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Old 03-12-2011, 08:15 PM   #1
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Duct layout questions


I have a two story home, approximately 1500 sqft, that I'm running all new ducts for. The house foot print is 25' wide, 40' deep. I was planning to run one large rectangular duct down the right side (long side) of the house, running in a soffit in the first floor, and branch everything off that with flexible ducts.

My contractor wants to do all flex, replacing the rectangular trunk with two separate flex ducts, one to feed only the upper floor, and one for lower floor. The claim is that this method isolates sound transmission between floors.

What are your opinions on which method is better? I feel my original method is more efficient for air flow since sheet metal ducts have less resistance than the flex. But if it does promote sound transmission, I will reconsider. I'm afraid he's trying to find ways to cut cost by using all flex. We already settled a price to redo the entire house. So your opinions are appreciated.

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Old 03-12-2011, 09:43 PM   #2
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After insulating some of my metal ductwork I now appreciate more the flex ducts I have which are naturally wrapped with insulation. Plus the flex are jointless - so no seams or gaps to seal with tape. I'm not a pro - just sharing those thoughts.

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Old 03-12-2011, 09:55 PM   #3
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Duct layout questions


Flex duct is faster to install but costs more it has less leakage but reduces air flow so it needs to be sized properly.as far as sound reduction not really going to see a big difference.the other draw back is it doesn't look that great if it isn't strapped well all and all I would use flex over metal if it is concealed metal if not
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Old 03-12-2011, 10:41 PM   #4
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The 2 ducts weather sheet metal or flex is the way to go.

Sheet metal trunks and then using flex supplies will also help to minimize sound transmission.
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Old 03-12-2011, 11:59 PM   #5
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Duct layout questions


Well, what I'm most concered about is airflow...and proper airflow reduces sound. So, I'm inclined to agree with your soffet should take less room. Sounds like you'll be making some 90'S which will cause static back pressure, so you'll need to make sure the metal duct and soffet are sized to deliver the needed cfm. If you're coming down, might want to consider dropping a chase close to the unit....no 90's and a lot less heat load since we are not on an outside wall.
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Old 03-13-2011, 12:33 AM   #6
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Yes, there will be 90's wherever I tap off of the main trunk. But otherwise the trunk runs in line with the furnace. I am hanging the furnace horizontally on the ceiling. Poppy, I'm not sure what you mean by coming down.
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Old 03-13-2011, 12:43 AM   #7
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Quote:
Originally Posted by beenthere
The 2 ducts weather sheet metal or flex is the way to go.

Sheet metal trunks and then using flex supplies will also help to minimize sound transmission.
Beenthere, I've followed many of your posts and you seem to know your stuff. You are a great asset to the forum, so I respect your opinion.

So 2 smaller ducts are better than one larger. Is it better to do it with metal or flex. I think the plan is the two trunks well be 8". Each will be feeding two 6" and two 4" supplies. 8" feels small to me. What do you think?
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Old 03-13-2011, 08:22 AM   #8
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Quote:
Originally Posted by acerunner View Post
Beenthere, I've followed many of your posts and you seem to know your stuff. You are a great asset to the forum, so I respect your opinion.

So 2 smaller ducts are better than one larger. Is it better to do it with metal or flex. I think the plan is the two trunks well be 8". Each will be feeding two 6" and two 4" supplies. 8" feels small to me. What do you think?
What size is the furnace and A/C again?

2-8"(sheet metal/hard pipe, (flex not even close to 1 ton quietly)) is barely enough for 1 ton, and its moving air fairly fast then. Large ducts move air slower, so it inherently is quieter. Sheet metal flows air better at smaller sizes, but still needs to be insulated if the chase is on an outside wall. And there is any chance that unconditioned air can seep/leak into the sofit.

Flex duct will require at least the next size up, of what ever is the correct size for hard pipe.

A 2 ton A/C using hard pipe as the trunk line. Would take 2-10"ers as the smallest trunk lines for 2 tons. Or 1-14"er.

Flex would take 2-12"ers or 1-14"er(the hard pipe actually comes out to a 13"er, but 13" hard pipe isn't made).

So2-8" hard pipes will make it noisy, and restrict air flow too much. I'm guessing the 4" are to bathrooms/powder rooms?

