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Old 02-01-2012, 09:01 AM   #1
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Duct (argh!)...


Total Duct Trunk run is 36 feet feeding a total of 9ea branch ducts.
These are all 6 inch round and about 10 feet long the register boot.

Five of these tap off the side of the trunk, then under a beam and up.
Three go all the way to the wall; two stop short (bathrooms).
Four of these ducts tap off the top of the trunk; go all the way to the wall.

Assuming that these 9 branch ducts and registers work and are staying put...
what size(s) of duct **should** the trunk they are attached to be?
Thanks.
-------

Here's the (rough) Trunk & Branch layout:

_|___________________|______|___________________|_ _______|__
-----|-------|--<80K>----------------------------------------|---------------|-

|<--3'->|<----8'---->|<-----6'------>|<-----9'----->|<-----10'---------->|


Last edited by TarheelTerp; 02-01-2012 at 09:08 AM.
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Old 02-01-2012, 09:53 AM   #2
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need to know what size furnace, house size, how many tons of air .

model of furnace would help too.

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Old 02-01-2012, 04:46 PM   #3
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Quote:
Originally Posted by hvac5646 View Post
need to know what size furnace ...lets say 70-80,000 Btu; 90%+
house size ...lets say 1300SF; single floor R32 ceiling etc
how many tons of air ...lets say 24,000 Btu or 2T
model of furnace would help too ...yet to be determined
Feel free to plug in (assume typical?) for anything else that you might need (CFM; SP, etc)
Thanks

Last edited by TarheelTerp; 02-01-2012 at 05:00 PM.
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Old 02-01-2012, 05:16 PM   #4
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Duct (argh!)...


1300 sq ft in NC only needs a 70,000 BTU 90% plus furnace if its made of all glass.
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Old 02-01-2012, 05:47 PM   #5
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Quote:
Originally Posted by beenthere View Post
1300 sq ft in NC only needs a 70,000 BTU 90% plus furnace if its made of all glass.
If you say so..
The question is about sizing duct.
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Old 02-01-2012, 05:50 PM   #6
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Tarhell, PM me your email and I wil send you a deal about duct work that may help you.
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Old 02-01-2012, 06:01 PM   #7
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Duct (argh!)...


If the furnace and coil is at one end ,20x8 would do it.If the furnace is somewhere in the middle that would change.
What is all that 3',8',6',9',10' all about?
Now while I have seen a few supply side mistakes,Return air ducting mistakes are way more common.
On 2 ton you would like to see at least a 24x8 drop.
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Old 02-01-2012, 06:06 PM   #8
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Quote:
Originally Posted by REP View Post
What is all that 3',8',6',9',10' all about?
These are the distances between the various areas where the trunk gets tapped.
I'm assuming that the whole trunk shouldn't be done with one size.
Do you get how the | indicates those 6" branch ducts?

The (rough) Trunk & Branch layout:
_|___________________|______|___________________|_ ________|__
-----|-------|--<80K>----------------------------------------|-----------------|-

|<--3'->|<----8'---->|<-----6'------>|<-----9'----->|<----7'---->|<---3'-->|

Quote:
If the furnace and coil is at one end...
It's not. See that 80K at about 1/4 way into the run?

Quote:
Return air ducting mistakes are way more common.
One bridge at a time. Thanks

Last edited by TarheelTerp; 02-01-2012 at 06:13 PM.
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Old 02-01-2012, 06:07 PM   #9
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Duct (argh!)...


Quote:
Originally Posted by TarheelTerp View Post
If you say so..
The question is about sizing duct.
The duct work is sized to the mode that requires the most air flow. A 70,000 BTU input 95% efficient furnace requires a lot more air flow then a 40,000 BTU 95% efficient furnace. 1000CFM for the 70, and 600 CFM for the 40.

So how can you size the duct work, without knowing the air flow you need.
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Old 02-01-2012, 06:18 PM   #10
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OP PMed me with this:
Quote:
Originally Posted by TarheelTerp
Ever install any Kelvinator stuff?
I'll probably end up with Lennox or Rheem...
but it would be great to have a benchmark to see how much the name brand premium comes out to.

Would you mind checking your wholesaler?
Thanks

Kelvinator Furnace KG7SC-072D-024; 66,000Btu; 95% (25B141)

you are not allowed to solicit contractors by PM.

And since when is it customary to ask a contractor what his costs on equipment are?
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Old 02-01-2012, 06:32 PM   #11
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Quote:
Originally Posted by beenthere View Post
So how can you size the duct work, without knowing the air flow you need.
You drop back ten and punt.
Isn't it your practice to install all new duct on every replacement job?

The Q: Assuming that these 9 branch ducts and registers work and are staying put... what size(s) of trunk duct **should** they be attached to? Thanks.
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Old 02-01-2012, 06:41 PM   #12
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Duct (argh!)...


Tarheel, send me your email address and I can send you a booklet helping you understand alittle better of how to size duct. PM it to me
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Old 02-01-2012, 06:59 PM   #13
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Duct (argh!)...


Quote:
Originally Posted by TarheelTerp View Post
You drop back ten and punt.
Isn't it your practice to install all new duct on every replacement job?

The Q: Assuming that these 9 branch ducts and registers work and are staying put... what size(s) of trunk duct **should** they be attached to? Thanks.

Is that 9-6" branches each carrying 70 CFM, 100 CFM, or do some of them carry less then others.

On replacements, I test the duct to see what it can carry, and put in the right size equipment for the house. not just some arbitrary size.

Now is the time to do the load calc, and get the right size equipment. Instead of just guessing at a size, and not having a good system.
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Old 02-01-2012, 08:16 PM   #14
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Quote:
Originally Posted by beenthere View Post
Now is the time to do the load calc, and get the right size equipment. Instead of just guessing at a size, and not having a good system.
I agree entirely. And I would even pay good money to have someone produce those numbers for me. But so far, and even with asking for it specifically...
the people from the top companies I've had in aren't doing this.

It's all "don't look for the man behind the curtain" and "this should be OK".
But that is still a whole other set of discussions.

Back to THIS discussion (if you're inclined to do so)... The Q remains:
Assuming that these 9 branch ducts and registers work (they do fine)
and are staying put (they are)...
what size(s) of trunk duct **should** they be attached to?

Feel free to plug in (assume? typical?) for anything else that you might need
(CFM; SP, blower speed, etc) or commonly find/input. Thanks

Last edited by TarheelTerp; 02-01-2012 at 08:21 PM.
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Old 02-01-2012, 08:19 PM   #15
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Duct (argh!)...


HVAC CALC.

Do your own load calc.

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