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Old 11-08-2011, 02:27 PM   #1
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Would like to Reduce one length of Ductwork?


Hello-
We would appreciate any advice anyone could offer- we are DIY'ers that might be in a little over our heads on this one

We are currently renovating our basement and have gotten to the HVAC. There is a 10' line of ductwork (27"Wx9"H) that runs across the middle of the basement. We would really like to cut the height in half to clear my hubby's height- he currently has to duck to walk under the duct (no pun intended). I saw somewhere on TV once - TOH maybe - about reducing the height of the run and doubling the width, but can't find it again. I found a local metal shop that will happily recreate the ductwork to 54"Wx 4.5"H, but we would like to know if anyone has successfully done this. We are concerned about efficiency, or any other issues that might arise.
We would also prefer pre-made ductwork, but have had very little success looking up this issue online. I've left messages with a couple of HVAC guys in the area to see if we could just hire it out, and no one will even return my call. That should probably tell us something.

A secondary question is regarding the actual installation of the current ductwork. The DIY'er that owned the home/renovated before us screwed his drywall directly into the ductwork- no framing, no kidding. We figured it out once we noticed that as we were removing some of the drywall that we were leaving holes in the ductwork itself (which we will tape over). Should we leave the rest as is, or again, for efficiency's sake, remove it all, patch all the holes, and re-drywall?

Thanks, and we really do appreciate any advice. This is one area where we just know enough to be dangerous...

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Old 11-08-2011, 03:18 PM   #2
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Would like to Reduce one length of Ductwork?


Quote:
Originally Posted by AlmondJoy View Post
Hello-
We would appreciate any advice anyone could offer- we are DIY'ers that might be in a little over our heads on this one

We are currently renovating our basement and have gotten to the HVAC. There is a 10' line of ductwork (27"Wx9"H) that runs across the middle of the basement. We would really like to cut the height in half to clear my hubby's height- he currently has to duck to walk under the duct (no pun intended). I saw somewhere on TV once - TOH maybe - about reducing the height of the run and doubling the width, but can't find it again. I found a local metal shop that will happily recreate the ductwork to 54"Wx 4.5"H, but we would like to know if anyone has successfully done this. We are concerned about efficiency, or any other issues that might arise.
We would also prefer pre-made ductwork, but have had very little success looking up this issue online. I've left messages with a couple of HVAC guys in the area to see if we could just hire it out, and no one will even return my call. That should probably tell us something.
They want to install furnaces, that is where the money is. You can get custom ductwork over the internet.

Anyway, you probably can't do that, duct work is more inefficient the more narrow it is.

54x4.5 has probably double the friction of 27x9, even though the number of square inches is the same. If your duct work was oversized to begin with, then you may be ok. But the duct transitions from 54 x 4.5 to and from 27 x9 also introduce friction.

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Old 11-08-2011, 05:25 PM   #3
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Would like to Reduce one length of Ductwork?


Quote:
Originally Posted by AlmondJoy View Post
Hello-
We would appreciate any advice anyone could offer- we are DIY'ers that might be in a little over our heads on this one

We are currently renovating our basement and have gotten to the HVAC. There is a 10' line of ductwork (27"Wx9"H) that runs across the middle of the basement. We would really like to cut the height in half to clear my hubby's height- he currently has to duck to walk under the duct (no pun intended). I saw somewhere on TV once - TOH maybe - about reducing the height of the run and doubling the width, but can't find it again. I found a local metal shop that will happily recreate the ductwork to 54"Wx 4.5"H, but we would like to know if anyone has successfully done this. We are concerned about efficiency, or any other issues that might arise.
We would also prefer pre-made ductwork, but have had very little success looking up this issue online. I've left messages with a couple of HVAC guys in the area to see if we could just hire it out, and no one will even return my call. That should probably tell us something.

A secondary question is regarding the actual installation of the current ductwork. The DIY'er that owned the home/renovated before us screwed his drywall directly into the ductwork- no framing, no kidding. We figured it out once we noticed that as we were removing some of the drywall that we were leaving holes in the ductwork itself (which we will tape over). Should we leave the rest as is, or again, for efficiency's sake, remove it all, patch all the holes, and re-drywall?

Thanks, and we really do appreciate any advice. This is one area where we just know enough to be dangerous...

The main problem that your going to run into is comprimizing crucial air flow, that can lead to much more serious issues with your furnace and a/c. I sugest that you make some more phone calls to local havac contractors........in the months following x-mas alot of companys get very slow and would welcome the work. Keep in mind that duct work requires shop time and shop time is exspensive. Another thing to consider is moving the entire trunk line to one side of the basement or another, completely out of the way of hubbys head.
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Old 11-08-2011, 05:56 PM   #4
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Would like to Reduce one length of Ductwork?


If your ductwork was closer to square to begin with then flattening it out would be a viable option. The aspect ratio you have now (3:1) is about the limit of stretching it out while still being efficient.

Changing it to 54 x 4.5 (or even something in between like say 36 x 7) will increase the friction in 3 ways:

-The perimeter of the duct goes up from 72" to 117" - think running your finger along a wall compared with running your hand along it
-Any take-offs along this section will cause a lot more turbulence
-The transitions to change to (and, if necessary, from) this size - again, turbulence (also cost)

I don't think that you should try to flatten this section of ductwork more - at least not as much as shrinking to 4-1/2". Rerouting it to a different area would be better. Could you run it along a wall on edge and give up a little floor area or build in something below the bulkhead?

Last edited by Dog5; 11-08-2011 at 05:59 PM.
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Old 11-08-2011, 09:47 PM   #5
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Would like to Reduce one length of Ductwork?


Thanks guys.

we did consider moving the ductwork to a back wall, but we would have to move it 8-1/2 feet, then 10' across, then back again 8-1/2. We thought that might be an even worse loss of efficiency?


Thank you!
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Old 11-09-2011, 10:12 AM   #6
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Would like to Reduce one length of Ductwork?


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Originally Posted by AlmondJoy View Post
Thanks guys.

we did consider moving the ductwork to a back wall, but we would have to move it 8-1/2 feet, then 10' across, then back again 8-1/2. We thought that might be an even worse loss of efficiency?


Thank you!
It is not the length but the turns that hurts the efficiency the most, a single turn may be equivalent to 80 feet of straight duct. You may want to read this

http://www.perfect-home-hvac-design....-fittings.html

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