[logan176]

Should I use 6" round ductwork or round oval ductwork that's designed for a 2x4 wall?

I just had central air installed in my 1200 sqft cape cod house. I have 2 bedrooms upstairs, one is fed from the end of the supply trunk and the other is fed from the beginning of the supply trunk. The room that is located at the end of the supply trunk is pumping out a lot more air than the other room. I don't know exactly how much, but when I put my hand in front of the vent, there is a noticeable difference in airflow.

Both rooms use 3.5"x10" rectangular ducts and both runs have two 90 degree bends. However, the room with the lower airflow also twists at each 90 degree bend. I think this might be the reason for the decreased airflow.

To eliminate the 90 degree bend & twist, I'm thinking if running new ductwork from the main trunk to the bedroom. The ductwork will travel through a small wall with 2x4 studding. The guys that installed my AC said that replacing the 3.5x10 duct with round oval duct would increase the airflow in the bedroom from about 100 cfm to about 150 cfm.

Should I switch to the round oval ductwork, which would be a pretty easy fix, or should I fir out the small 2x4 wall and resheetrock to allow for a 6" round duct?

[Joshdta]

All ducts at the end of the run will blow harder then ones at the start. Especially if the first lead is installed to close to the plentum it could be bypassing it all together. As far as your question I tend to use the oval stack more then the wall stack. Seams to work better and easier to install. If you are running 6 in odds are you will not get more the 125 cfm weather you use 6'' round or 6'' oval. You could go to 8'' oval to get around 200 to 220 cfm

[logan176]

Would the 8" oval fit inside a 2x4 wall?

[Joshdta]

Yes 8'' oval is just about the same width as 6'' oval just wider

[logan176]

Thanks for the help. Now I just have to figure out exactly where to attach it to the main trunk and I'll be good to go. I'll post some pics on that dilemma soon enough.

[logan176]

As far as your question i tend to use the oval stack more then the wall stack. seams to work better and easier to install. if you are running 6 in odds are you will not get more the 125 cfm weather you use 6'' round or 6'' oval. you could go to 8'' oval to get around 200 to 220 cfm

With the ductwork you mentioned, do you mean just plain old oval or round oval? Can you post a picture?

When it comes to putting this in the wall, will I be intentionally squishing the oval to get it to fit into a 2x4 wall or will it pop right in like the rectangular duct?

Thanks

[Joshdta]

This is not round duct smashed oval, it is actually oval duct

http://www.mcintoshair.com.au/oval_fittings.jpg

[Joshdta]

http://www.controlaer.com/images/pro...al_duct_27.jpg

Can't find any of the actuall duct here are the fittings

[logan176]

this is not round duct smashed oval, it is actually oval duct

http://www.mcintoshair.com.au/oval_fittings.jpg

I know I wouldn't be buying round duct to smash, but I was talking about the difference between these two. They might be the same thing but one looks more flexible than the other.





BTW, what's the minor axis of the 8" oval? If it's going to fit into a 2x4 wall, the minor axis would have to be 3.25". I'm still trying to figure out how something is 8x3.25 oval is going to push more air than a 10x3.25 rectangular duct.

[Joshdta]

The bottom picture is the one I was refuring too

[logan176]

It's amazing how every new answer leads to another question.

Okay, so I know I need to use oval ductwork in the 2x4 wall. Now I'm wondering about the other stretches of duct on the same run. I'll divide the whole run into 3 parts, the basement section, the wall section, and the attic section. I figured I would branch off the main trunk in the basement with round duct. Then I would need the 90 degree connector below to convert the round duct into oval duct so it can travel up the wall. Then in the attic I need another 90 degree bend to get the ductwork to the second floor bedroom wall.

Once I get into the attic, should I keep the duct oval or convert it back to round?

[Joshdta]

I would leave it oval upstairs, too much going back and forth will reduce pressure. Also in the round part in the basement your could install a duct boster fan if you need even more air flow.
http://images.bizrate.com/resize?sq=400&uid=625690819

[logan176]

How far away from the 90 degree bend should I place the fan?

[Joshdta]

12 to 18''

[Joshdta]

http://www.netropolus.com/Pictures/4...ionDiagram.gif

[logan176]

Thanks for all the advice. This info is really going to help me this summer.

[ffakr]

I'll preface this by saying I'm not an HVAC guy, I do have a background in the physical sciences though. :-)
It seems counterintuitive that a 8"x3.25" oval might push more air than 10"x3.25" rectangular BUT you have to account for turbulence. There is drag inside a duct. More drag, less flow.
In nature, the most efficient shape is a circle. It's got the highest area to circumference ratio.. and, it turns out, apparently a lower amount of turbulence. At least that's the way it looks if you check equivalent duct info.
http://www.engineeringtoolbox.com/equivalent-diameter-d_443.html

The cross section of a round duct, x" square, moves as much air as a rectangular duct with a larger cross section. Round ducts are more efficient than rectangular ones.
I suspect the rounded sides of the oval duct present less drag than a square duct.