Jon;
You have encountered the the infamous ductboard. You're right about it being
folded insulation, with a bit of stiffener thrown in. Lasts long enough to get
by a warranty, but not much.
Duct bending tools are called, shears, brakes, seamers, rollers, and snips. :)
Rectangular duct is fabbed to order. Bigger shops even have CNC layout
printers that spit out patterns just like your mother (my mother anyhow)used to
make dresses with.
Measure the old stuff, inside dimensions, and have a sheetmetal shop build it.
Might be a bit spendy. You could use a sheet metal plenum and then run flex
duct. flex has its detractors, but I think it's just fine for reidential use.
It's in my house.
Make sure the inside volume of the new is the same or greater than the old.
Most metal duct is lined on the inside for both thermal and sound insulation.
Three six inch diameter round ducts are approximately 85 sq.in. A 10 inch
diameter duct is about 79 sq.in. so you'd need a 12 inch (113 sq.in.)to feed
them. Always go to the bigger size, even for 1/2 a square inch. Make the
plenum big enough to tap off at right angles to the airflow. Tapping with the
airflow gives a supercharger effect to that branch and starves the rest of the
runs. It would be smart to put dampers in every tapping collar and wye branch
to balance the airflow. Plan it out on paper and figure each run. You don't
have to make a home run for every leg. Wyes (Y connection)are fine.
You can do the same with hard pipe, very time consuming, screwed together
joints and durable as all get out. You insulate it on the outside after
installation. Nasty job, that's how flex duct got so popular.
Do the attic in the winter and the cellar in the summer.
My arms itch just thinking about that insulation.
The above is very simplified and books have been written about duct design but
if the previous system worked well enough before the crapboard fell apart you
ought to be OK duplicating it in flex.
Cheers,
Matt
> My HVAC ducts, made from folded insulation, are less than 12 years old and
> falling apart everywhere. The ducts themselves are pulling apart at the seams
> and
> the tape on the joints is coming unstuck. Also the supporting brackets are
> cracking from fatigue.
>
> All the runs are straight with no complicated jogs or bends. Would replacing
> them
> with more insulated duct, or better yet galvanized steel duct, be within the
> realm of a do-it-yourselfer, assuming I could rent the necessary bending
> equipment? I probably couldn't do a worse job than what was done already.
>
> Has anybody ever tackled a job like this that could share some wisdom?
Assuming
> I
> couldn't get the sheetmetal bending equipment, is there a way I could mix
folded
> insulation and steel to obtain stronger ducts?
>
> Thanks
> Jon
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