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Discussion Starter · #1 ·
Hi all. I've got a bsmt. washroom job coming up, where the main heat duct coming off the furnace runs right through where the new washroom will go.

It's about 8 x 14". I will of course be getting my sheet metal guy to do the work, but my question is - can I have him reduce this to a 4" x 28" to get some headroom, then have it go back to the 8x14" ?

If we keep the same square inch total, the cfm should remain ok?

Hope I explained this properly. Just want the heads up before I call him in.

Thanks
 

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Bad idea.

Its not just the sq in of free area of the duct that counts.
A 8X14 duct at 100' TEL and .1" friction rate has a CFM of 625.
A 4X28 duct at 100' TEL and .1" friction rate has a CFM of 490.
The 4X28 would have 1-2/3 more sq ft of surface area per linear foot.
So on a short section 4 foot long. A 8X14 has 3.66 sq ft of surface area. And a 4X28 has 5.33 sq ft of surface area.

First though may be to just make the 4" duct wider yet to get more air flow. But, once you past a 7:1 ratio. Friction rate charts and ductulators become inaccurate.

A 6X20 will carry as much as a 8X14 at the same friction rate.

And yes. Make your transition angles 15° or less.

Changing from one size duct down, and then back up will add about 70 foot of total equivalent length to your duct system.
 

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Discussion Starter · #4 ·
Beenthere - am I correct then, that you're saying don't go down to the 4". That I should only reduce this to a 6"
 

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Good math, Katoman... most guys think that you can just add the same amount to one side as you subtract from the other, and keep the perimeter the same a la changing a 20x20 duct to 24x16... obviously you'll lose out on the total area. An example I used to throw out is, "Okay, you have 20x20 with an area of 400 square inches... add/subtract 10 and you'll have 30x10, a 25% loss with area 300 square inches. But hey, why not add/subtract 19 and call it 39x1? Then you'd have 39 square inches... a 90% reduction in area."

BUT as Beenthere pointed out, it's not a raw area calculation. A minor change would be no big deal but the "equal friction loss method" is based on the perimeter length of a cross section of the duct, the amount of metal that comes in contact with the flow of air. (This is why round pipe is the ideal shape for duct: max. area with min. perimeter = min. friction loss. And of course for obvious reasons this never comes up for the transfer of water...) Beenthere used a sliderule device called a "ductulator" which shows how we figure the ductwork. And he's also right that when the length is more than 7 times (or inversely, less than 1/7th) the width, the ductulator isn't very accurate. SO while I have installed 90x10 oval duct spiral pipe, but I do not know how they sized it.

If you absolutely MUST have 4 inches of duct height, you are not out of options, but you'll need a good shopman tinknocker to make this work. In a nutshell, you split the 14x8 duct into TWO 14x4 ducts, keeping the top of the fitting flat. It's a kinnnd-of tough fitting, but doable. I made them back when I was working with my hands in this trade. This type of fitting is called a "pair of pants" because there is one duct "wye-ing" into two (usually smaller) ducts, like a pair of pants has a big hole for the waist then wyes into to smaller sections, like pants legs. [Beenthere: Do you agree with this? Please tell us.]

And yeah, don't exceed 15 degrees taper... I think I was taught to use 21 degrees per SMACNA or whatever, but the longer the fitting is, the shallower the angles are, the more laminar flow you'll have for your air.

I spent 12 years in the field (on the job sites and in the shop fabricating duct, fittings, etc.) so I'm not just talking down from the desk side of a blueprint... if anyone has any other issues with duct, please contact me. I'm always glad to help.

Gugh, it's New Years Eve and I'm here talking about ductwork. Maybe I DO need to get out more!
 

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Uh-oh... I just realized that my above rationale might not make sense: whereas ONE 28x4 duct would have 64 (28+28+4+4) inches of perimeter, TWO 14x4 ducts would have 72 (14+14+4+4+14+14+4+4) inches! What the hell was I thinking? Well, it WOULD keep you far from the 7:1 max. length/width... but still you'd have the cross section surface area increase. I don't have a ductulator at home... let me look at this at work on Monday and get back with you.

AND... as for that 90x10 spiral pipe installation I mentioned, it was probably sized with a higher friction loss (0.25" instead of the common, low-pressure 0.08") as spiral often is.

Thanks, everyone!
 

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Umm...I think you may be going a bit overboard with all the calculations for a little bit of residential duct work. The guy has what? Maybe 5 ft. of this duct to run? Six feet? He will have some pressure loss but jeez guys...it's not that big a deal. This probably doesn't need to be over analyzed.

Juss get'er dunn, times'a'wastin.

Andy.
 

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Umm...I think you may be going a bit overboard with all the calculations for a little bit of residential duct work. The guy has what? Maybe 5 ft. of this duct to run? Six feet? He will have some pressure loss but jeez guys...it's not that big a deal. This probably doesn't need to be over analyzed.

Juss get'er dunn, times'a'wastin.

Andy.
Seen lots of residential duct systems that don't work because of that kind of thinking.

Like it or not, believe it or not.
That 5' you you don't think is important . Will have an equivalent length of roughly 100 foot when you add in the 2 wyes.
So that some pressure loss will be more then enough to make the rooms after that 5 foot not heat or cool right.

Currently the pressure in the duct past this room is somewhere between .08 and .16'wc.
If the wyes and 5 foot were installed as he originally wanted. He would loose between .04 and .08"wc of pressure.
Meaning he would loose between 200 to 300 CFM of air flow. At his current blower speed.

To make up for this. His blower speed could be increased. And then he could go and install dampers in all of his supply runs. And hope the noise isn't too bad.
 

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Uh-oh... I just realized that my above rationale might not make sense: whereas ONE 28x4 duct would have 64 (28+28+4+4) inches of perimeter, TWO 14x4 ducts would have 72 (14+14+4+4+14+14+4+4) inches! What the hell was I thinking? Well, it WOULD keep you far from the 7:1 max. length/width... but still you'd have the cross section surface area increase. I don't have a ductulator at home... let me look at this at work on Monday and get back with you.


Thanks, everyone!
You would need 2- 20X4 ducts to move the same amount of air.

He'll be fine with 6" high duct.
 
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