In the ductadd dialog box under duct shape is a "Undefined" choice. This
allows the user to draw single line, un-sized, duct at specified
elevations. Many companies have a DD phase (Design Development) and a
CD phase (Construction Document)...I've seen these names a bit different
at different companies, but what you do in these phases are essentially
the same. In DD, the Architects are still (Ok, they never stop)
changing the layout...well, they slow down and make minor ones in CD,
but they do slow down enough engineers can solidify their design in CD
phase. So, back to DD phase, layouts are changing and engineering wants
to schematically show their concept of where the duct is going to route,
where diffusers go, where & what equipment is planned. Sizes are added
as the design is refined and knowledge is gained. Some sections may be
sized at different times. Until the engineer knows the CFM's at
diffusers and equipment, the duct can remain "Un-defined" single line.
It serves it's purpose to coordinate and plan duct routes with other
disciplines...and it is a placeholder so that down the road (closer to
CD phase or in CD) when CFMs are better known, the duct can be very
quickly sized and in 3D...one button click. There's always engineering
judgment and tweaks required, but the sizer roughs it it in saving time.
The duct size dialog looks at the duct system setting to get the
friction and velocities for that type of system or you can override them
at the sizer dialog. You can select a section of duct to size or do the
whole thing. Any connected diffusers need to know their CFM's prior to
sizing the section of duct. In the sizer dialog, you can specify the max
height for oval ducts and max diameter for round. You can set all
diffuser branches to get their shape and size from the diffuser allowing
mains to be what you specify. There are many options in the sizer to
quickly get the duct size and shape where you want. Click of a button
will take your selected duct from Un-defined to fully sized and you can
always check it with a ductulator...even though, I've found that there
are subtle differences between different manufacturer's ductulators.
We've compared our results to each of the various different ASHRAE
sizing methods and we're close. We are also, like other features,
continuously making improvements, which as an Autodesk employee, I
cannot tell you when or what you may see in a future release. Ideas and
wish items are always welcome.
If you do not like a size, the built-in sizer is best used as a quick
"Rough-in"...it gets most of the duct sizes close to what is expected
and it's up to the engineer/tech to do the due diligence, checking
sizes. When or if a change is desired, the duct can be selected on and
manually changed, or re-run the sizer.
Hope this answers your question.
Joshua Benoist, PE (Autodesk)
ptodorov wrote:
> What is the purpose of drawing the system by 1-line duck and then calculate the system?
>
![Autodesk Design & Make Report](https://forums.autodesk.com/t5/image/serverpage/image-id/1350519iCB55845EBFC43016/image-size/large?v=v2&px=999" "Autodesk Design & Make Report")
