You can get them to do it right

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
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On 01/18/2014 08:23 AM, Bill Gill wrote:

I am sure it's a waste of money. Putting in a /slightly/ large return duct will probably have negligible benefits.
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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.
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On Saturday, January 18, 2014 1:39:48 PM UTC-5, snipped-for-privacy@attt.bizz 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.
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On Sat, 18 Jan 2014 12:19:23 -0800 (PST), " snipped-for-privacy@optonline.net"

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).     

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On 1/18/2014 8:51 AM, philo wrote:

That is not slightly smaller. The diameter is 20% greater and the cross sectional area is 44% greater. Not too 'slight'.
Bill
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On 1/18/2014 8:23 AM, Bill Gill wrote:

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.
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Agreed, flex is horrible. I have to rip down an entire ceiling in a long hallway to replace about 35 feet of flex that was damaged by rats.
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Your problem is *not* the flex.
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On 1/18/2014 9:55 AM, gonjah wrote:

Are you going to spec galvanized, not flex?
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On 1/18/2014 12:13 PM, Stormin Mormon wrote:

Probably with flex and be sure they use the right materials with metal angles. Sounds like any other route would be too expensive right now.
Oh the joys of home ownership.
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On 1/18/2014 3:20 PM, gonjah wrote:

Best wishes. Sounds like you've got some understanding of what works, what doesn't. And that you will keep an eye on what the workers do. Or, do it yourself.
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On 1/18/2014 9:23 AM, Bill Gill wrote:

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 common situation.
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.
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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 more maintenance.
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