In article , "Smarty" wrote:
Thanks to all for your help. I have a large return duct already installed
using rigid rectangular duct (I think it is roughly 12 to 15 inches wide and
about 5 inches tall) which brings the hot air from the top of the attic room
back to the air handler / furnace. It passes thru a first floor closet and
is maybe 20 feet long including a couple 90 degree turns it makes coming up
to the attic. No doubt it is less optimal than using a 2nd floor separate
cooling system, but I am totally opposed to the idea of adding all the extra
equipment and complexity and cost.

The original question really is trying to understand how well or how poorly
flexible duct compares to rigid duct. I was able to find a good engineering
article on the web comparing the two, and have now learned that flexible
duct can be fairly similar to rigid duct ***IF***** it is not compressed,
and is properly supported without sag or severe bends. The tests conducted
by this PhD ASHRAE engineer showed a difference of as much as ******TEN
TIMES THE PRESSURE DROP***** for improperly / compressed flex duct. This is
most likely my problem, since the CFM I can create in the attic seem far
lower at the outlets than the CFM available at the other end of the 3 runs.
See:

http://www.mmmfg.com/pdfs/060601_CC-...tTechPaper.pdf

Thanks for that very interesting info.

I have a lot of rather crappy flexible ducting in my crawlspace.
This confirms all of my own impressions.

In practical domestic HVAC applications, the situation gets
even worse. In my case, the ducting does not appear to be
terribly well insulated and in some spots the insulation
is clearly damaged. I strongly suspect there is some
significant leakage as well. In fact, when I purchased
this property (it's ~15 years old) the home inspector noted
that the ducting had become completely detached from two
of the registers. So a significant amount of air was just
heating/cooling the crawl space.

When you consider those problems AND the likely pressure
loss, it tells me flexible ducting is pretty much a
disaster area to be avoided!

I've made a few repairs here and there but I don't think
there's much else I can do in short term other than
ripping the whole lot out. Then I might was well
replace the 15 year old furnance and AC too with
high-efficiency units and some independently
controllable zoning. And that gets fairly expensive :-(

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