HVAC distributor and/or ask somebody to do a calculation for you. Actually you don't need a calculation because I can tell you that wider and shorter is not going to work out, unless of course the blowing capacity of the fan is so low that it just doesn't matter. -B

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It's a small bungalo and my gas bills are around $60 per month budget. I also have a gas water heater and dryer using some of this. I'll get someone in though to check it out though. I like the round duct idea and will mention that. In a worst case scenario, I'll just leave it the way it is. Not a huge deal, although some of my friends who are taller have to slouch slightly to avoid bumping their heads.
Thanks for the help.
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At a 0.1 resistance, 20X8 rect. duct will carry about 950 CFM. Taking that to 4 inch duct means you would have to have nearly 55 inch to get the same carrying capacity.

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See what I mean defecto? I didn't want to sound snarky but this is the type of project where you're better off having a pro come in, at the very least, to spec it out.
Good luck

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Yes, I know what you mean. I'm only in the planning stages right now and was just wondering if such a thing was possible. I won't be making any changes until I get a new furnace and central air (sometime later this year). At that time I will be getting lots of advice as to what exactly to do as they will be able to see my exact setup. I'm convinced now to have a professional do the construction of such duct work, and I thank you all for your more than helpful advice.

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