Furnace return plenum "bangs" when blower motor starts/stops

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The sheet metal return plenum on one of our 2 otherwise identical gas furnaces gives out a loud "bang" as the duct contracts inwards when the blower starts and again when the duct returns to its original conformation when the blower stops.
The other furnace that doesn't "bang" seems to have a relief "X" on its broad surface while the one that does "bang" doesn't have the "X".
The return plenum is 24"x8"x60".
I would prefer not to reduce the blower speed since the furnace is in the basement and needs to provide forced air all the way to the 2nd and 3rd floors and the ductwork is highly non-optimal (the previous owner had some hack do it with a maze of flex duct -- one day I will have it replaced but not now...)
I have heard about people using "banding" to stiffen up the plenum and reduce the movement but am not sure of the best way.
So... 1. What is the recommended way to eliminate such movement and "banging"?
2. If some type of banding is appropriate, what are the appropriate materials and techniques to do it in a professional and neat manner?
THANKS!!!!
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Well there you go. Those Xs are there for a reason. Get the installer back to do the job right. Otherwise, you'll have to add some external stiffening.
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Wish I could but it was installed 10 years ago - under the previous owner. External stiffening is probably the simplest solution though I could use some pointers on specifics...
(ultimately, I will need to redo all the ductwork to replace the flex duct hack)
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Go to lowes and get a small piece of angled iron or aluminum. Can be 1/2" x 1/2". Cut to fit diagonally, drill holes, and put some screws into the sheet metal.
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Do I really need angle iron? Wouldn't that stick out and look awkward (and potentially be dangerous by sticking out that way)
Would using thick enough iron banding work? I was thinking of making one or two bands wrap around the 3 accessible sides.
I imagine all I need to do is overcome the net suction effect of the blower (net of the air flowing through the returns and net of the static bending resistance of the sheet metal plenum).
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I think that the poster recommending use of angle iron meant to put it parallel to the plenum over a considerable distance to stiffen it. Or, if you know where the bowing occurs, the angle iron could be cut to length and placed at rightr angles to the plenum to stiffen it at that point. But if the plenum is in the ceiling rafters of the basement, 1/2 inch sticking down shouldn't make any difference unless you basketball team members using your basement.
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The plenum is actually vertically oriented, sitting on the floor adjacent to the furnace which is why I was concerned about having something sticking out that people could bang into (given that the space is a bit cramped there).
The bowing and banging (which I have been told is properly called "oil-canning") occurs very prominently across the middle of the "belly" of the plenum so I'm not sure that angle iron along the edges will help enough since the problem doesn't seem to be along the edges per-se.
I bought some more "ess cleat" last night and it actually seems to be about as stiff as 1/8" flat iron (note "drive cleat" was significantly less stiff which is understandable since it has 2 (at most) layers vs the 3 plus layers for ess cleat). I figure if I need more stiffness I could even interdigitate two pieces of ess cleat to get an even more ridgid band.
I will let you know how this works...
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If it's about as stiff as 1/8" flat steel, you're wasting your time. Even 1/16"x1/2" angle aluminum will provide way more stiffening than 1/8" flat steel.
1/2" ain't gonna kill ya. Just round off the corners with an angle grinder or belt sander and screw it on, either in an X pattern across the panel that's bowing, or horizontally across the middle of the panel, making sure to overlap the ends.
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On Jan 22, 10:19am, snipped-for-privacy@rochester.rr.com wrote:

One piece of angle stock placed diagonally across the middle where it is oil canning will fix it.
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On Fri, 22 Jan 2010 07:19:59 -0800 (PST), snipped-for-privacy@rochester.rr.com wrote:

evvective as either horizontal or vertical.
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snipped-for-privacy@rochester.rr.com writes:

I ended up using aluminum angle iron but mounted it on the inside so I got all the stiffness I needed without any protrusions -- all I had to do was to remove one of the ducts which gave me access to the interior of the plenum. I then used sheet metal screws to secure two horizontal pieces of angle iron.
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That way, you're not ripping clothes when you walk by. Good idea.
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Christopher A. Young
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I've seen it done with a section of allthread, with washers and nuts to establish a 'pinched' distance, and the nuts on the outer portion to snug it in.

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blueman wrote:

Aluminum angle iron? Could you tell me where I might find such an interesting metal alloy? *snicker*
TDD
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Maybe returns and vents are to small
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flexduct. But I can't replace it all now for many reasons so I need an interim fix that works.
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Cutting a hole for a basement return might do it, I did it.
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On 1/21/2010 13:02, ransley wrote:

That may cause problems depending on the proximity to the furnace with the return air duct drawing in combustion air. As for the angle iron idea, this doesn't have to be iron; aluminum may be used. It only needs to be strong enough to stop the sheet metal from flexing.
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Since when do furnaces need ducted returns, Ive seen no return ducts many times, just the furnace intake.
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In colder cliimates a return plenum is almost always necessary to circulate ari in rooms far from the furnace. Houses you saw must have been 40 - 50+ years old.
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