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mid787@ptd.net wrote:
>
> anyone familliar with using flex duct in a two-story home for heating
> and ac? I need to make a decision on whether to use sheet-metal duct
> work or flex duct. all heating contractors will charge more for
> sheet-metal. Any drawbacks to using flex-duct?
>
> thanks mid787
Yes! Flex duct restricts air flow much more than rigid duct. Therefore
less of the cold air gets to the living space due to the pressure losses
and since what does make it out has had time to warm up, the A/C is less
efficient. On the other hand, for short runs flex may be adequate, but
it's especially important to make sure the duct is well sealed to the
main line using mastic compound in addition to duct clamps.
Insulation should be on the outside of the duct to avoid spreading
fiberglass fibers though the house. This is important for rigid duct as
well. When using rigid duct it is also especially important to ensure
all joints and duct caps are sealed with mastic compound and tyvek
tape. Use a flashlight to double check the joints, especially in the
hard to see areas of the ducts where the duct installers figure nobody
will ever see whether or not they did a good job. The expensive, cold,
pressurized air blowing through the ductwork will definitely find the
leaks and your electric bills will too.
For ductwork in the attic picture this: the coldest air under the
highest pressure running through unsealed ductwork in the hottest place.
Every pin hole in the ductwork blows right outdoors. In Florida where
they measure these things, I'm told they loose 13% of their total
electrical generating capacity through duct leaks.
Where ductwork passes through a floor or ceiling you should make sure
the gap is foamed to avoid additional loss of conditioned air.
But it gets worse. When leaking supply ducts blow air into the attic,
it's lost. The A/C blower becomes starved for air and this creates a
vacuum in the living areas. Now the little holes throughout the entire
house shell will be sucking hot, wet outdoor air into the framing. Guess
what happens when 90 degree 80% RH outdoor air hits the backside of 70
degree drywall.
I guess this got into more than just the question of whether or not to
use flex duct, but I hope this is helpful to you in thinking through
this project.
Jim Baker
There are several experts on this topic who lurk the newsgroup from time to time. One who does not, I believe is Phil Morey. He can be contacted at:
Philip R. Morey, Ph. D. CIH
Director of Microbiology
Air Quality Sciences
1337 Capital Circle
Atlanta, GA 30067
770-933-0638
fax 770-933-0641
Phil is among the best known in the field, because he has been a pillar of knowledge and strength on the ASHRAE Committee working on Standard 62, which has wrestled with these issues extensively for the last 10 years.
On Thu, 10 Apr 1997, mberkhei@GP908.jsc.nasa.gov wrote:
I am in Industrial Hygiene and would like to know more about sampling of toxins in air, experiences with growth in air handling systems(mold fungi),hypersensitivity pneumonitis diagnosis and other related topics
