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.
On Saturday, January 18, 2014 1:39:48 PM UTC-5, firstname.lastname@example.org wrote:
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.
On Sat, 18 Jan 2014 12:19:23 -0800 (PST), " email@example.com"
Geez, larger than the 36% difference between 14" and 12" duct would
imply (or the 44% difference between 12" and 10").
The free area goes up as the square of the diameter (area) but the
drag only linearly with the diameter (circumference).
Great link. Thanks.
I have flex duct and I don't like it. I made a lot of the repairs myself
using common sense. I could not believe the amount of money the previous
homeowners must of been wasting. Some of the tubes were crimped (or
leaking) and one didn't function at all. I really need all of them replaced.
On a project like that, I agree, it's best to
get it right. I don't have the stats, but I do
know that flex duct has a lot more air resistance.
It should ought to be possible to do with sheet
metal, as you had it done. When I worked for a
HVAC company years ago, the guy who was training
me said that under sized return air is a very
Years after that, I got a call to a house where the
furnace kept cutting out. Turns out that the only
RA vent had been removed, and plywood put over the
opening. With pretty much zero air to work with,
the furnace kept over heating, and the house was cold.
Those floor registers are expensive. I think it was
about $75, my cost. Ouch!
Sounds like you were informed, polite, and got the
results you desired. And thank you for posting, here.
On Sat, 18 Jan 2014 13:11:42 -0500, Stormin Mormon
Sure. They cost money.
When I bought this house, we were having AC problems. The HVAC guy
noted (not the issue) that the returns were too small so I had him
install another 12" return from the living room (an easy run and
central to the house). It does take another 20" filter but really no
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