Contractors....HVAC ductwork question

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4spdragtop

4spdragtop

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What Im contemplating doing is "changing" the duct work in our basement for our NG furnace, for either the cold air return and/or the supply. I don't like the way I finished the ceiling in the hallway and am looking to increase the ceiling height if possible. Is it possible to change the dimensions of the ductwork? Length stays the same, but can I change the wxh as long as I keep the "area" the same?
Ex. lets say duct is 12"H X 15"W.....can I change it so it is 9"H x 20"W??
The volume is the same, just the area changed....

Whacha think??
 
WELL, that's a question of a different color on FABO!!!! LOL

Question is, square duct or round? I think you mean its square. is this intake or supply?

Remember if you are doing a intake duct, as long as its a clear way, making the "area" smaller, it will increase the intake power, and if you do that to the supply, it will make the outward pressure seem more as you walk past. I think you'd be OK, BUT be sure to check with your local building inspector or Building Code inspector, to make sure its up to snuff with them, BEFORE you do anything. Those "codes" are there, for a reason......

ALWAYS when doing HVAC, SAFTY FIRST, and FOREMOST!

Asking questions to these people should NOT cost a cent......IF they insist to inspect, then it might cost a couple bucks, BUT, BEFORE you go to their office, go to the local supply house, (place of parts purchased) Like Home Depot, OR Lowes, they can tell you, what the codes for such is, IF you have educated persons manning that area of the store.....WHICH if they are, and can help you, with the questions on your local codes, and then you wouldn't have to or need to go to the local building codes office at your town hall.
 
Steve, duct design is something I'm not "into" I did mostly the "R" side of AC, and service / repair. "I don't hang duct"

But the thing is you can only get it so thin, as you run into what is called "boundary layer." This is why you can't hide duct in 2x4 stud spaces, as the wall thickness is just too thin to be effective.

To put this simply, this is a sort of stagnant area of air that "stays glued" to the sides of the duct, and in effect makes the useful area smaller.

This can be overcome, IE small duct, high pressure systems, but the entire system needs to be designed around it, otherwise, you'll have "one duct" with "no flow"

Frankly, this is one time I'd suggest you pay someone to look at it who's qualified "locally."
 
Your reasoning seems sound Steve.

I don't think that the boundary layer is going to play that much role. You are going from a perimeter of 54" to 58", so there may be a little more 'drag' on the airflow, but I think that it will be minimal.

Boundary layer increases as surface area increases. You are only going from a perimeter of 54" to 58". boundary layer is only on the surface of the flow, not the center.

Usually most of your resistance/loss is when you turn bends. Like as in porting heads, straighter is better.
 
KK part of my reasoning was that it might be on the small side to begin with. We ran into that a lot, retrofitting older, especially heat pump installs. I remember one customer who went from a heat pump to gas / AC had replaced THREE compressors (total of FOUR) on a system about 20 years old!!!!!

Kinda makes ya wonder who that "service" man was!!!!
 
Thanks guys my dimensions are an estimate, just throwing numbers at ya. It is also rectangular duct. I haven't ripped ceiling down but Im pretty sure it s output air/duct and its a straight line
 
Steve, at least you are thinking. You probably would not believe the "home improvements" I"ve seen over the years

One of the potentially dangerous was showing up to finish an AC install in time to see one of our crew "wondering" about a sawzall. He'd plugged it into an extension cord the homeowner left in the basement and nearly burned up the tool

Turns out the homeowner had a 120V cord he'd cut off and hard-wired to a 240V breaker pair to operate a window mount AC unit!!!!! Which also means he'd taken the trouble to chop off the AC plug on the windowbanger and change it from a 240 to a 120V connector!!!

The homeowner was unhappy with me when he stopped by and found his cord had been yanked out of the box. I told him "I'd be happy" to get the inspector over here if he wanted. I also made it clear that, above and beyond the job bid, he would probably be paying for a sawzall.
 
67Dart273 said:
Steve, at least you are thinking. You probably would not believe the "home improvements" I"ve seen over the years

One of the potentially dangerous was showing up to finish an AC install in time to see one of our crew "wondering" about a sawzall. He'd plugged it into an extension cord the homeowner left in the basement and nearly burned up the tool

Turns out the homeowner had a 120V cord he'd cut off and hard-wired to a 240V breaker pair to operate a window mount AC unit!!!!! Which also means he'd taken the trouble to chop off the AC plug on the windowbanger and change it from a 240 to a 120V connector!!!

The homeowner was unhappy with me when he stopped by and found his cord had been yanked out of the box. I told him "I'd be happy" to get the inspector over here if he wanted. I also made it clear that, above and beyond the job bid, he would probably be paying for a sawzall.
Click to expand...

What the heck was the home owner thinking?? Up here our residential "window shakers" as we call them are 120v. BUT I gotta say that worker shoulda been checking both ends before pluggin into it. Bet he does that now! LOLCeiling in the basement is low, and we are restricted obviously with the ceiling joist height. The current rect duct(heat) hangs lower than the joists, so if I could once Im home be able to "raise" the ceiling if possible.ThanksSteve
 
4spdragtop said:
What the heck was the home owner thinking?? Up here our residential "window shakers" as we call them are 120v. BUT I gotta say that worker shoulda been checking both ends before pluggin into it. Bet he does that now! LOLCeiling in the basement is low, and we are restricted obviously with the ceiling joist height. The current rect duct(heat) hangs lower than the joists, so if I could once Im home be able to "raise" the ceiling if possible.ThanksSteve
Click to expand...

go for it, I WOULDN`T BE AFRAID OF THAT AT ALL.-------bob:supz:
 
I am a Sheet Metal Worker and the size you want to change to will be ok if you have the duct already. It is a tad undersized. 8x25 is the exact match for your 12x15. If you can't go that wide and want to stay with your 9" height then increase the width to 22" wide.
AL
 
ram250098 said:
I am a Sheet Metal Worker and the size you want to change to will be ok if you have the duct already. It is a tad undersized. 8x25 is the exact match for your 12x15. If you can't go that wide and want to stay with your 9" height then increase the width to 22" wide.
AL
Click to expand...

Thanks Al, the sizes I give were approxiamte, Im not sure what the exact dimensions are.I will be in touch when I get home and tackle this project.Thanks for the experienced advice AL
 
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