HVAC Plenum and Duct Design

db4570September 26, 2012

I'm making some improvements to my noisy and undersized ductwork, and seeing big improvements to the areas I have done so far.

The next step is to install a correct plenum and ducts to a family room that is over a crawl space, which has a very inadequate system. My plan is to used insulated flex duct, connected directly to the furnace plenum, starting at 10" diameter, and running it through this space, wyeing off 6" ducts for individual registers along the way, and reducing the diameter of the main plenum along the way. Similar to the attached picture. The rest of the house is supplied by a traditional rectangular main trunk off the opposite side of the furnace plenum.

I'm trying to follow good duct design practice as I've learned by reading, but something mystifies me. In a rectangular trunk installation, we are told never to install a branch duct off the very end of the trunk, but when I see diagrams of reducing trunk systems, there is a takeoff off the end.

What exactly is the reason for not taking a branch duct off the end of an extended plenum? And why does this rule not seem to apply to a reducing trunk layout? I want to do this right, assuring the best airflow and least noise.

Any opinions?



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no pic attached.
you don't tap off the end of the supply plenum
because it reduces static pressure.
as the air enters the plenum it hits the end cap
if a duct is taken off the end cap the total pressure
is reduced.

also you don't tap off the plenum within 18" of the coil
not sure why...but just one of the things I was taught
in class.

10" to tri box then 6"? you sure about those sizes?
6" is usually a bathroom. do you have a ductalator to
know how many cfm of air each size duct can move?
this along with room size determines how much cmf
to provide. then you can divide it between two ducts.

that pic would help.

make sure you mastic seal all duct take offs, where
flex connects to supply boxes, where supply boxes
penetrate floor & plenum to equipment connections.
duct leakage in a crawl space creates a lot of problems.

best of luck.

    Bookmark   September 28, 2012 at 11:18AM
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" don't tap off the plenum within 18" of the coil
not sure why"

You do not want to increase flow velocity over the coil.

It reduces effectiveness and efficiency.

    Bookmark   September 28, 2012 at 1:04PM
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Uniformity of flow can also be disturbed.

    Bookmark   September 28, 2012 at 1:05PM
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