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
I think I have perhaps answered my own question in part, and now need to
determine if replacement ducts, booster fans, or some approach makes more
sense.
Again, thanks for suggestions and assistance.
Smarty
"clint" wrote in message
news:zdmxh.2036$yH3.1469@trndny07...
"Smarty" wrote in message
...
Thanks for your reply. I'm a little perplexed. Putting an air handler on
the
second floor is a possibility, but I then have a configuration where the
evaporator coil is sitting in the basement in the furnace plenum, and a
secondary blower is running on the second floor being used as a "booster
fan". I take it your suggestion doesn't also include adding a second
coil,
outdoor compressor, etc. for cooling the second floor. This would be a
highly unusual and extreme solution for the climate I live in, where a
single central (basement) furnace with evaporator coil provides adequate
2nd
floor cooling for both older and new build homes.
I am in a Northeast U.S. climate where summer temps (July-September)
seldom
go above 90, are most often in the mid 80's, and the temperature drop /
difference I need to achieve in the 2nd floor is typically 15 degrees. An
8000 BTU/hr window air conditioner used to cool the space very
adequately,
and I was (improperly) advised by the HVAC "expert" that 3 ducts each
with
10 inch diameter could deliver way more CFM and BTU/hr cooling capacity
than
required. This has not proven to be the case, and my first (and perhaps
erroneous) conclusion is that the ducts cannot deliver enough CFM because
they are restricted. My intuition is that these flexible ducts impede the
air flow a great deal more than rigid ducts would. And thus my
question........
Smarty
wrote in message
...
On Sat, 3 Feb 2007 20:38:57 -0500, "Smarty" wrote:
About 10 years ago, I had three 10" diameter runs of insulated flexible
duct
installed by an HVAC contractor to go from my basement forced air
furnace
plenum to a 2nd floor attic room, hoping to provide adequate cooling
and
heating to the 2nd floor attic area. Each of the 3 runs is about 25
feet
long.
I have never been able to get really adequate cooling or air flow on
the
2nd
floor, and can only assume that the static pressure drop on these
flexible
ducts must be much higher than rigid ducts of the same approximate
diameter
and length. The installer was very careful to avoid any sharp bends or
turns
in the ducts when installing them, but they really seem to have a lot
of
pressure drop.
Is there any easy way to predict how much of an improvement I might
expect
by tearing out these flexible ducts and replacing them with the same
size
rigid 10 inch circular ducting?
The HVAC contractor who designed and installed this arrangement
apparently
over-stated / over-estimated the flow / volume capacity, and the CFM
delivery rates were highly optimistic. I get very little cooling, and
better
but not great heating.
Thanks for any opinions in advance.
Smarty
The real problem is in trying to push that much cold air up into the
attic. Bigger/better ducts might help a little but you should really
have an air handler up there if you are serious about cooling it.
I understand that this may be a foreign concept in places where the
heat is on more than the A/C but from a physics aspect they are two
entirely different things.
I also had the same problem with 2 upstairs bedrooms (new house-lousy
builder). I consulted a HVAC contractor. They added an additional return
from the basement through a 1st floor closet, into the attic (insulated
duct)and down to the ceiling between the 2 rooms. I was skeptical, but the
problem was solved. There was enough supply but not enough return.
Clint