So depending how much space you have for your sofit, you might be stuck with one trunk line, and in the interest of air flow, sheet metal would be better. A single trunk line won't let you balance air flow as well as 2 trunks lines would.

2-8" flex ducts moves roughly 360CFM quietly, 4-6" flex ducts moves roughly 320 CFM quietly, 4-4" flex ducts moves 140 CFM quietly.
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Old 03-13-2011, 01:23 PM   #9
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Quote:
Originally Posted by beenthere

What size is the furnace and A/C again?

2-8"(sheet metal/hard pipe, (flex not even close to 1 ton quietly)) is barely enough for 1 ton, and its moving air fairly fast then. Large ducts move air slower, so it inherently is quieter. Sheet metal flows air better at smaller sizes, but still needs to be insulated if the chase is on an outside wall. And there is any chance that unconditioned air can seep/leak into the sofit.

Flex duct will require at least the next size up, of what ever is the correct size for hard pipe.

A 2 ton A/C using hard pipe as the trunk line. Would take 2-10"ers as the smallest trunk lines for 2 tons. Or 1-14"er.

Flex would take 2-12"ers or 1-14"er(the hard pipe actually comes out to a 13"er, but 13" hard pipe isn't made).

So2-8" hard pipes will make it noisy, and restrict air flow too much. I'm guessing the 4" are to bathrooms/powder rooms?

So depending how much space you have for your sofit, you might be stuck with one trunk line, and in the interest of air flow, sheet metal would be better. A single trunk line won't let you balance air flow as well as 2 trunks lines would.

2-8" flex ducts moves roughly 360CFM quietly, 4-6" flex ducts moves roughly 320 CFM quietly, 4-4" flex ducts moves 140 CFM quietly.
This will be furnace only. It didn't get hot enough often enough around here to require a/c, most rarely will you find a house with a/c. The furnace quoted is a 95% 80000btu.

The 4" are for bathrooms and a small kitchen.

I have about 10"of vertical space for the soffit. Horizontal is not an issue. So will two 10" trunks be adequate?
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Old 03-13-2011, 01:59 PM   #10
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Quote:
Originally Posted by acerunner View Post
This will be furnace only. It didn't get hot enough often enough around here to require a/c, most rarely will you find a house with a/c. The furnace quoted is a 95% 80000btu.

The 4" are for bathrooms and a small kitchen.

I have about 10"of vertical space for the soffit. Horizontal is not an issue. So will two 10" trunks be adequate?
Not even close.
A 95% 80,000 BTU input furnace would REQUIRE a min of 1000 CFM to be at the upper limit of its allowable temp rise.
A 60,000 BTU 95% needs a min of 753 CFM.
A 60,000 BTU 80% would need a min of 634 CFM, and a 40,000 BTU 80% would need a min of 423 CFM.

So 2-8" flex trunks would be a disaster on a 80,000 BTU 95% furnace.
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Old 03-13-2011, 07:09 PM   #11
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Duct layout questions


Quote:
Originally Posted by beenthere

Not even close.
A 95% 80,000 BTU input furnace would REQUIRE a min of 1000 CFM to be at the upper limit of its allowable temp rise.
A 60,000 BTU 95% needs a min of 753 CFM.
A 60,000 BTU 80% would need a min of 634 CFM, and a 40,000 BTU 80% would need a min of 423 CFM.

So 2-8" flex trunks would be a disaster on a 80,000 BTU 95% furnace.
Now I have doubts about this guys ability altogether. Is 80k btu even the right size furnace for 1500sqft? I know it's impossible to calculate without seeing the house, but ball park? Entire house will be insulated. Windows will be low e, double pane. Only fenestrations are on the short side of the 25'x40' foot print: four 4x6 windows, one 6' wide and one 3' wide full glass patio doors, one 3' wide half glass patio door.

I remember doing one of those very basic online calculators a while back and came up with 60k btu. I never trusted that number though because I didn't feel like that formula had enough parameters to be accurate.

Last edited by acerunner; 03-13-2011 at 07:20 PM.
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Old 03-13-2011, 07:48 PM   #12
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Many 1500 sq ft homes only need a 40,000 BTU furnace.

Try this load calc program, its worth the 49 bucks. http://hvaccomputer.com/talkref.asp

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