On Saturday, January 18, 2014 1:39:48 PM UTC-5, wrote:
On Sat, 18 Jan 2014 08:51:28 -0600, philo� wrote:
On 01/18/2014 08:23 AM, Bill Gill wrote:
I had some work done on my heater and the tech checked the
air flow. He found that one of the 2 air return ducts was
smaller than it should have been. So I had a man come out
to give me an estimate on putting in a larger duct. His
proposal was to replace the 10" metal duct with a 14" flex
duct. I looked at it and the very idea of flex duct seemed
to be kind of iffy. So I did some research and found that
the general consensus is that flex duct is ok, if it is
installed right. And hat there is a lot of flex duct that
is not installed right. That involves mostly short straight
sections. Well, my installation involves a right angle
bend and a 1 foot offset near the end. Not exactly straight.
It took some doing but I finally got the salesman to understand
that it is going to be hard to get the duct 'fully extended'
with that much play in it. So you would get a lot of
turbulence and reduced flow. He thought that by oversizing
it from 12 to 14 inches it would make up for it. But
he did come around after a while. So they will put in
12 inch metal duct.
It costs more, but I figure you can do it cheap or you
can do it right.
In my search the site that I found to be about the best
was http://askweldin.com/Flex.html. There were a number
of places but I thought he provided the best over view.
Bill
I am sure it's a waste of money. Putting in a /slightly/ large return
duct will probably have negligible benefits.
A 14" duct is 36% larger than a 12" duct. If there is a problem with
turbulence because of the ridging of flex duct, the difference will be
even larger.
Larger than what? And I'd point out that if he has a sharp 90
turn that can be turned into a gradual turn using flex,
that is a significant benefit
in reducing airflow resistance. The problems with flex are as the
OP pointed out, mainly if it's poorly installed. If you chuck it
over a cross brace in an attic and let gravity compress it, then
it sucks. If you shove it in where there isn't enough room and
compress it, then it sucks. But for short run where you can install
it properly, I think it's fine, especially if you can take advantage
of some of it's features.