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.